How I prevented QRM to HF reception from my solar charger and AC inverter at Quartzfest

Figure 1:
White board from Quartzfest!
Click on the image for a larger version

As it happens, I found myself at QuartzFest in Arizona in the latter half of January, 2026 where we set up some banners proclaiming the existence of the Northern Utah WebSDR (link) - but I also scribbled on a small white board the words "QRM-Free Solar is possible - Ask How!".

Between the SDR, this message and the diverse portable HF antennas erected, I have had a lot of conversations over the past several days about these and many other topics, meeting new people and re-acquainting myself with others that I've seen on and off over the past several years of my attending QuartzFest (this is year #4 for me.)

RFI-less solar IS possible 

During the "Solar Walkabout" - an on-foot tour to look at how others camping have deployed their solar panels - I volunteered to have folks look at what I'd set up:  It's nothing obviously special - a glass-panel 200 watt Renogy folding array and another Renogy "flexible" solar array - but there is one major difference:  It does NOT produce HF QRM, meaning that I can plant my portable antennas near my panels and not get any interference on HF.

As I've done some previous articles on this, what I'll present here is mostly a set of links to those articles with a quick overview, but this effectively puts that information in one, handy place.

Let's start with quieting the Renogy solar charge controllers:

Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system- Link

Figure 2:
My humble, RF-quiet solar array at 2026 Quartzfest
Click on the image for a larger version.

The main issue with Solar charge controllers is that you have a "dipole + transmitter" situation:  The panels themselves do NOT cause RFI, but the charge controller is effectively a transmitter - especially if it's a PWM and/or MPPT-type - and the legs of the "dipole" are the solar panels (possibly long wires connected to large, rectangular pieces of metal) and another set of wires going to the battery - which also find their way around your RV/campsite via the inverters, DC wires, etc.:  It is no surprise at all that RF finds its way out of these things!  By adding filtering, we are effectively "shorting out" the the RF at the feedpoint of this hypothetical dipole and preventing it from radiating.

To quiet these panels, I added bifilar-wound ferrite toroids - but also bypass capacitors:  The toroids (ferrite) alone will probably knock down the QRM by 2-3 "S" Units, but if you are getting S-9+ interference from your solar, simply knocking it down to S-6 or S-7 when you are in the boondocks - where the natural noise floor is closer to S-1 or S-1 - is still pretty bad!

The key here is adding capacitors in addition to the ferrites and this method is perfectly capable of quieting even the noisiest of solar chargers.  It is also vitally important to put this filtering physically close to the noisy device and use good-quality bypass capacitors. 

Figure 3:
Filtering on the bottom of the Renogy controller
making it RF-quiet.
Click on the image for a larger version.

While the above blog entry showed a modest (200 watt) system, the above can be scaled up for higher-power systems:  Larger wire will handle more current and larger toroids will accommodate it!

RF Quieting a Samlex 150 watt Sine Wave inverter - Link 

Another component of RV/camping with power is the inverter to run mains-voltage devices, and these can be terrible noise sources.  The article above shows how it's possible to make one of these devices completely quiet.  For the older Samlex inverter - which was terribly noisy out-of-the-box, it is now quiet enough that I can power LED Christmas lights from it that are strong from the same mast as the antenna and I get NO RFI (the LED Christmas themselves don't produce QRM).

I was fortunate that there was enough room in the Samlex's case to be able to add this filtering, but it may be added externally as well, provided that the leads are kept short.

What follows below are some methods for quieting UPSs (Uninterruptable Power Supplies).  These are very much like the inverters in an RV in that they produce mains voltage from battery power - and the same problems with RFI occur:

A high-current DC (and AC) noise filter for UPS or RV use - Link

This shows a rather extreme example (an 8kVA UPS) where high currents are involved:  Such would be the case with a kilowatt-class DC-AC inverter or even a large PV system.

Containing RF noise from a sine wave UPS - Link

This article shows the techniques involved in quieting a lower-power UPS, but it also introduces some other components:  Rather than winding your own filter using toroids and wire, you can get "Line Filter" modules from electronic parts supplies (e.g. Digi-Key, Mouser) with brand names like "Corcom" or "Delta" (among many others.)  These are self-contained modules with the components built-in - available in a wide variety of voltage and current ratings - that can do an excellent job of filtering.

 

Completely containing switching power supply RFI - Link

This is an extreme example, but it shows how one might be able to make even the noisiest switching power supply quiet - and this might be important to someone who is trying to get every device in their ham shack - whether it be at home or on the road - quiet.  This method is foolproof in its effectiveness, but it is also likely overkill for many applications, but it discusses the "how and why" these techniques can work.

* * * * * * 

I hope that this helps those who venture out in the wild with their RVs, solar power and battery system and still be able to operate HF.

This page stolen from ka7oei.blogspot.com

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Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system

Figure 1:
Renogy 200 watt folding panel, in the sun
Click on the image for a larger version.

Update - 10/24: 

I recent spent most of a week in a national park where I extensively used the solar panel and charger - along with some LiFePO4 batteries - as my main power source.  The panel was set up with 3-6 feet (1-2 meters) of my portable HF antenna.  I did not notice any interference from this system at all.  I did notice a bit of QRM from my fridge/freezer cooler to which I have since added its own RF filtering.

A year or so ago I got a 200 watt foldable solar panel system.  This unit - made by Renogy - consists of two glass panels in metal frames equipped with a sort of "kickstand" assembly to allow it to be angled more favorably with the sun to improve its output.  I use this panel when "car camping" to charge the batteries to run the sorts of things that one might bring:  Lights, refrigerator, amateur radio transceivers and who knows what else.  

