The title serves to illustrate a problem frequently encountered when trying to power a device that operates with a high peak current: Your energy storage - or your power source - may have plenty of capacity, but not enough current capability!
One such example of a power source that has plenty of capacity, but rather limited power capability is that of the "cigarette lighter" in a typical vehicle: As long as the engine is running, you can pull power - but not too much: More than 10-15 amps is likely to blow a fuse, and even if you were to replace the original fuse with, say, a 20-30 amp fuse (not smart!) the rather light gauge wiring would likely result in a voltage drop that would render a typical 100 watt HF rig unusable on voice peak.
For another, more extreme example let us consider a set of alkaline "D" cells, referring first to some online data:
- The "Energizer" D-cell data sheet - link.
- "Duracell" D-cell data sheet - link. Does not include direct amp-hour rating, but such may be inferred from the graphs presented.
- "'D' Battery" Wikipedia page - link.
One thing that jumps out is that a "D" cell has somewhere between 5 and 30 amp-hours per cell, but if you study the graphs, you'll also note that this apparent capacity drops like a rock with increasing current. Why is this?
At least part of this is due to internal resistance. If we examine the data for a typical alkaline "D" cell we see that on a per-cell basis that the internal resistance is 0.15-0.3 ohms per cell when it is "fresh", but this increases by 2 or 3-fold near the end of life of the cell (e.g. <=0.9 volts/cell) and increases dramatically - and very quickly - at still-lower voltages. Interestingly, the manufacturer's data used to include graphs of internal cell resistance, but these seem to have disappeared in recent years.
If we take a general number of 0.2 ohms/cell and expand that to a 10-cell series battery pack we get a resistance of 2 ohms which means that if we attempt to pull even one amp we will lose 2 volts - and this doesn't take into account the contact and wiring losses related to these batteries!
If you look at the graphs that relate battery capacity to discharge current you will notice something else: If you draw twice the current, your apparent capacity - in "run time" decreases by more than half and if you convert this to amp-hours, the more current drawn, the fewer available amp-hours.
These two facts together tell us two things:
- We cannot draw too much current or else resistive losses will cause excess voltage drop.
- Higher current consumption will cause a marked drop in available amp-hour capacity.
As you may have inferred from the title of the article, our particular application implies a usage where our typical, resting (or average) current consumption is quite low but our peak current consumption could be very high. Clearly, with a string of "D" cells, alone, we cannot tolerate the latter!
A power source with good longevity:
What we need is a low-impedance power source, and as it turns out almost any type of rechargeable cell - whether it is lead-acid, NiCd, NiMH or lithium - has a lower impedance than alkaline cells so the obvious question that one might ask is why not use one of those other types?
The obvious problem with lead-acid is that of longevity: If you own a lead-acid battery that is more than three years old (and certainly more than five!) it is likely lost a significant percentage (at least 30%) of its rated capacity - probably more unless it has been treated really well (e.g. controlled temperature at or below 70F, 21C) and always kept at a proper floating voltage when not in use (e.g. 13.5-13.7 volts for a "12 volt" battery at nominal room temperature.)
On the other hand, modern alkaline cells will retain the vast majority of the capacity for at least 5 years, just sitting on a shelf - a period of time that is beyond the likely useful lifetime of either lead-acid or most rechargeable lithium-ion cells! What's more is that alkaline cells are readily available practically anywhere in the world and they come "fully charged". To be sure, there are other types of "primary" (non-rechargeable) cells that have excellent shelf life such as certain types of lithium, but these are much less-available and would likely "break the bank" if you were to buy a set with comparable capacity!
A low-impedance voltage source:
An advantage of Lead-Acid, NiCd, Lithium Ion and some NiMH cell types is that they have quite low internal resistance compared Alkaline cells: Even an aging lead acid battery that is near the end of its useful life may seem to be "OK" based on a load test as its internal resistance can remain comparatively low even though it may have lost most of its storage capacity!
One could ask, then, why now simply parallel Alkaline cells, with their ready availability, long life and high storage capacity with one of these other cell types and get the best of both worlds? In theory you could - if you had some sort of charge control circuitry that was capable of efficiently meting out the energy from the alkaline pack and using it to supplement the "other" storage medium (e.g. lead-acid, lithium-ion, etc.) but you cannot simply connect the two types in parallel and expect to efficiently utilize the available power capacity of both types of storage cell - this, due to the wildly different voltage and charge requirements.
