The solar saga - part 2: Getting the system online

In part one (March 2, 2016) I wrote about why I chose to use a series string inverter system.  (Hint:  It was to prevent radio-frequency interference.)  To read part one, click on the link here:  The Solar Saga - Part 1:  Avoiding Interference (Why I did not choose microinverters.)

Eventually, I was able to get the system "online" in late April even though everything solar-related had been in place for over 2 months.  Why the delay?

Figure 1:
Slip-sliding around, the work crew clearing ice and snow off
the metal garage roof.  Later, they wielded a propane
"weed burner" to loosen the remaining ice and snow and
dry the metal roof panels.
Click on the image for a larger version.

As is often the case in life, things don't always go exactly according to plan!

Installing the panels:

Going back a bit, the solar panels and inverter were actually installed in February.

In the winter.

In the snow.

Think about that for just a moment, particularly considering when the system was actually put online.

Figure 2:
The mounting rails for the solar panels, installed.  If you
look carefully in the background you can see where
someone fell hard on the ridge cap, slightly crushing it!
Click on the image for a larger version.

To be sure, there was a lot to do, but everything seemed to be going according to plan, with no obvious trouble (of which I was ever informed) with the city in pulling permits.

Part of the install actually began a few weeks before this when a survey crew came out to check out the "sun situation" where the panels were to be installed.  As soon as they arrived they placed a ladder against the garage roof and then I heard some muttering:  They'd just realized that they had left the device that analyzes the sun's path and potential shadows at the previous job some 15-20 miles north in another county.  Instead of being able to plop this device down in the locations of the solar arrays they squinted with expert eyes at the trees and sky and declared more or less "I don't think that shade will be a problem."  Who was I to argue - they were professionals!

Figure 3:
The installed eastern solar array  At the time that the picture
was taken the system was wired up, but without a net meter it
couldn't be "officially" used.
Click on the image for a larger version.

Aside from lots of slipping and sliding on the snow-covered metal roof, the installation seemed to go well with the rails having been installed the first day and the roof penetration - a set of rubber "Flasher" bushing type thing being used to seal about the electrical conduit that emerged from from within the garage itself, one for each array.

A few days later the panels were on the roof and the inverter wired up and connected to the new electrical sub-panel that I'd put in the garage a few weeks before.  While the system worked, I really couldn't use it as the "Net Meter" was not yet installed and any excess power that I produced would be charged to me just as if I'd actually used it!

Figure 4:
A screen shot of the system producing just under 1400 watts
in "Standalone" or "Island" mode - a configuration that allows
the solar electric system to produce useful energy even if the power
grid was down, unlike a microinverter system where the potential
electrical solar power from the panels is completely
inaccessible if the power is out!
Click on the image for a larger version.

While I could not do the "net meter" thing, one feature provided by the Sunny Boy inverters - but not with microinverter systems - is the "Secure Power System" or "SPS" (tm) that will allow power "islanding".  In other words, if the main breaker is shut off and a switch is flipped, the inverter will provide up to 1.5kW of power (12 amps, 125 volts) - even if the mains power is unavailable, provided that there is adequate sun, of course:  Just try that with a microinverter system!

Figure 5:
The installed Sunnyboy 5+kW inverter and the garage
sub-panel to which it is connected.   Below and
to the left of the inverter is the DC disconnect switch
for the two independent MPPT solar panel strings
(the "East" and "West") and just to the right
of it is the "SPS" or "Standalone" power outlet capable of
providing up to 12 amps at 125 volts (1.5kW) even
if the electrical grid is offline.
Click on the image for a larger version.

A problem with the electrical service entrance:


From the beginning I was informed that my main electrical panel - the place where the power from underground gets to the house - would have to be replaced.  Fortunately, this cost was "baked into" the cost of the system itself and since I'd replaced the sub-panel in my garage myself, the installers would cover it.  In asking around I determined that in my case, the typical cost for this would be in the $1000-$1500 range including all parts and labor.