On that last point, I've done some "in the field" operating on the HF amateur bands while the battery is being charged and noticed that the charge controller (and not the panel itself!) produced a bit of "hash" on the radio - mostly in the form of frequent "birdies" that swished around in frequency as the solar insolation and temperature varied - as well as a general low-level noise at some frequencies.   

This problem is not specific to the Renogy panel's charge controller, but common to almost any panel+controller combination that you will find.

Nearly all "portable" and RV solar power systems cause QRM:

You will find similar systems built into RVs and campers and these are also well known (notorious, even!) for generating RFI.  The techniques described here to quiet interference from these devices applies equally to those as well - but note that one may have to "scale up" the inductors/capacitors to accommodate higher voltages and currents that may be found in those systems.

By placing the solar panel with charge controller and the battery being charged some distance away from the antenna, this interference could be reduced, but that fact that it was even there in the first place annoyed me, so I did what I have done many times before (see the links to other blog entries at the end of this article) and mitigated it by making fairly easy, reversible modifications to the panel's controller.

Portable solar panels and RFI In my travels, I've been around other users of portable solar panels of various brands and I have yet to find any commercially-available portable panel+controller combination that does NOT produce noticeable RFI on HF/VHF among the half-dozen or so brands that I have checked.  In comparison with most of the others that I've been around with radios, the Renogy is comparatively quiet - producing less overall QRM with fairly long wires between the panel/controller and battery - than the others - but I decided that I could make it even quieter!

Where does the QRM come from?

It is NOT the solar panel itself that produces the radio frequency noise, but rather the charge controller attached to it.

Modern charge controllers electronically convert the (usually higher) voltage from the solar panels down to something closer to the battery voltage and this is typically done using PWM (Pulse Width Modulation) which means that these devices contain high-power oscillators:  This is true for simple "PWM" types of charge controllers as well as those using "MPPT" techniques.  It's this oscillation / switching action that produces a myriad of harmonics that can extend through the HF spectrum - and even into VHF/UHF!

The "Antenna" in this case consists of two parts of the PV system as depicted in the drawing below:

Figure 2:
A typical solar charging system showing the separation of the two major components that can radiate interference:  The panel itself, connected to the input of the charge controller and the wires and load connected on the output side.
If there is even a slight amount of differential between the two at radio frequencies, the system will radiate.
Click on the image for a larger version.

 

In other words:

• The wires connecting to the load.  Typically a battery being charged - which can be connected to other things (e.g. vehicle, inverter, etc.)  The wires connecting the panel to these other things - and those devices themselves - act as part of the "antenna" that potentially radiates noise.
  

• The solar panel itself.  The solar panel consists of large plates of metal - not only the silicon of the panel, but any metal frame and wiring:  This large area of conductive material offers a suitably large aperture to permit radiation of HF RF.
  

The "load" and solar panel constitute two different parts of the charge controller's system when it comes to radiation of RF:  The panel is connected to the INPUT of the PWM circuitry while the wiring is connected to the OUTPUT of the PWM circuitry, effectively forming a dipole antenna.  To a degree, the electrical lengths of these two conductors - which can include power cords or even a vehicle - overall can broadly resonate, affecting certain frequency ranges more than others.

The reason for the generation of the interference is due to the fact that the PWM circuitry (which is operating at a frequency of 10s or 100s of kHz) uses square waves, rich in harmonics.  As the voltage input (from the panel) and the output (to the battery/load) are different parts of the PWM circuit, they necessarily have different waveforms on them.

Figure 3:
Charge controller with additional filtering showing added
bifilar-wound chokes/caps on both the input and output leads.
Click on the image for a larger version.

While this device does have some filtering to provide a degree of input impedance reduction (fairly high capacitance) and smoothing of the PWM waveform of the output (more capacitors and likely some inductance) the extent to which this filtering is implemented is suitable for the purpose of providing clean DC power to the load and maximize power conversion efficiency.  This filtering - and likely the controller's circuit board itself - was likely not intended to provide the high degree of RF suppression needed to make it quiet enough to avoid the conduction of RF energy onto its conductors which is then picked up by a nearby receiver.

Containing the RF energy

 

As the controller itself is potted with a silicone material, it's not practical to modify it directly to make it RF-quiet - and there is no need to do so:  Instead, we must take steps to eliminate any differential RF currents that may exist between DC Input and DC output terminals.

Ferrite alone is NOT the answer!

One may presume that the answer to this problem is the implementation of RF device such as snap-on or toroidal ferrite devices - and you would be partially correct.  Any practical inductor - such as that formed by the introduction of a ferrite device onto an existing wire - will have rather limited efficacy in quashing RF currents.

Snap-on devices (e.g. those through which a wire passes) have very limited usefulness at HF frequencies (<30 MHz) - especially on the lower bands - as they simply cannot impart a significant amount of reactance in the conductor onto which they are installed.  At higher frequencies (VHF, UHF) they can have a greater effect - but their efficacy will usually be disappointing at HF.

Using a device that can accommodate multiple turns through its center such as a toroid (or even a larger snap-on device) it may be possible to get up to a few hundred ohms of reactance on a conductor across a fairly wide frequency range - but even this will be capable of reducing the amount of RF by 10-20 dB (2-3 "S" units) at most:  Depending on the intensity of the RFI from the solar controller, this may not be enough to quash the interfering energy to inaudibility - particularly in a remote and otherwise "RF Quiet" location.

To be sure, it's worth trying just the ferrite devices by themselves to see if - in your situation - it reduces the RF interference from the controller to your satisfaction, but remember that the location where you are likely to be using this panel is probably far quieter (RF-wise) than your home QTH:  A "quiet" Solar charging system may seem quiet enough at your noisy home QTH, but could still be noisy in the middle of nowhere.