Even if you do use a fairly small-ish (e.g. 7-10 amp-hour) lead-acid or lithium-ion battery pack, even though its internal resistance may be low compared to that of alkaline packs, it likely cannot source the 15-20 amp current peaks of, say, a 100 watt SSB transceiver without excess voltage drop, particularly if it isn't brand new.
This is where the use of "Ultracapacitors" come in.
In recent years these devices have become available on the market and for reasonable prices. These capacitors, typically with maximum voltages in the range of 2.5-2.7 volts per unit, may have capacitance values as high as several thousand Farads in a reasonably small package while at the same time offering very low internal resistance - often in the units of milliohms. What this means is that from this capacitor one may pull many 10's of amps of current while losing only a small percentage of the energy in heat: Indeed, many of these capacitors have current ratings in the hundreds of amps!
What this means is that we can use these capacitors to deliver the high, peak currents while our power source delivers a much lower average current.
The difference between peak and average current - and power:
With a simple "thought experiment" we can easily realize that with our transmitter, one second at 100 watts (about the) same total power as ten seconds at 10 watts. If, in each case, we averaged the power over ten seconds, we would realize something else: In both cases, the average power is 10 watts per second.
Clearly, the power requirements for a radio operating at 10 watts are different than at 100 watts: For the former you'll likely need 4-5 amps of current, but for the latter you'll need 18-20 amps (at 12 volts or so, in each case.)
Here, we have a problem: If we have a given resistance somewhere in our DC supply, we lose much more power at, say, 5 amps than at 18 amps according to the equation:
P = I2R
In other words, the ratio of the power loss is equal to the square of the ratio of the current differences - in other words:
182 / 52 = 12.96 (or 13)
That means that power losses at 18 amps are 13-fold worse than those at 5 amps.
Clearly, the best way to mitigate this is with heavy cables to minimize resistance, but what if your power source is, by its very nature, fairly high in resistance in its own right?
This is where the "ultracapacitors" can be useful. By acting as a reservoir to handle peak currents, but rely on the battery - whatever form it may take - to make up the average.
With SSB (Single SideBand) transmission we have an almost ideal situation:
- There is no RF power when we aren't saying anything
- The amount of RF power is proportional with voice peaks, and
- Speech has comparatively rare peaks with a lot of empty spaces and lower-energy voice components interspersed.
- SSB is 6-12 times more power efficient in conveying voice than FM and occupying 1/3-1/4th of the space (bandwidth) as one FM signal.
Compare this with 100 watt FM transmitter:
- When you are "keying down" it is always putting out 100 watts, no matter what your voice is doing.
- 100 watts of FM is less effective in conveying voice than even 10 watts of SSB and it takes at least 3 to 4 times as much bandwidth as an SSB signal.
For the purposes of this discussion the point that I was really trying to make was the fact that the use of these "ballast" capacitors is appropriate only for relatively low duty-cycle modes such as SSB or, possibly CW: If you tried this with FM or digital modes such as PSK31 the long duty cycle (e.g. key-down) would quickly drain the energy stored in the capacitors and they would "disappear", putting the load back on the battery bank.
This technique is not new: For many years now one has been able to buy banks of capacitors intended for high-power car audio amplifier installations that provide that instantaneous burst of current required for the "thud" of bass without causing a similar, instantaneous voltage drop on the power supply. Originally using physically large banks of "computer grade" electrolytic capacitors, these systems are now much smaller and lighter, using the aforementioned "Ultracapacitors".
There are also devices on the amateur radio market that do this: Take as an example the MFJ-4403 power conditioner (link). This device uses ultracapacitors in series to achieve approximately 5 Farads of parallel capacitance across the output leads of the device.
Now, a few weasel words:
- The device(s) described on this page can/do involve high currents and voltages that could cause burns, injury, fire and even death if improperly handled if reasonable/proper safety precautions are not taken and good building techniques are not followed.
- The device(s) described are prototypes. While they do work, they may (likely!) have some design peculiarities (bugs!) that are unknown, in addition to those documented.
- There are no warranties expressed or implied and the author cannot be held responsible for any injury/damage that might result. You mileage may vary.
- You have been warned!
How this may be done:
For a variety of reasons (physics, practicality) cannot buy a "16 volt" ultracapacitor: Any device that you will find that has a voltage rating above 2.7 (or, in some cases 3.something volts) is really a module consisting of several lower-voltage capacitors in series. What is more, you cannot simply throw capacitors in series as there is no guarantee that the voltage will always divide equally amongst them unless there circuitry is included to make this so.