The reason that the old panel had to be replaced was ostensibly due to the "20% rule", and in my case it went something like this:

My panel, originally installed when the house was new (early 1970s) was rated for 100 amps on the bus.  The "20% rule" said that it was permissible to have 20% above this value, or up to 120 amps.  The problem was that my photovoltaic system would, being capable of 5.3 kilowatts, could in theory of being capable of putting 22-24 amps (depending on voltage) on the bus and this, combined with a 100 amp main breaker, meant that I could put a total of 124 amps on the bus.

This would not pass muster - or inspection - so the panel was to be upgraded to a 125 amp unit with a 100 amp breaker which, according to the same rule, should allow a total of 150 amps on the bus.

A few days before the date in late February when the service upgrade was scheduled I got a call from the contractor saying that they couldn't do it:  The power company would not sign off because of the location of the gas line and meter with respect to the electrical panel itself.  After being informed of this I took a walk through my neighborhood and observed that about a third of the houses had their electrical panels "on top" of the gas meters.

Figure 6:
 The old meter and below it, the gas meter for the
house:  Since the solar was now connected, it was
required that the red warning tag be attached even
before the net meter was installed.
The power company ultimately determined that the
electrical panel and its underground conduit had to be
relocated to a minimum of 36" (about 92cm) distant from
the gas meter and any of its piping.  I couldn't be sure,
but it looked as though the gas meter was installed
after the electrical with the original riser pipe for the
electrical being wedged between the
house and the gas meter!
Click on the image for a larger version.

As it turns out, current code in this area requires at least a 36 inch separation between the closest part of the gas line and meter and any part of the electrical entrance.  What was surprising is that none of the contractors that had visited my place to assess the scope of work had caught this, let alone planned for it!

In my case the gas meter was literally touching the conduit from the underground power feed and the panel itself was about 3 feet above the gas meter.  Since the panel was to be upgraded, it had to meet current code so it would have to be moved.  This also meant that the underground power feed, which was a length of "direct burial" wire would also have to be redone, placed in 4" conduit and run to the location of the new meter.

All of this meant a delay - about 4 weeks, as it turned out - as the plans had to be revised and arrangements had to be made to coordinate the schedule of the electrical contractor with the schedule of the city inspector along with having "Blue Stakes" come out and mark the utilities so that yet another contractor could dig a trench in my front yard from the power junction to the location of the new meter.  While this meant that it would be another 4 weeks or so before the work would be done - and likely another 2-3 weeks after that before my "Net Meter" would be installed - it also meant that the company in charge of all of the work would end up "eating" the difference in cost which was probably something in the area of an extra $1000-$1500 on top of the already-allocated cost for panel replacement.

Comment:

There is a device called a "Connect DER" (tm) that may be used in many locales to bypass the need to upgrade or replace the house's electrical service when solar power is installed.  This device plugs into the original meter base and is sandwiched between the original panel and the power company's electrical meter so that the additional current of the solar power system does not appear on the house's electrical bus since the connections to the solar is made on the Connect DER itself with a built-in circuit breaker for electrical protection and to allow it to be disconnected.

For various reasons this sort of device was apparently not an option in my case - possibly because of the fact that the gas and electrical piping were co-located. 

Figure 7:
The narrow trench that magically appeared in my yard,
running between the underground junction from the
electrical utility to the approximate location of
the new panel.
Click on the image for a larger version.

Finally, the day arrived where there appeared a trench in my front yard (figure 7).  A few days later the electricians arrived and installed the new panel and a conduit in the trench - but I couldn't help but notice that it was only about 23" from the gas lines, a fact that I mentioned to them when they arrived the next morning (figure 8).  After a bit of digging and drilling, the new panel and conduit was suddenly another 18" or so farther away from gas meter than it had been.  At about this time the power company and city inspector showed up to disconnect the power from the mains and pull new conductors into the conduit under the watchful eye of the inspector who gave preliminary approval to the work plan.