The addition of capacitors to the circuit can improve the efficacy over ferrite alone by orders of magnitude.  Consider the diagram below:

Figure 4:
Diagram, including additional filtering.  L1 and L2 are the bifilar chokes seen in Figure 3, above while the capacitors (C1a, C1b, C1c and C2a, C2b and C2c) and their implementation are described below.
Click on the image for a larger version.

 

Ferrite devices L1 and L2 are comprised of bifilar-wound inductors on the DC input/output lines, respectively.  These inductors will suppress common-mode RF energy that may appear - but these alone are not likely to be quite enough.

 

In order to force the RF energy to common mode to maximize L1/L2's effectiveness, capacitors C1a, C1b do so for the "external" connections (e.g. those connected to large devices, long wires) while C2a and C2b do so for any RFI emanating from the controller itself.

 

C2c - which is placed between the DC input and output of the charge controller - effectively shunt RF energy differences between the in/out terminals to minimize the differential currents.  Figure 3 shows C2a placed between the two positive terminals, but it could have been placed in any combination (+ to -, - to -, etc.) and been just as effective since the capacitors C2a and C2b effectively short the + and - terminals together at RF frequencies.  If your OCD bothers you, could could add additional capacitor combinations, but the three shown above for C1 and C2 proved to be adequate.

 

The real work for our filtering magic is actually done by C1c.  As seen from the diagram it's shunting RF currents that might appear on the "external" sides of L1 and L2 - which will have been significantly reduced in amplitude by L1 and L2 anyway:  The low impedance of the C1c at RF (a few ohms) coupled with the high RF impedance of the conductors through L1 and L2 work together to make sure that differential RF currents that might exist between the input and output of the charge controller are minuscule, and thus there is effectively no RF energy that can be radiated.

 

Figure 5:
Three 0.1uF monolithic capacitors placed across the
controller's terminals (C2a, C2b, C2c).
Click on the image for a larger version.

Implementation

A glimpse of what was done may be seen in Figure 3.  Some 14 AWG paired copper wire (red/black) was wound on two FT140-43 ferrite toroids - about 6 bifilar turns in this case:  Individual wires could have been used other than "zip" cord - just be sure that the two parallel conductors are laid in parallel to maximize the effectiveness of the bifilar configuration.  Two of these wire/bifilar devices were constructed - one for the DC from the panel and the other for the output to the battery/load.  "Spade" lugs were installed on one end of the red/black wires - two lugs per wire/bifilar assembly.  (FT240-43 or FT240-31 toroids could also have been used, but the FT140-43 is a fraction of the cost, half the diameter, and perfectly suitable for this application.  The FT240 size may be more appropriate if such a filter network is constructed for a higher-current system with larger-gauge wire.)

On the solar controller itself, small 0.1uF, 50 volt monolithic capacitors were installed (C2a, C2b, C2c) to form part of the filter circuitry:  Minimal lead length is important for maximum effectiveness.  While monolithic ceramic capacitors are preferred because they are small (and will fit more easily in tight spaces) and have very low ESR (Effective Series Resistance) one could use disk ceramic capacitors instead.  Film/plastic capacitors are less effective at higher frequencies.

Figure 6:
Terminal strip with capacitors C1a, C1b and C1c.
As described, these capacitors do much of the "bypassing"
of RF differential currents between the input and output.
Click on the image for a larger version.

As can be seen from this picture and Figure 5, the terminals are the "clamp" type and are connected in the same manner as the lugs on the cable on the bifilar toroid assembly. - and also note that this "modification" is completely reversible as nothing at all was changed on the controller itself.

The other end of the red/black wires were soldered to a four-position screw terminal strip as seen in Figure 6 - similar to the one on the back of the charge controller.  As with the terminal strip on the controller, three 0.1uF 50 volt capacitors were soldered (C1a, C1b, C1c) on the back for RF bypassing.  It is possible to have connected the capacitors under the clamps as was done on the controller, but soldering them to the back means that they would not be prone to falling out or being lost if the cables were changed.

 

With these connections made, the wire on the toroids and the connections to the added terminal strip were covered with "Shoe Goo" - a robust rubber adhesive (which may be used to fix shoes, as the name suggests) both as mean of strain relief and to provide electrical insulation.

 

The reader may have noted that we have physically brought together the input/output cables again at this terminal strip - and this was intentional.  By keeping the leads with the bifilar inductors as short as possible and then bringing them back together, we can use the shortest-possible leads on our capacitors to effectively "short" the input and output cables together at radio frequencies, making it impossible for the wires to radiate effectively at HF.  With this, the RF energy is contained within the area of the charge controller itself and the terminal strip/cables and since this is a very small aperture at HF, it can't radiate effectively and additional metallic shielding is unneeded.

 

At VHF/UHF frequencies - where the physical size of the controller+bifilar chokes is a larger proportion of the size of the wavelength (plus the fact that the components used won't work as well at these frequencies) means that some RF energy could radiate, but testing shows that the amount of VHF/UHF RF energy conveyed by the panel and cables was reduced below the point of detection more than a few feet (a meter) or so away from the system.

Spectrum analysis plots - updated 12/24

In the original version of this post I "loosely coupled" a spectrum analyzer (the "Tiny SA Ultra") to the leads to/from the controller by wrapping them around - but this only tells part of the story, showing the worst of the interference.  In this update, I connected  to the DC leads "directly" - using a 0.0022uF capacitor as a DC block - before and after the filtering to show the difference.  (Ignore the markers in the following plots - I forgot to shut them off)

First, the bad (no filtering):

 

Figure 7:
Un-filtered 0-30 MHz spectrum from the controller, directly coupled.

Figure 7 shows what the spectrum looks like from 0 to 30 MHz - each horizontal division representing 5 MHz - covering the extent of the HF spectrum but as expected, the worst of it is found in the first several MHz as shown in the next plot:

 

Figure 8:
Un-filtered 0-10 MHz spectrum from the controller, directly coupled.

