Author Topic: Supercapacitor balancer  (Read 448 times)

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Offline Mr Evil

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Supercapacitor balancer
« on: January 13, 2020, 11:02:45 pm »
I've been doing some stuff with supercapacitors recently, mostly using them to store power from solar panels. One of their shortcomings is their low voltage rating, typically <3V, so using several in series may be necessary. Like any polarized capacitor, they have to be balanced when connected in series, to ensure the voltage is shared equally. There are some off-the-shelf parts for this, but all the ones I have seen don't actually balance until the full voltage is reached. This is no good for me, as the unpredictable nature of solar means that they may never reach full charge.

Thus I put together this circuit, which will start balancing as long as there is enough voltage for the op-amp to function - the MCP6041 is specified down to 1.4V, and it still functions well enough to provide some balancing down to 1V per capacitor.

The schematic shows one section. To balance a string of capacitors, one section is needed per capacitor, with the three marked nodes connected together between sections. With n sections in series like that, the resistors labelled R5 form a voltage divider with exactly 1/n of the total voltage across each resistor, which is the target voltage per capacitor for perfect balance. The op-amp is simply connected as a differential amplifier, comparing the target voltage with the actual voltage, and turning on the transistor to drain charge if the actual voltage is too high. Since perfect balance is impossible with real components, the feedback values are chosen so that it doesn't drain charge if the actual voltage is only slightly high, otherwise it would constantly try to drain a little bit. The diodes are to prevent reverse bias, since that's still possible if the capacitors are drained much below the point where the op-amps stop working.

908316-0

With SMT parts, it easily fits inside the footprint of a big supercapacitor.

908320-1

I put four of them in series for 10.8V total, suitable for being charged by a nominally 12V solar panel. Note that I designed it to use tiny SMT reverse polarity protection diodes, but I have a drawer full of big schottky diodes that I'm never going to use up otherwise, so I soldered some of those on instead.

The green text on the capacitors is the capacitance that I measured. I bought a bunch of these "500F" capacitors, and although some are within 500F ±20%, some of them are lower. The are all within 400F ±20% though, so I suspect that they are really meant to be 400F and have been mislabelled.

Here's a thermal image of it being charged at 5A. I have no experience with thermal imaging, so the temperatures might not be right, but it shows that the two capacitors on the right, the smallest ones, increase in voltage the fastest, and so need charge to be drained from them.

908324-2

And the reverse, being discharged at 5A. As expected, it's the ones on the left that get hottest.

908328-3

And an image of it after it has been sat fully charged for a while, showing nothing since it's all balanced.

908332-4

After all that, I'm not entirely sure how necessary this really is. There's not a lot of hard data about the effects of reverse charging supercapascitors. There's a paper by NASA showing that it reduces capacitance, but they only tested reverse charging to full voltage. If small reverse voltages are tolerable, then a simple diode may be sufficient, in combination with the voltage clamping type of balancer that you can find on eBay. however, my design does have the advantage of dissipating power over the whole charging cycle instead of only at full charge, which should mean the peak temperature is lower. It's also just as simple.

Offline Mr Evil

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Re: Supercapacitor balancer
« Reply #1 on: January 13, 2020, 11:06:44 pm »
Something happend to the schematic attachment, so here it is...
908364-0
EDIT: And I forgot to mention that at the top of the string, Div+ must be connected to V+, while at the bottom Div- must be connected to V-.
« Last Edit: January 15, 2020, 05:40:24 pm by Mr Evil »
 

Offline NANDBlog

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Re: Supercapacitor balancer
« Reply #2 on: January 13, 2020, 11:17:10 pm »
what is the point of balancing it, if it is not at full voltage? You dissipate energy, but what do you gain?
 

Offline Mr Evil

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Re: Supercapacitor balancer
« Reply #3 on: January 13, 2020, 11:20:07 pm »
what is the point of balancing it, if it is not at full voltage? You dissipate energy, but what do you gain?
If the capacitors never reach full charge, so the balancing is never applied, then it could end up with one being more or less permanently reverse biased, clamped only by the diode.