Off came the old panel to be replaced with a weatherproof junction box, connected to the new panel with a run of conduit.  In the new junction box - at the location of the original panel - a "horse tail" of wires appeared representing the individual circuits in the house, each of which had to be spliced with a new set of conductors wired to the new panel.  After about 4 hours of work everything was turned back on and I was back in business.

Figure 8:
Oops!  That ain't no 36" separation between the gas
and electrical!  The vertical pipe on the left, against the
brick was the conduit conveying the electrical to
the old meter.
Click on the image for a larger version.

A day or two later the city inspector came back to meet with me and a representative of the contractor to survey the work done both with the installation of the new panel and the photovoltaic system.  Finding everything to his satisfaction he gave his approval which also meant that the power company was notified so that the "Net Meter" would be installed.  A couple days after this a small work crew from a landscaping company appeared, filling in the trench in the front yard and replanting the sod that had been removed.

About two and a half weeks later I came home from work to find a notice from the power company stuck to my front door indicating that the net meter had, in fact, been installed so I happily closed the necessary breakers to put the system online.  Since it was already the evening, not much power was produced that day, but it was now ready for the next days' sun!

* * *

Figure 9:
In mid-job, the "horse tail" wires from the original
breaker panel emerging from the junction box that
had been installed in the approximate location of the
original breaker panel.  New wires were run
between it and the new service entrance/breaker panel.
I have since painted the new junction box and conduit a
red color to somewhat match the brickwork.
Click on the image for a larger version.

A minor shading problem:

As of this posting it has now been about four months since the system was put online and it has been working quite well.  One minor complication - something that I would have addressed earlier had I been aware of it - has to do with the fact that some of the eastern panels were located where they get shade until a bit after noon, reducing the output of the east array by 15-20% during that time.

By late March I was noticing that the northern-most panel was starting to be shaded by a nearby pine tree and by June and July, the angle of the sun had precessed to the point where at least three panels were being completely shaded in the morning, the shade finally clearing about an hour before "local noon" - or around 12:45.  Had I been aware of this I might have requested to have the east panels arranged somewhat differently to reduce this effect as there is plenty of room on the roof to do this

Would microinverters have improved this situation?  Perhaps by only 5% or so:  The real problem is that the panels get shadowed lengthwise, equally affecting each of the three sections of the panel isolated by the built-in "shade tolerance" diodes so these diodes don't have a useful effect when shading occurs in that aspect which means that it would be more difficult to extract power from it by any means.

A month or so after installation I was able to get them to come back out and do the formal shade analysis that they hadn't done before starting the job and it confirmed what was empirically observed - plus it gave a bit of information as to what problems could arise in the future in terms of tree growth.

* * *

No RF noise at all!

As far as my one of my original concerns - that of generated RF noise - I can detect absolutely nothing from the photovoltaic system at all at any frequency.

The "quiet-ness" of the system can be borne out by the fact that even if one brings a portable receiver right up to the panels or the inverter, nothing at all can be heard from it except when its antenna gets within a few inches of the inverter's LCD panel.

Where I do get some RF noise is from sources unrelated to the solar power - switching type "wall warts" scattered throughout my house, powering various things, but most of the "problem" devices have already been quieted as described in previous postings on this blog, see:

• December 17, 2015 - Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - Link
• August 18, 2014 - Completely containing switching supply RFI - Link
• September 4, 2013 - Quieting high-current switching power supplies used in the ham shack - Link
• December 2, 2012 - Reducing switching power supply racket (RF Interference) - Link

* * *

Generation of power - observations:

In the (over) four months of operation the cumulative amount of power has exceeded my actual usage by about 300 kWh so the recent power bills have been low - just the "minimum charge" of less than $10.  According to my calculations based on past and current usage I expect to use up that surplus in the winter when the "production" of the the photovoltaic system will be much lower due to the lower sun angle, shorter days, occasional snow cover and the tendency for there to be extended temperature inversions that can block sun for days at a time.