Figure 8 shows from 0 to 10 MHz - with each horizontal division representing 1 MHz - and we can see that the peak energy at around 5 MHz is stronger than -40dBm - a level which correlates with an S-meter reading of about "30 over S-9".  This plot along with that of Figure 7 show that even as high as 20 meters (14 MHz) the noise can exceed "10 over S-9".

Remember that we are connecting the analyzer directly to the terminals of the equipment - something that you would not normally do, but these plots give you an idea as to how much energy is available to be radiated on the wires that go between the panel and the battery:  Even if the signals at 5 MHz were down by 30dB (1000-fold) due to coupling, they would still be about "S-9" in strength (e.g. pretty strong).

Below are the same plots - this time with the filtering depicted in figures 4-6:

Figure 9:
0-30 MHz, directly coupled to the controller - after filtering.

 

This shows a dramatic decrease in the amount of RF present from 0-30 MHz and as can be seen above, the noise floor above 5 MHz is VERY much reduced.  Let's look at the 0-10 MHz range, below:

Figure 10:
0-10 MHz, directly coupled to the controller - after filtering.

 

As expected, the filtering is less effective at frequencies below 1.5 MHz (more inductance and larger capacitors could help with that) but even at 2 MHz we see that the "grunge" is attenuated by about 20dB (100-fold, around 3 S-units worth), and is further reduced to be down by roughly 40dB (10000-fold, or about 7 S-units) around 80 meters (3.5-4.0 MHz) and higher, approaching the noise floor of the spectrum analyzer (approx. -100dBm with an RBW of 100 or 300 kHz) around 7 MHz, rising slightly in the 13-16 MHz range.  As can be seen in Figure 9, the worst-case noise output anywhere above 7 MHz are only around -85 dBm (S-7) and this would be with the receiver connected directly to the solar panel leads:  Again, no-one would ever do that!

 

The above measurements show how much RF energy is inputted into the wires connecting the panel and the battery bank - both of which will act as halves of a dipole antenna - and these measurements show that we can very significantly reduce this energy, likely reducing it to the point of inaudibility. 

Figure 11:
The same filtering - this time, applied to a Renogy
Rover 20 amp MPPT solar controller.
Click on the image for a larger version.

As noted in the update at the top of this posting, I've operated the solar panel and controller right next to my HF antenna and could not hear any interference from it!

 

Figure 11 shows the techniques depicted in figures 4, 5 and 6 applied to a different Renogy controller, also using FT140-43 toroids and 0.1uF monolithic capacitors.  In this case, the only capacitors used were those attached directly to the back side of the four-screw terminal strip (C1a, C1b, C1c) which works because this particular controller already has the other set of capacitors (C2a, C2b, C2c) on its circuit board.

 

Conclusion:

 

Prior to the modification, getting within several feet/meters of the solar panel with a portable shortwave receiver equipped with SSB revealed drifting "birdies" from the controller's normal operation and holding the antenna against either the panel or the output cable made this orders of magnitude worse.

After the modification these "birdies" were inaudible on the cables:  It took holding the portable receiver's antenna within a few inches/cm of the charge controller to hear its operation.  By the addition of these nine components (two bifilar inductors, six capacitors and the terminal strip) the RF energy is confined to the (small!) physical space of the controller itself and is no longer being introduced differentially to the panel and output cable, causing it to be unable to radiate effectively at HF, making it very quiet and "Radio Friendly".

While the supplied charge controller for the Renogy panel was a simple PWM type rather than an MPPT (Maximum Power Point Tracking) and is thus somewhat less effective at extracting all-possible energy from it, there is no reason why this sort of filtering could not be applied to either types.

 

This shows how a typical portable solar panel+charge controller can be made to be RF-quiet and "POTA" or "SOTA" compatible.  This (reversible!) modification has rendered this panel completely quiet across the HF spectrum and inaudible on VHF/UHF frequencies as well at distances of more than a few feet (a meter or so) as well.

* * * * *

Related articles:

• The December 8, 2012 entry about "Reducing Switch Supply Racket racket (RF Interference) - Link 
• See also the September 4, 2013 entry, "Quieting High-Current switching supplies used in the ham shack - link
• The article "Minimizing VHF and HF RFI from electronic ballasts and fluorescent tubes - link - from December 17, 2015 may also be useful in quashing interference from these sources.
• The December 1, 2017 article "Containing RF noise from a sine wave UPS" - link - also details how interference from an RF-noisy device may be suppressed.
• Quieting an insanely noisy LED floodlight - link.  This describes how a constant-current LED supply that produced enough interference to quash HF reception was quieted down to the point of undetectability. 
• Quieting a 150 watt Samlex sine wave inverter - link.  This page talks about making an otherwise RF-noisy sine wave inverter quiet at radio frequencies. 
• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave. 
  

This page stolen from ka7oei.blogspot.com

 

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An RV "Generator Start Battery" regulator/controller for use with a LiFePO4 power system

I was recently retrofitting my brother's RV's electrical system with LiFePO₄ batteries (ReLi³on RB-100's).  This retrofit was done to allow much greater "run time" at higher power loads and to increase the amount of energy storage for the solar electric system while not adding much weight, not needing to vent corrosive fumes.  (These types of batteries, LiFePO₄,  are very safe - e.g. they don't burst into flame if damaged or abused.)

While I was doing this, I began to wonder what to do about the generator "start" battery.