Another consideration is that if you have such a device - a large bank of capacitance - that is discharged, you cannot simply connect it across an existing power source because a (theoretically) infinite amount of current will flow from the power source into the capacitors, if their voltage is lower, to force equilibrium. Practically speaking, if you were connect the battery bank "suddenly", if the resistance of the wires themselves didn't serve to limit the current you'd likely blow the fuse, trip the breaker and/or cause the power supply to go into some sort of overcurrent (or shut down) mode - none of which are at all helpful.
Finally, this capacitor bank will be (theoretically) capable of sinking or sourcing hundred of amps if applied to a high current source/shorted out, so some sort of protection is obviously needed.
The diagram in Figure 2, below, provides these functions.
Schematic diagram of the capacitor bank/power ballast/conditioner and charging circuit - see the text for
a circuit description.
Click on the image for a larger version.
Capacitors C1-C6 are "Ultracapacitors": Their exact values are not important, but they should all be identical values and models/part numbers. In the example of the MFJ-4403, six 25 Farad capacitors are used yielding a total of 4-1/6 Farads while the drawing in Figure 2 depicts six 350 Farad capacitor being used to yield a total of 58-1/3 Farads. The greater the capacitance, the more energy storage, but also the longer the "charge" time for a given amount of current and, of course, the larger the size and the higher the initial cost of the capacitors themselves.
Zener diodes D1-D6, each being 2.7 volt, 1.3 watt units, are placed across each capacitor to help equalize the voltage. As is the nature of Zener diodes they will be conducting at lower voltage than nominal, increasing dramatically at their rated voltage - which, itself, can vary significantly.
Originally, I experimented with the use of a series-connected resistor, diode and green LED across each capacitor to equalize the voltage as depicted by components Ra, Da and LEDa. In this circuit the LED, a normal, old-fashioned indicator-type "non-high brightness" LED was used, taking advantage of its 2.1 volt threshold voltage along with the 0.6 volt drop of an ordinary diode with a 5.1 ohm resistor in series to provide a "knee". While this circuit did work, providing a handy, visual indication of a "full charge" state of each of the six capacitors, it did not/could not safely conduct enough current to strongly force equalization of the capacitors' voltages. The Zener diodes, with their maximum current of more than 400mA, as compared to 15-25mA for the LED-based circuit, seemed to be more effective. I left these in place after constructing the prototype since there was no reason to remove them and the illumination of the LEDs serves to indicate that the capacitors are charging up with the equal illumination being generally indicative of equal charge distribution.
It will be noted that the "top" of the capacitor bank is connected to the positive side of the power source, "straight through" at all times via TH1 and TH2, current-inrush limiters. Because of this, when this unit is "off" it is, for all practical purposes, transparent, consuming no current. These devices act as self-resetting fuses, limiting the current to a sane amount, somewhere in the 30-50 amp region, if the output (or input!) is shorted: Ordinary "slow blow" fuses could be used here, but if so, the advice is to keep spares on hand!
It is only the "bottom" of the capacitor string that is connected/disconnected to enable or disable the "ballast" (and filtering) capability of this circuit: When "off" the bottom of the capacitor bank is allow to float upwards.
If the capacitors are discharged when switch SW1 is turned ON while power is applied the first thing that happens is that the gate of Q1 is turned on via resistor R3. When this happens current flows through R1, but when it exceeds approximately 0.6 volts - a voltage commensurate with approximately 2.5 amps of "charge" current - transistor Q3 begins to be turned on, drawing down Q1's gate voltage. When this circuit reaches equilibrium 2-3 amps will flow through Q1, R1 and TH3 and thus charging the capacitor bank comparatively slowly and preventing a "dead short".
While in this equilibrium mode the gate voltage on Q1 is necessarily reduced to keep it partially "off", to maintain the current at approximately 2.5 amps, and there will be a voltage drop across R3: This voltage drop is detected by Q5, a PNP transistor via R4 which, if Q2 is turned on, will also be turned on which, in turn, turns on LED2, the "Charging" LED which also turns on Q4 which, in turn pinches off the drive to the main capacitor bank switch, Q2, forcing it off.
(In the event of a circuit malfunction self-resetting fuse TH3 limits the maximum current through Q1 to 5 amps before "blowing", at which point the current will be reduced to a few 10's to 100's of milliamps.)