At the moment I do not have a "refrigeration" whole-house air conditioner - only an evaporative (a.k.a. "swamp") cooler and a wheel-around "room" air conditioner for those relatively rare days that it is both hot and humid, but I'm considering getting a whole-house A/C sometime in the future:  When I do that I may consider increasing the capacity of my photovoltaic system.

While the system has eighteen 285 watt panels which are theoretically capable of 5130 watts, the slant of the roof (north-south ridge line with panels mounted flat on the east and wet sides), the operating temperature of the panels (an output power reduction of 0.45% per degree C panel temperature) and the actual solar insolation (e.g. the actual amount of solar energy) has limited the peak power to around 3800 watts on hotter, crystal clear days and about 4300 watts on cooler days.

If one does the  numbers this should not be too surprising.  For example, the 285 watt panel rating assumes a cell temperature of 25C.  On a hot summer day where the ambient air temperature is around 38C and the panels themselves are around 50C (a fairly modest temperature as my roof is metal and very light-colored which keeps it quite cool) that means that assuming a temperature derating of about 0.5%/C that I have lost - from heat alone - 12.5% of power, or can expect only about 250 watts per panel, or 4500 watts from the system - and that would assume that the sun was illuminating the panel at optimal right-angles - which it really cannot at any time of year.

Since my panels are on a roof with a moderate east-west pitch, I lose another 15% or so of solar insolation on a typical summer day due to the angle, yielding a number that is actually pretty close to the 3800 watt peak.  What I have observed is that because of the east/west angle of the two strings that I have a slight "double" peak around "local" noon when, before noon the angle is nearly optimal for the east array and then similarly, after noon, for the west array.  To make matters worse, during many days of the summer in recent years the valley's air is a bit murky due to some smog and the frequent wildfires that seem to be a regular occurrence in the western U.S. these days, knocking off another 10-15% of production.

What this means is that, in theory, I could have used a 4kW inverter if I was willing to tolerate a bit of "clipping" (e.g. more available photovoltaic power than the inverter will produce) on optimal days (e.g. cool spring or fall days with clear air) and possibly have averted all of the hassle with the 20% rule and the replacement of the electrical panel, but considering the state of the older electrical distribution panel, its replacement was probably for the best! If I wished to do so, I could (in theory) add another 4 panels to my system which would just about bring it to clipping under optimal conditions and to around 4600 watts on a normal, summer day.

The other option - if I needed more capacity after, say, adding a house air conditioning unit - would be to simply install another set of panels - maybe 14-16 or so - and a separate inverter - a 3.6 kW unit, perhaps:  This would still stay within the 20% rule for the new electrical panel and add a degree added redundancy.  Since the "hard" work (e.g. update of the electrical, etc.) has already been done, such an addition would be comparatively easy.

Comment:

As of the time of this writing (mid August, 2016) it would appear that the company that I used for the installation of my system (Auric Solar) will no longer consider the use of series-string photovoltaic systems, at least for residential customers - a statement based on a conversation a friend and fellow amateur radio operator had with a company representative.  The impression given - perhaps unintentionally - was that they had enough business that they didn't necessarily need to offer flexibility or other system options to their potential customers.

What this means is that for fellow amateur radio operators who wish to avoid an "RF noisy" installation, I've recently been suggesting another company.

Update - May, 2024:

It has been about eight years since the above system was installed and I have had ZERO maintenance issues during the entire time other than occasionally trimming back tree branches that block/hang over panels.

I have since had a Tesla Powerwall 2 installed as well as increased the solar production to 10 kW - again using a SunnyBoy series string inverter, still with no interference from my own solar and only very slight interference from the Powerwall 2 when it was running on some of the lower bands:  If you want more details on this and other solar-related topics at this blog, peruse the links below.