Charging LiFePO4 batteries in an RV The voltage requirements for "12 volt" Lead-Acid batteries are a bit different from those needed by LiFePO4 "12 volt" batteries: Lead acid batteries need to be kept at 13.2-13.6 volts as much as possible to prolong their life (e.g. maintained at "full charge" to prevent sulfation). LiFePO4  batteries may be floated anywhere between 12.0 and their "full charge" voltage of around 14.6 volts - but they will not be fully recharged unless they are held near the high end of this range. Routinely discharging lead-acid batteries below 50% can impact their longevity - and they must be recharged immediately to prevent long-term damage. LiFePO4  batteries may be discharged to at least 90% routinely - and they may be left there, provided their voltage is not allowed to go too low. Lead acid batteries may be used without any management hardware:  Maintaining a proper voltage is enough to ensure a reasonable lifetime. LiFePO4 batteries must have some sort of battery management hardware to protect against overcharge and over-discharge as well as to assure proper cell equalization.  Many modern LiFePO4 batteries (such as the "Rel3ion" devices used here) have such devices built in. Conventional RV power "converters" are designed to apply the proper voltage to maintain lead-acid batteries (e.g. maintain at 13.6 volts.) Because LiFePO4 batteries require as much as 14.6 volts to attain 100% charge (a reasonable charge may be obtained at "only" 14.2 volts) connecting them directly to an existing RV with this lower voltage means that they may never be fully-charged!  Modern, programmable chargers (e.g. inverter-chargers, solar charge controllers) have either "LiFePO4 " modes or "custom" settings that may be configured to accommodate  the needs of LiFePO4 batteries.  While the lower voltage (nominal 13.6 volts) will not hurt the LiFePO4 batteries, they likely cannot be charged to more than 40-75% of their rated capacity at that voltage.  (approx. 13.6-13.7 volts is the lowest voltage were one can "mostly" charge a LiFePO4 battery.) Because of Peukert's law, one can only expect 25-50% of the capacity of a lead-acid battery to be available at high amperage (e.g. 0.5C or higher) loads. With LiFePO4 batteries, more than 80% of the battery's capacity can be expected to be available at similar, high-amperage.  What this means is that at such high loads, a LiFePO4 battery can supply about twice the overall power when compared with a lead-acid battery of the same amp-hour rating.  At low-current loads the two types of batteries are more similar in their available capacity. In short:  Unless an existing charging system can be "tweaked" for different voltages and charging conditions, one designed for lead-acid batteries may not work well for LiFePO4 batteries.  In some cases it may be possible to set certain "equalize" and "absorption" charge cycle parameters to make them useful with LiFePO4s, but doing this is beyond the scope of this article.

Originally the RV had been equipped with two "Group 24" deep-cycle/start 12 volt batteries in parallel (a maximum of, perhaps, 100 amp-hours, total, when brand new, for the pair of "no-name" batteries supplied) to run things like lights, and the pump motors for the water system, jacks and slide-outs and as the "start" battery for the generator.  Ultimately we decided to wire everything but the generator starter to the main LiFePO₄ battery bank.

Why?

Suppose that one is boondocking (e.g. "camping" away from any source of commercial power) and the LiFePO₄ battery bank is inadvertently run down. As they are designed to do, LiFePO₄ battery systems will unceremoniously disconnect themselves from the load when their charge is depleted to prevent permanent damage, automatically resetting once charging begins.
 
If that were to happen - and the generator's starter was connected to the LiFePO₄ system - how would one start the generator?

Aside from backing up the towing vehicle (if available), connecting its umbilical and using it to charge the system just enough to be able to get the generator started, one would be "stuck", unable to recharge the battery.  What's worse is that even if solar power is available, many charge controllers will go offline if they "see" that the battery is at zero volts (e.g. when they are in that "disconnected" state) - even if the sun is shining, preventing charging from even starting in the first place!

What we needed was a device that would allow the starting battery be be charged from the main battery, but prevent it from back-feeding and being discharged.


Note:

It is common in many RVs for the generator to not charge its own starting battery directly, via an alternator.  The reason for this is that it is assumed by the makers of the generators and RVs that the starting battery will be charged by the towing vehicle and/or via the RV's electrical system via its AC-powered "voltage converter", powered from "shore" power or via the generator's AC output.

But first, a few weasel words:

• Attempt to construct/wire any of the circuits only if you are thoroughly familiar with electronics and construction techniques.
• While the voltages involved are low, there is still some risk of dangerous electric shock.
• With battery-based systems extremely high currents can present themselves - perhaps hundreds or even thousands of amps - should a fault occur.  It is up to the would-be builder/installer of the circuits described on this page - or anyone doing any RV/vehicle wiring - to properly size conductors for the expected currents and provide appropriate fusing/current limiting wherever and whenever needed.  If you are not familiar with such things, please seek the help of someone who is familiar before doing any wiring/modifications/connections!
• This information is presented in good faith and I do not claim to be an expert on the subject of RV power systems, solar power systems, battery charging or anything else.
• You must do due diligence to determine if the information presented here is appropriate for your situation and purpose.
• YOU are solely responsible for any action, damage, loss or injury that might occur.  You have been warned! 

Why a "battery isolator" can't be used:

If you are familiar with such things you might already be saying "A device like this already exists - it's called a 'battery isolator'" - and you'd be mostly right - but we can't really use one of these devices because LiFePO₄ batteries operate at a full-charge voltage of between 14.2 and 14.6 volts, and the battery isolator would pass this voltage through, unchanged.  If you apply 14+ volts to a "12 volt" lead-acid battery for more than a few days, you will likely boil the away electrolyte and ruin it!

What is needed is a device that will:

• Charge the generator start battery from the main (LiFePO₄ ) battery system
• Isolate it from the main battery, and 
• Regulate the voltage down to something that the lead-acid chemistry can take - say, somewhere around 13.2-13.6 volts.