Once the capacitor bank has become (mostly) charged and the voltage across it is nearly the same as the applied voltage, the charging current will begin to drop and as it does, transistor Q3 will start to turn off, causing less voltage drop across R3 in an attempt to make Q1 conduct more. At some point, when Q1 is already "fully on" this will no longer work and the voltage drop across R1 will start to drop, turning off Q1 entirely. When this happens the voltage at the "bottom" of R1 (the side connected to the gate of Q1) will be equal to that of "V+_SW" and this will cause transistor Q5 to turn off.
Once Q5 turns off, the "Charging" LED will also turn off, as will Q4 and the voltage on the gate of Q2, being pulled up by R5 and "slowed" by capacitor C8, will start to rise. As the gate voltage on Q2 crosses the "on" threshold it will conduct strongly, connecting the bottom of the capacitor bank to ground with only a few milliohms of resistance.
If switch SW1 is turned off, the voltage at "V+_SW" drops and via R5, C8 and the voltage at the gate of Q2 drops, turning it off and disconnecting the capacitor bank.
The ultracapacitors were wired together outside the enclosure (a Hammond 1590 series die-cast box) using multiple pieces of #12 AWG bare wire, both for low ohmic resistance and mechanical support. Additional pieces of wire were used on the capacitors' support pins for spacing and support when the two banks of three were arranged to be parallel to each other, terminals facing - but separated by a safe distance as seen in the pictures. The balancing Zeners, etc. were also installed across the capacitors at this time.
Prior to mounting the capacitors in the bottom of the box the holes for the LEDs and switches were drilled/filed - this to eliminate the possibility of the tools damaging them.
The two parallel banks of three series capacitors were prepared and then placed in the bottom of the box, held in place with RTV (Silicone (tm) seal).
There is no circuit board: The actual control circuitry is mounted on the lid, with Q1 and Q2 being bolted to the lid itself for heat-sinking using electrical insulating hardware (washer, grey insulating pads) that were scavanged from a dead PC power supply. The circuit itself was constructed on a piece of glass-epoxy circuit board material.
Without making a circuit board, there are several ways that the circuit could have been constructed, but I chose to use a variation of the "Manhattan" style which involved islands of copper. In some instances - such as for the large resistor(s) comprising the 0.25 ohm unit on Q1 and for the connections of the power in/out leads and TH1 and TH2, islands of copper were isolated on the board by first drawing them out with a pen and then slitting both sides of a narrow (1/16th-1/8th of an inch) trace with a sharp utility knife and straight edge and then using the heat from a soldering iron to aid in the lifting of the narrow strip to isolate the area of board.
For other portions of the circuit I used "Me Squares" available from QRPMe.com (link): These are small pieces of glass-epoxy circuit board material with nice squares etched on them that one glues down using cyanoacrylate (e.g. "super") glue and then uses as points for soldering and connection.
The nice thing about these "Me Squares" is that they are very thin, look nice and are very flat - which makes them easy to solder and glue down, but one could also cut out squares of ordinary circuit board material and solder those down, instead, provided that they were also made flat on the back side and de-burred for maximal surface contact. Finally, one could use the utility knife and isolate islands - or even use an "island cutter" tool - to produce isolated lands on the piece of circuit board material itself.
The main reasons for using this technique were that it was quick, and also that it was surface-mountable: Had I wired it on perforated board or even made a conventional circuit board, I would have had to stand it off from the lid to insulate the circuit from it: Using this technique the board itself was simply bolted flat to the lid and was quick to wire up.
For interconnects between the circuitry on the lid short lengths of #12 AWG stranded wire were used for the high-current leads connecting the capacitors and input/output leads and much smaller (#24 AWG or so) for the wires that connected to the switch and LEDs. For the "outside world" connections 30 amp Anderson Power Pole (tm) connectors were used mounted in the standard manner.
A few caveats with this circuit:
- This circuit consumes some current (at least a few 10's of milliamps - maybe more) whenever it is set to "on", even after the capacitors have equalized. What this means is that it will slowly drain your battery if the switch is left in the "on" position and because of this it is recommended that one switch it to the "on" position ONLY when intermittent, high current is going to be needed. In other words, if you are going to receive (only), leave it "off", turning it "on" only if you plan to transmit, knowing that it may take 10's of seconds for the capacitors to charge.