* * * * *

Other articles at this blog on related topics:

• It *IS* possible to have an RF-quiet home PV (solar) electric system - link
  
• The Solar Saga Part 1: Avoiding Interference - (Why I did not choose Microinverters) - link
  
• Does the Tesla Powerwall 2 produce Radio Frequency Interference? - link
• Reducing RFI from the Tesla Powerwall 2 - link 
• Tesla Powerwall susceptibility to RF Transmissions - and how to deal with it - link 

[End]

This page stolen from "ka7oei.blogspot.com".

A latching low-voltage disconnect for 12 volt lead acid and lithium batteries (non auto-resetting)

Figure 1:
The as-built and working prototype constructed.
This version does NOT automatically reset itself - by design.
Click on the image for a larger version.

There are two things that you don't want to do with any rechargeable battery on a routine basis:

• Overcharge it.
• Overdischarge it.

While the above are true for lead-acid batteries, they are particularly true of Lithium-Ion chemistries, but for different reasons.

With Lead-acid batteries:

• Lead-acid batteries - particularly the "flooded cell" types (e.g. those to which you can add water) can handle quite a bit of overcharging as long as the electrolyte level is maintained.  "Sealed" batteries (e.g. AGM, or those that many mistakenly called "gel" cells) can handle some overcharging, but only to an extent before their pressure vents release accumulated gasses, reducing the amount of usable electrolyte, which is why they should never be "equalized".
• Lead-acid batteries can also handle being run (almost) completely down - as long as you don't keep them in that state for very long (a few days at most - as little time as possible) and don't do it very often.  In other words, if you run an otherwise healthy lead acid battery completely dead and immediately recharge it, little actual damage is likely to have been done other than taking a bit of life off it farther down the road.  In cold-weather environments, while the degradation (primarily sulfation) is dramatically slowed, extremely deep discharge also reduces the specific gravity, raising the electrolyte's freezing point, increasing the possibility of the battery being damaged/destroyed at very low temperatures if it does freeze.

 With Lithium-ion batteries:

• If a battery is overcharged, it will start to chemically decompose.  Gross overcharging - while tolerated at least briefly by lead-acid batteries - may result in a lithium-ion battery venting and/or exploding, possibly catching fire.
• If a battery is over-discharged it will chemically decompose, often with the contained lithium changing into a more volatile - and not useful - state.  Severe over-discharging (e.g. below 2 volts per cell - this voltage varies depending on chemistry) can mean that the battery can never safely be charged again or, in some cases - if the voltage is only allowed to get down to this general area and not lower (again, the voltage varies according to chemistry) some special charging precautions are required (e.g. a specific, very low-rate trickle charging regimen) to "recover" this battery.
• There are also some specific temperature-related restrictions with lithium-type batteries regarding their use and charge/discharge and these are noted by their respective manufacturers.

Avoiding over-discharge:

The avoidance of overcharging is usually pretty easy:  Just use the appropriate charging system - but over-discharge is a bit more difficult, particularly if the battery packs in question don't have a "protection board" with them.

Lead acid batteries (almost) never come with any sort of over-discharge protection - one must usually rely on the ability of the device being powered (e.g. an inverter) to turn itself off at too-low a voltage and hope that the threshold is sensible for the longevity of a 12 volt battery system.  For low-to-moderate loads (e.g. 1/10th "C" or so) a pretty safe "dead battery" voltage for a 12 volt lead-acid battery is around 11.7 volts - or somewhat higher for heavier loads.  Again, after disconnect, it is not a good idea to keep it in a discharged state for any longer than possible.

Many larger (e.g. >10 amp-hour) lithium-iron phosphate (LiFePO4) do not routinely come with "protection" boards unless it is ordered specially or includes some sort of "Battery Management System":  Batteries in this category can include the "Lead Acid" replacements sold for use with motorcycles and off-road vehicles  and some of the "raw" LiFePO4 batteries available from many vendors, such as the 20 amp-hour modules made by GBS.

While it is also important to equalize LiFePO4 batteries when charging (refer to this post - Lithium Iron Phosphate (LiFePO4) batteries revisited - Equalization of cells - link) the more immediate danger in routine use is accidental over-discharge.