In this case the main LiFePO₄ battery bank will be maintained via the AC-powered (generator or shore) charging system and/or the solar power converters at its normal float voltage, so it makes sense to use it to keep the start battery fully-charged.

The solution:

After perusing the GoogleWeb I determined that there was no ready-made, off-the-shelf device that would do the trick, so I considered some alternatives that I could construct myself.

Note:  The described solutions are appropriate only where the main LiFePO₄ bank's voltage is just a bit higher (a few volts) than the lead-acid starting battery:  They are NOT appropriate for cases where a main battery bank of a much higher voltage (e.g. 24, 48 volts, etc.) is being used to charge a "12 volt" starting battery.

Simplest:  "Dropper diodes":

Because we need to get from the nominal 14.2-14.6 volts of the LiFePO₄ system down to 13.2-13.7 volts it is possible to use just two silicon diodes in series, each contributing around 0.6 volts drop (for a total drop of "about" 1.2 volts) to charge the starting battery, as depicted in Figure 1, below.  By virtue of the diodes' allowing current flow in just one direction, this circuit would also offer isolation, preventing the generator's battery from being discharged by back-feeding into the main battery.

To avoid needing to use some very large (50-100 amp) diodes and heavy wire to handle the current flow that would occur when the starter motor was active - or if the start battery was charging heavily - one simply inserts some series resistance to limit the current to a few amps.  Even though this would slow the charging rate somewhat, the starting battery would be fully recharged within a few hours or days at most - not a problem considering the rather intermittent use of the starting battery - more about that later.

Figure 1.
This circuit uses a conventional tungsten-filament "1157" tail/turn signal bulb (NOT an LED replacement!) with both filaments tied together, providing more versatile current limiting.  Please read notes in the text concerning mounting of the light bulb.
The diodes (D1 and D2) should be "normal" silicon diodes rather than "Shottky" types as it is the 0.6 volt voltage drop per diode that we need to reduce the voltage from the LiFePO₄ stack to something "safe" for lead-acid chemistry.  If one wished to "tweak" the voltage on the starting battery, one could eliminate one diode or even replace just one of them with a Shottky diode to increase the lead-acid voltage by around 0.2-0.3 volts.
The use of a current-limiting device like a tungsten light bulb allows lighter-gauge wire to be used to connect the two battery systems together.
Click on the image for a larger version.

In lieu of a large power resistor, the ubiquitous "1157" turn signal/brake bulb is used as depicted in Figure 1.  Both filaments are tied together (the bulb's bayonet base being the common tie point) providing a "cold filament" resistance of 0.25-0.5 ohms or so, increasing to 4-6 ohms if a full 12 volts were placed across it.  The reason for the use of a light bulb will be discussed later.

Although not depicted in Figure 1, common sense dictates that appropriate fusing is required on one or both of the wires, particularly if one or more of the connecting wires is quite long, in which case the fuse would be placed at the "battery" end (either LiFePO₄ or starting battery) of the wire(s) to provide protection should a fault occur between that source and the charge controller:  Fusing at 5-10 amps is fine for the circuit depicted.

This circuit is "good enough" for average use and as long as the LiFePO₄ bank is floated at 14.2 volts with occasional absorption peaks at 14.6 volts, the lead-acid starting battery will have a reasonably long life.

A regulator/limiter circuit:

As I'm wont to do, I decided against the super simple "dropper diode and light bulb" circuit - although it would have worked fine - instead, designing a slightly fancier circuit to do about the same as the above circuit, but have more precise voltage regulation.  While more sophisticated than two diodes and a light bulb, the circuit need not be terribly complicated as seen in Figure 2, below:

Figure 2:
The schematic diagram of the slightly more complicated version that provides tight voltage regulation for the starting battery.  As noted on the diagram, appropriate fusing of the input/output leads should be applied!
This diagram depicts a common ground shared between the main LiFePO₄ battery bank and the starting battery, usually via the chassis or "star ground" connection.  In the as-built prototype, Q2 was an SUP75P03-07 P-channel power MOSFET while D1 was an MR750 5 amp, 50 volt diode. A circuit board is not available at this time.
NOT SHOWN is the fusing of the input and output leads, near-ish their respective batteries/source connections, with 10 amp automotive fuses.
Click on the image for a larger version.

How it works:

U1 is the ubiquitous TL431 "programmable Zener".  If the "reference" terminal (connected to the wiper of R5) of this device goes above 2.5 volts, its cathode voltage gets dragged down toward the anode voltage (e.g. the device turns "on").  Because R4, R5 and R6 form an voltage divider, adjustable using 10-turn trimmer potentiometer R5, the desired battery float voltage may be scaled down to the 2.5 volt threshold required by U1.

If the battery voltage is below the pre-set threshold (e.g. U1 is "seeing" less than 2.5 volts through the R4/R5/R6 voltage divider) U1 will be turned off and its cathode will be pulled up by R2.  When this happens Q1 is biased on, pulling the gate of P-channel FET Q2 toward ground, turning it on, allowing current to flow from the LiFePO₄ system, through diode D1 and light bulb "Bulb1" and into the starting battery.

By placing R1 and R2 on the "source" side of FET Q2, the circuit is guaranteed to have two potential sources of power:  From the main LiFePO₄ system, through D1, and from the starting battery via the "backwards" intrinsic diode inside Q2.  The 15 volt Zener diode (D2) protects the FET's gate from voltage transients that can occur on the electrical system.

Figure 3:
The completed circuit, not including the light bulb, wired on a small
piece of perforated prototype board.
A printed circuit board version is not available at this time.
Click on the image for a larger version.