- If the power source cannot deliver the amount of current required during the "charge" cycle - or if it briefly "blinks" - the source voltage will sag and it will likely switch from "charge" mode to "operate" mode and connect the ultracapacitors directly across the power source. If the power source has limited current in the first place, this will simply mean a "dip" in voltage while the capacitor bank charges, but if this occurs with a high-current source that has had a momentary glitch, a premature switch to "operate" mode could, in theory, blow a fuse. This premature switching would likely happen if you had this connected via a cigarette lighter and started a vehicle while it was in a charge phase.
- This capacitor bank should be treated like a battery: If it is shorted out you will get lots of current - more than enough to burn open small wires and blow fuses - or even burn open large wires or small tools if you do an "oops" on the unfused/unprotected side of the circuit, or neglect to include any fusing/current protection!
- Neither this capacitor bank (or any battery) should ever be placed on the output with any power supply that has a "crowbar" voltage protect circuit (such as is present on many power supplies such as the Astron "RS" and "RM" lines) as a power line or transient on the output could cause the crowbar to trigger and short the output of the power supply - including the capacitor bank or whatever battery you might have on the output.
- This sort of device must be contained within a rugged enclosure/container. The picture shows the prototype being built into a Hammond 1590 series die-cast aluminum box that is both very rugged and also provides heat sinking for Q1 and Q2. In the unlikely event of a catastrophic failure due to a wiring or a capacitor going bad this enclosure will not melt and is likely to contain the "mess" - even if it were to get very hot!
This circuit should be disconnected/disabled when the vehicle is started since these capacitors can supply "car starting" current on their own and it is possible to blow fuses, pop breakers, damage switches, circuits, relays and other mischief if the current from the capacitor bank were to "back-feed" through wiring that was not designed for that sort of current!
- It is the nature of power FETs such as Q1 and Q2 to have an intrinsic "reverse diode" - even when turned off. Be aware that if it so-happens that the bottom of capacitor bank is more negative than the "ground" (Battery -) side, these diodes - particularly the one in Q2 - will conduct.
While it is possible to use this to allow an HF rig to be powered from a set of D cells, it is more practical to use smaller lead-acid or lithium-ion packs as the primary power source and use the capacitor bank as the "ballast" to supply the peak currents.
I'm certain that this device could be improved, but it seems to function as it is.
How usable is it, in the real world?
It depends a lot on how - and with which radio you plan to use it. For example many older-vintage Kenwood HF transceivers will fail to function properly (e.g. operate with distorted audio, "FMing" of the signal, etc.) much below 12-12.5 volts while more modern, compact HF radios like the Yaseu FT-100 and FT-857 will happily run at 10-11 volts - perhaps at slightly reduced output power, but fine otherwise. The upshot is that if you are considering a radio to be operated from "marginal" power sources, be certain that you have done your research and consider how your candidate radios operate at low supply voltage and how/if they degrade "gracefully" or not!
How about running an HF rig from alkaline "D" cells? As it turns out I can happily transmit 100 watt (peak) SSB using my old Yaesu FT-100 with the device described on this page using 10 "D" cells in series.
To do this effectively, one must minimize the contact resistance of the battery contacts which pretty much rules out using cheap, spring-loaded battery holders which, by themselves, can have almost as much resistance as the cells themselves. Aside from spot-welding tabs onto the alkaline cells (the heat of soldering would likely cause some damage and slight loss of capacity) the best holders are aluminum with heavy bus bars and the fewest number of springs and contacts (e.g. multiple cells directly in series) such as the Keystone four cell holders, models 158 (two of them) and one two cell holder, Keystone model 186.
One cannot "key down" with a carrier without significant voltage sag, but the FT-100 seems to work OK on typical SSB with voice peaks - but under such heavy loads don't expect to get much longevity before the cells' internal resistance increases: As noted previously not all radios (such as older Kenwood mobiles) behave so well at lower voltages, so do your homework!
As detailed in the article, a more practical use of this sort of device is as a "power conditioner" to help compensate for voltage sags due to resistance in the interconnecting cables, somewhat underrated power sources, aging battery packs and/or "small"-ish batteries.
How about a solar power source? Assuming that voltage from the panel is regulated to a safe value (15 volts or below) the capacitor bank could, in theory, maintain voltage if the solar array provided at least the average current, but considering that solar illumination can vary wildly it would be recommended that additional storage capacity (remember that 53-1/2 Farads has only the theoretical storage capacity of a single "AA" alkaline cell!) be used as well, such as a 7-20 amp-hour lead-acid or lithium-ion based pack - but this sort of system could well be the basis of another article!