A simple "latching" low-voltage disconnect circuit:

Again, for lithium batteries one may install "protection" boards that prevent accidental over-discharge and, in some cases, provide charge equalization - but such things are much rarer for lead-acid batteries, but such a circuit is quite simple and is applicable to either Lithium or Lead Acid batteries.

Note that this particular circuit does not automatically reconnect the battery again after its voltage has been restored by charging (directly at the battery, anyway - and this is the "latching" part) and this was intentional - both to keep the circuit as simple as possible and because it draws attention to the system on which it is used when it trips out.

Figure 2:
Schematic diagram of the low-voltage disconnect circuit.
Not shown is overcurrent protection (e.g. fusing) that should be present on the output of the battery - see text below. 
If desired, LED1 can be placed in series with R2 which could be changed to 2.2k and R7 be omitted as an indicator that the circuit has actually latched, not just that there is voltage present on the Load+/- terminals.
Click on the image for a larger version.

These days it is rather easy to construct a low-voltage disconnect circuit using readily-available components:  The diagram of one such circuit may be found in Figure 1.

How it works:

The RESET button is pressed, applying a positive voltage to the gate of N-channel power MOSFET, Q1, turning it on, which then connects the "BATT -" output terminal to the "LOAD -" terminal.

If the voltage at the "Ref" terminal of U1 is above 2.5 volts, as determined by the voltage divider consisting of R4, R5 and R6, the cathode of U1 is connected to its anode (e.g. "Load -"), pulling the base of Q2 down, making it negative with respect to its emitter, R2 limiting Q2's base current to a safe value and providing enough current for U1 to function, and turning it on.  With Q2 turned on, Q1 is "latched" on, even when the RESET button is released.

If the voltage at the "Ref" terminal of U1, representative of the voltage across the LOAD terminals, drops below 2.5 volts, U1 turns off and the base of Q2 gets pulled positive to the emitter voltage by R1, turning it off.  With Q2 turned off resistor R8 pulls the gate of Q1 down to its source, also turning it off and disconnecting the load.  Because of the "latching" effect, once this has happened the load will never be turned on again until the RESET button is pressed.  This happens because with Q1 turned off, U1 is without voltage (e.g. "Load -" rises to the same voltage as "Load +) and can never turn Q2 (and thus, Q1) back on again.  Even though pressing and holding the RESET button will connect the load even if the voltage is below the threshold, until the voltage rises above the threshold the circuit will not stay "on" once the RESET button is released.

To accommodate a range of voltages, U1's "Ref" terminal is connected across the output (Load +, Load -) with R4 and R6 to "scale" the range of potentiometer R5 to have a threshold in the 8-16 volt range:  Without R4 and R6 the usable range of R5 would be compressed to a very small portion of the overall rotation and make adjustment touchy, but with these resistors setting R5 at mid-rotation yields a threshold of around 11 volts.

Note the presence of capacitors C1 and C2:  C1 provides a bit of filtering of the sampled output voltage to prevent brief current transients that might momentarily drag the voltage down below the threshold, "falsely" causing an undervoltage condition from being detected.  Similarly, C2 slows the "fall" time of Q1's gate voltage, preventing it from shutting off instantly in response to a brief spike of current - and it also provides some degree of protection of Q1's gate in response to possible voltage transients.

While not explicitly tested, the presence of C1 and C2 should provide a modicum of RFI protection:  If your environment includes high RF fields - such as powering a 100 watt amateur transceiver - this could be considered in testing and the construction/layout, knowing that such a transceiver can also impose very brief, high-current loads on the battery can causing momentary brown-outs due to I*R drops in the wiring and battery which could also trip this circuit.

Finally, the combination of R7 and LED1 provide an indication of power-on to the user - see the note on modification of this circuit, below.