Once the starting battery has attained and exceeded the desired float voltage set by R5 (typically around 13.5 volts for a "12 volt" lead-acid battery) U1's reference input "sees" more than 2.5 volts and turns on, pulling its cathode to ground.  When this happens the voltage at the base of Q1 drops, turning it off and allowing Q2's gate voltage, pulled up to its source by R1, to go high, turning it off and terminating the charge.

Because the cathode-anode voltage across U1 when it is "on" is between 1 and 2 volts it is necessary to put an additional voltage drop in the emitter lead of Q1, hence the presence of LED1 which offsets it by 1.8-2.1 volts.  Without the constant voltage drop caused by this LED, Q1 would always stay "on" regardless of the state of U1.  Capacitor C1, connected between the "reference" and the cathode pins of U1 prevent instability and oscillation.

In actuality this circuit linearly "regulates" the voltage to the value set by R5 via closed loop feedback rather than simply switching on and off to maintain the voltage.  What this means is that between Q2 and the light bulb, the voltage will remain constant at the setting of R5, provided that the input voltage from the LiFePO₄ system is at least one "diode drop" (approx. 0.6 volts) above that voltage.  For example, if the output voltage is set to 13.50 volts via R5, this output will remain at that voltage, provided that the input voltage is 14.1 volts (e.g. 13.5 volts plus the 0.6 volts drop of diode D1) or higher.

Because Q2, even when off, will have a current path from the starting battery to the main LiFePO₄ bank due it its intrinsic diode, D1 is required to provide isolation between the higher-voltage LiFePO₄ "main" battery bank and the starting battery to prevent a current back-feed.  Were this isolation not included, if the main battery bank were to be over-discharged, current would flow backwards, through FET Q2, from the generator starting battery and discharge it, possibly to the point where the generator could not be started.

Again, D1's 0.6 volt (nominal) drop is inconsequential provided that the LiFePO₄ bank is at least 0.6 volts above that of the starting battery, but this will occur very frequently if the charge on that bank is properly maintained via generator, solar or shore power charging.  A similar (>= 5 amp) Shottky diode could have been used for D1 to provide a lower (0.2-0.4 volt) drop, but a silicon diode was chosen because it was on hand.

Testing the device:

Assuming that it is wired/built correctly, connect a variable power supply to the input lead to simulate the LiFePO₄ battery bank.  Setting the voltage a volt or two higher than the expected float voltage (e.g. 14.5-16 volts) adjust R5 to attain the desired start battery float voltage (13.50-13.7 volts is recommended - I use 13.55 volts) as measured on either side of "Bulb1".  Adjust the power supply voltage up and down a bit (e.g. below 12 volts and up to 17 volts) and if working correctly, the output voltage from the circuit should be rock-steady as long as the input voltage is about 0.6 volts above the set output voltage.

Now short the output leads (e.g. the "positive" output lead should be going through "Bulb1") and the light bulb should illuminate fully - assuming that your variable voltage supply is capable of supplying the 3-ish amps needed for the lamp.  Measuring directly at the circuit board's "ground" (common "battery negative") terminal and at the connection between Q2 and "Bulb1" you should still have the voltage set by R5 within a few hundredths of a volt.

Note:  If you were to measure connect the negative lead of the voltmeter to the power supply or the shorted output leads the measured voltage would be a bit lower owing to voltage drop along the wires.

Shorting the output leads and measuring the voltage as done in the previous step demonstrates two important design points:

• That the voltage at the output of Q2 remains steady from no-load to maximum current conditions.
• That the light bulb is properly acting as a current limiting device.

While doing this "short circuit" test, make sure that the heat from the light bulb rises away from the circuit board itself and that the means of mounting it is capable of withstanding the bulb's heat without burning or melting anything.

Connecting the device:

On the diagram only a single "Battery negative" connection is shown and this connection is to be made only at the starting battery.  Because this circuit is intended specifically to charge the starting battery, both the positive and negative connections should be made directly to it as that is really the only place where we should be measuring its voltage!

Also noted on the diagram is the assumption that both the "main" (LiFePO₄ ) battery and the starting battery share a common ground, typically via a common chassis ("star") ground point which is how the negative side of the starting battery ultimately gets connected to the negative side of the main LiFePO₄ bank:  It would be rare to find an RV with two battery systems of similar voltages where this was not the case!

Finally, it should go without saying that appropriate fusing be included on the input/output leads that are located "close-ish" to the battery/voltage sources themselves in case one of the leads - or the circuit itself - faults to ground:  Standard automotive ATO-type "blade" fuses in the range of 5-10 amps should suffice.  In order to safely handle the fusing current and to minimize voltage drop while charging the connecting wires to this circuit should be in the range of 10 to 16 AWG with 12-14 AWG being ideal.

What's with the light bulb?

Figure 4:
The circuit  board mounted in an aluminum chassis box along with the
light bulb.  Transistor Q2 is heat-sinked to the box via insulating hardware
and the board mounted using 4-40 screws and aluminum stand-offs.  The light
bulb is mounted to a small terminal lug strips using 16 AWG wire soldered
to the bulb's base and the bottom pins:  A large "blob" of silicone (RTV)
was later added around the terminal strip to provide additional support.
Both the bottom of the box (left side) and the top include holes to allow
the movement of air to help dissipate heat.  Holes were drilled in the back
of the box (after the picture was taken) to allow mounting.
This box is, in this picture, laying on its side:  The light bulb would be
mounted UP so that its heat would rise away from the circuitry via
thermal convection.
Click on the image for a larger version.

The main reason for using a light bulb on the output is to limit the current to a reasonable value via its filament.  When cold, the parallel resistance of the two filaments of the 1157 turn-signal bulb is 0.25-0.5 ohms, but when it is "hot" (e.g. lit to full brilliance) it is 4-6 ohms.  Making use of this property is an easy, "low tech" way to provide both current limiting and circuit protection and, when the filament is cold (e.g. charging battery "mostly" charged), increase the amount of charging current that can flow.  Taking advantage of this changing resistance of a light bulb allows higher charging current that would be practical with an ordinary resistor.


In normal operation the light bulb will not glow - even at relatively high charging current:  It is only if the starting battery were to be deeply discharged and/or failed catastrophically (e.g. shorted out) that the bulb would begin to glow at all and actually dissipate heat.  

Limiting the charging current to just a few amps also allows the use of small-ish (e.g. 5 amp) diodes and reduce the heat that could be dissipated during regulation allowing the use of an aluminum box as Q2's heat sink, but more importantly it allows much thinner and easier-to-manage wire (as small as 16 AWG) to be used since the current can never be very high in normal operation.  Limiting the charging current is just fine for the starting battery due to its very occasional use:  It would take only an hour or two with a charge current to top off the battery after having started a generator on a cold day!

As noted on the diagram and in previous text the light bulb must be mounted such that its operating temperature and heat dissipation at full brilliance will not burn or melt any nearby materials as the glass envelope of the bulb can will easily exceed the boiling temperature of water!  With both the "simple" diode version in Figure 1 and the more complex version in Figure 2 it is recommended that the bulb is mounted above the circuitry to take advantage of air convection to keep the components cool as shown in Figure 4.  If a socket is available for the 1157 bulb, by all means use it, but still heed the warnings about possible amount of heat being produced.

In operation:

When this circuit was first installed, the starting battery was around 12.5 volts after having sat for a week or two (during the retrofit work) without a charging source and having started the generator a half-dozen times.  With the LiFePO₄ battery bank varying between 13.0 and 14.6 volts with normal solar-related charge/discharge cycles, it took about 2 days for the start battery to work its way up to 13.2 volts, at which point it was nearly fully charged - and then the voltage quickly shot up to the 13.55 volts as set by R5.  This rather leisurely charge was mostly a result of the LiFePO₄ bank spending only brief periods above 13.8 volts.

Even though this doesn't very quickly charge the battery under normal conditions, as we'll see below, this isn't really important.

How much of the starting battery's capacity is being used?

If one were to assume that the generator was set to run once per day and pull 100 amps (a current likely seen on a very cold day!) from the battery for 5 seconds this would represent (100 amps * ( 5 sec/3600sec )) = about 0.14 amp-hours: - This happens to be about the same amount of energy as is contained in 4 fresh hearing-aid batteries or about 1/10th of the capacity of a single AAA cell!

From this we can see that this "100 amps for 5 seconds" is an average current of just over 5 milliamps (1/200th of an amp!) when spread across 24 hours - a value likely comparable the self-discharge rate of the battery itself.   By these numbers you can see that it does not take much current at all to sustain a healthy battery that is used only for starting!  Because this battery is never used for running things like lights or motors, it really never gets abused by being deeply discharged.

A standard group 24 "deep cycle starting" battery was used since it and its box had come with the RV.  In this particular application, for generator starting only, a much smaller battery - such as one used for starting 4x4s or motorcycles - would have sufficed and saved a bit of weight and space.

The advantage of the group 24 battery is that it, itself, isn't particularly heavy and it is readily available in auto-parts, RV and "big box" stores everywhere.  Because it is used only for starting the generator, it need not have been a "deep cycle" type, but rather a normal "car" battery - although the use of something other than an RV-type battery would have necessitated re-working the battery connections as RV batteries have handy nut/bolt posts to which connections may be easily made.


Final comments:


There are a few things that this simple circuit will not do, including "equalize" the lead acid battery and compensate for temperature - but this isn't terribly important, overall in this application.


Concerning equalization:

Even if the battery is of the type that can be equalized (many sealed batteries, including "AGM" types - those mistakenly called "gel cells" - should never be equalized!) it should be remembered that it is not the lack of equalization that usually kills batteries, but rather neglect:  Allowing them to sit for any significant length of time without keeping them floated to above 2.17 volts/cell (e.g. above 13.0 volts for a "12 volt" battery) or, if they are the sort that need to be "watered" and not keeping their electrolyte levels maintained.  Failure to do either of these will surely result in irreversible damage to the battery over time.

It is also common practice to adapt the float voltage to the ambient temperature, but even this is not necessary as long as a "reasonable" float voltage is maintained - preferably one where water loss is minimized over the entire expected temperature range.  Again, it is more likely to be failure of elementary battery maintenance that will kill a battery prematurely than a minor detail such as this.

Practically speaking, if one "only" maintains a proper float voltage and keeps them "watered" the starting battery will likely last for at least the 3-5 year expected lifetime, particularly since, unlike battery in standard RV service, this starting battery will never be subjected to the deep discharge cycles which can really take a toll on a lead-acid battery.  While an inexpensive, no-name "group 24" battery, when new, may have a capacity of "about" 50 amp-hours, it won't be until the battery has badly degraded - probably to the 5-10 amp-hour range - where one will begin to notice starting difficulties.

Important also is the fact that the starting battery in this RV is connected to part of the main LiFePO₄'s battery monitoring system (in this case, a Bogart Engineering TM-2030-RV).  While this system's main purpose is to keep track of the amount of energy going into and out of the main LiFePO₄ battery, it also has a "Battery #2" input connection where one can check the starting battery's voltage - always a good thing to do at least once every day or two when one is "out and about".

Finally, considering the very modest requirements for a battery that is used only for starting the generator, it would take only a very small (1-5 watt) solar panel (plus regulator!) to maintain it.  While this was considered, it would have required that such a solar panel be mounted, wires run from it to the battery (not always easy to do on an RV!) and everything be waterproofed.  Because the connections to the main battery bank were already nearby, it was pretty easy to use this circuit, instead.

[End]

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