Additional circuit notes:

The "high voltage" limitation of this device is primarily that of the gate voltage rating of Q1.  Most power FETs are rated for only +/- 20 volts gate-to-source voltage which means that it is suitable for no more than a "12 volt" bus (e.g. 10-16 volts or so):  If a higher operating voltage is required it will be necessary to add additional circuitry around the FET's gate to keep its voltage safely below its rating.  For an example of such circuitry see this article:  A Simple, effective, yet Inefficient Solar Charge Controller - link and taking note of components D1, R7, R8 and C4 surrounding Q3 in Figure 3 on that page.

If a lower cut-off than 9 volts  (or higher than 15) is required it will be necessary to recalculate the values of R4 and R6 (in Figure 2, above) to appropriately scale the adjustment range.

It should also be noted that if voltages below 10 volts are routinely required one should pay close attention to the saturation (e.g. "full on") gate voltage required for the FET that you plan to use:  Typical FETs do not achieve their lowest resistance until 8-10 voltage of gate-source voltage is present but there are "logic level" FETs available that will be fully "on" at around 5 volts.

Finally, there is a slight modification to the circuit depicted in Figure 2 that could be made:  Place LED2 in series with R2 and decreasing the value of R2 to 2.2k or so, omitting R7 entirely.  This modification not only saves a few milliamps of "on" current, but it also provides an indication of when the circuit is actually latched in its "on" state - particularly useful if the load has its own, separate power source which would cause LED1 to illuminate no matter the state of the disconnect circuit if wired according to Figure 2.

Construction:

None of the components are critical, save the possible exception of R4, R5 and R6 which are selected to scale the adjustment range of R5:  While it is the ratios of these components that are important (e.g. one could use 4.7k, 1k and 1k for R4, R5 and R6, respectively) going much higher than the stated values may violate the minimum reference current specifications of U1 resulting in temperature/device variations of the set voltage thresholds.

The TL431 (U1) is a rather ubiquitous chip, found in practically every PC-type power supply made in recent years and is available in single quantities for well under $1.

Q2 may be practically any silicon PNP transistor with a rating of at least 30 volts while Q1 may be any N-channel MOSFET with a voltage rating of at least 30 volts and a current rating of at least 3 times the current that you plan to draw and an "ON" resistance of a fraction of an ohm.  For the prototype I used an F15N05 FET - a 15 amp, 50 volt device, more than adequate for the 3 amp load that was to be used, but one could use as "large" a power FET as you wish.  For "12 volt" operation make sure that the FET that you choose has at least a 20 volt gate-source voltage rating.  Higher-current FETs include the IRFZ44 (50 amp max.) and the PSMN2R7-30PL (100 amp max.) to name but two out of hundreds of possibilities.  If even more current is required one can parallel multiples of the same-type FET as needed, potentially providing many hundreds of amps of capacity, provided the wiring is appropriately considered.

Device layout is not critical aside from the use of appropriately heavy conductors to the source and drain leads of Q1 to carry the current.  For most applications a heat sink is not even required for the FET - particularly if one chooses a device with milli-Ohm range "on" resistance but there is never any harm in doing the calculations yourself to verify that this is true in your case with the FET that you choose.  Note that the "Batt+" and "Load+" lead is straight-through and the wire connecting this circuit to that "through" connection may be of light gauge:  The only caveat is that it is recommended that the connection to this circuit be connected closer to the "BATT+" terminal than the "LOAD+" terminal to minimize the resistance of that connecting wire which could cause the circuit to sense a slightly lower voltage than is actually present.

Finally, note that this circuit works by disconnecting the "BATT-" from the "LOAD-":  Your battery's negative terminal must be completely isolated from the load for this circuit to work properly and protect your battery!

(Comment:  It is possible to reconfigure this circuit to disconnect in the positive lead, but this requires the use of a P-channel power FET:  A not-yet-built or tested circuit design is available on request.)

Adjustment and Operation:

For proper set-up an adjustable power supply is required and the procedure is as follows:

• Set the power supply to a volt or two higher than the desired drop-out voltage.
• Adjust R5, the potentiometer so that the wiper is closest to R6 to set the drop-out voltage to maximum (e.g. highest voltage measured betweenU1's REF terminal and LOAD- while the RESET button is being pressed).
• Connect the device to the power supply using the BATT- and BATT+ connections.  No load is required for testing.
• Press and release the RESET button:  The LED should stay on, but if not, check the adjustment of R5 to verify that it is providing the maximum voltage to U1's REF terminal.  If this checks out, check for proper resistor values of R4, R5 and R6 as well as proper wiring of U1.  Note that the circuit will not stay on if U1's REF terminal is below 2.5 volts.
• Lower the power supply to the desired drop-out voltage.  The LED should stay on, but if not check the setting of R5.  (Remember that the useful range of R5 with the specified values of R4 and R6 is in the 8-16 volt area.)
• Slowly rotate R5 until the LED just turns off.
• Increase the power supply voltage slightly, press and release the RESET button and verify that the LED turns on and then goes off again when the voltage drops below the threshold, repeating the above steps as needed.

If a device is connected that has a high "starting" current it is possible that - particularly if the battery is weak or near the cut-off voltage and/or the cut-off device is located at the end of a long run of rather small-gauge wiring - it will drop-out before the voltage gets to the pre-set threshold.  If this happens and it is not practical to move the device closer to the battery or increase wire size to minimize lead resistance one can increase the value of C1 (to as much as 47uF) to slow the response time, allowing a momentary "brown out" to occur without tripping the device.  Note that with such a capacitor it will take longer to respond to such changes, but this should not be an issue from the viewpoint of protecting the battery.  The value of C2 can also be increased, but not much more than 1 uF should be used as this will excessively slow the "turn off" time of Q1, causing it to spend more time out of saturation and potentially dissipating more heat in the process.

Additional comments:

In this particular application Anderson Power Pole (tm) connectors were used on the input and outputs allowing this device to be easily removed from the circuit and configured as needed.

This device should also not be left connected to a battery in long-term storage as it draws several milliamps when it is in its "ON" state due to the LED and the current consumption of Q1/Q2 and associated resistors, R4-R6 and U1.

When in its "OFF" state its current consumption is negligible (likely in the nanoamp range) so if it is left connected and the battery gets drawn down, it will still do its job, disconnecting the load - and itself - from the battery and protecting it.  Note that if the load is "back-fed" from another source - say an AC/solar charger or power supply - and the voltage rises above the threshold, this will have the same effect as pressing the RESET button, turning the circuit on.  Again, if the voltage is back-fed, the LED will be drawing a few milliamps whenever voltage is present whether the circuit is "on" or not - unless the modification noted above is made.

It is recommended that one NOT attempt to charge the battery "through" this device - at least at higher currents:  In theory it should work, but the current will flow backwards through the FET.  The reason for this is that while a FET that is turned "off" has an intrinsic "backwards" diode, it will drop 0.5-0.8 volts across this diode causing the FET to dissipate far more power than it would if it were actually "on".  If the charge rate is limited to a rather low current - perhaps less than 3-5 amps - the amount of heat dissipated by the FET should be tolerable.

Until the voltage rises above the cut-off threshold the FET will exhibit this 0.5-0.8 volt drop, but above this - when the circuit turns the FET on - this diode drop will largely disappear.   If you do this it would be a good idea to test it at your intended charge current in the worst-case scenario (e.g. highest current and adjust R5 so that the circuit will not trigger "on" during this charge, forcing the "diode drop" across Q1 to exist) and note if additional heat-sinking of U1 is needed.  Note:  If this is done, the "LED1-R2" modification noted above is recommended so that the LED will properly show the state of the circuit.

Not shown - but recommended - is the use of some sort of fuse or other overcurrent protection on the output of the battery.  It is recommended that the fuse rating be no higher than a third of the current rating of the FET to increase the chance that the FET will survive the surge current required to blow the fuse in the event of a dead short on the output.

[End]

This page stolen from "ka7oei.blogspot.com".