Offline fcb

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Re: Supercapacitor balancer
« Reply #4 on: January 15, 2020, 09:35:57 am »
what is the point of balancing it, if it is not at full voltage? You dissipate energy, but what do you gain?
If the capacitors never reach full charge, so the balancing is never applied, then it could end up with one being more or less permanently reverse biased, clamped only by the diode.
Be interesting to see what effect it has on the charge/discharge efficiency of the pack?

It's a simple solution to keeping them all at the same voltage, but works be bringing every parts energy storage down to the storage of the smallest capacitor.  This may suit your design goal, but isn't optimal if you care about energy density.
« Last Edit: January 15, 2020, 09:40:10 am by fcb »
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Offline phester

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Re: Supercapacitor balancer
« Reply #5 on: January 15, 2020, 01:41:19 pm »
Ive seen some active balance circuits now available that take excess from higher cells on a DC-DC to close the imbalance. I believe aliexpress sell them. Would be better than dissipating the excess energy as heat.
 

Offline Mr Evil

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Re: Supercapacitor balancer
« Reply #6 on: January 15, 2020, 05:38:18 pm »
Be interesting to see what effect it has on the charge/discharge efficiency of the pack?
I haven't tried measuring it, but theoretically, if you consider two capacitors charging in series with the worst case of one being +20% and the other being -20%, and given that I = C * dv/dt, to keep dv/dt the same across both capacitors requires that the current into the smaller one be 0.8/1.2 the current through the larger one, thus 1/3 is wasted by the balancer. The opposite is true when discharging.

Ive seen some active balance circuits now available that take excess from higher cells on a DC-DC to close the imbalance. I believe aliexpress sell them. Would be better than dissipating the excess energy as heat.
Do you have a link? With AliExpress's mostly useless search, the closest I found was a device that uses a supercapacitor as part of a charge pump to balance batteries. I suppose the same principle could be used with a normal capacitor to move charge between a string of supercapacitors. There were also some sellers selling LTC3128, which is a buck/boost converter that can switch the output between either of two capacitors in series to keep them balanced, but it's only for two in series, and part of a charger rather than an independent device.

Offline schmitt trigger

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Re: Supercapacitor balancer
« Reply #7 on: January 15, 2020, 05:52:21 pm »
Mr Evil;
Excellent circuit! I'll keep it in my useful circuit collection folder.

It would be interesting if you could rig a simple scanning circuit, to be able to measure the actual voltage tracking while in actual operation.
I understand some of the concerns other posters have mentioned, but testing would prove/disprove the real world performance.
 


Offline Mr Evil

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Re: Supercapacitor balancer
« Reply #9 on: January 16, 2020, 05:58:19 pm »
Mr Evil;
Excellent circuit! I'll keep it in my useful circuit collection folder.

It would be interesting if you could rig a simple scanning circuit, to be able to measure the actual voltage tracking while in actual operation.
I understand some of the concerns other posters have mentioned, but testing would prove/disprove the real world performance.
While I haven't logged the voltages over a whole cycle, I did note down the voltages after charging to 10.8V at 1A: 2.68, 2.72, 2.71, 2.68. And after then discharging to 0.1V at 1A: 0.22, -0.02, 0.12, 0.09. And charging halfway to 5V at 1A: 1.27, 1.24, 1.25, 1.24. I'm confident that it would keep everything well balanced in real use.

Yes, it's inefficient. For solar powered applications like mine, that's quite important, but for now my aim is to get something as simple as possible that works. I can improve on it later.

I'm sorry , The "active" balance that I remembered was for Lithium ion. I guess it could be modified for a lower voltage. I saw some reviews of these on youtube
Those are interesting. The first one says "Operating voltage range: 2.0V-4.5V", which might be low enough to work. I might get one and see what happens. Also I didn't notice before, but the one I linked to earlier actually states "super capacitor" as one of the things it can be used to balance. It's too expensive to be worth it though... unless I build a gargantuan bank.

Offline fcb

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Re: Supercapacitor balancer
« Reply #10 on: January 17, 2020, 11:45:38 am »
So I had a thought whilst trying to goto sleep last night - so it's not fleshed out AT ALL. And almost certain this will have been thought of before.

What if you had a 5th smaller capacitor which 'shuttled' any excess charge between the 4 main capacitors?  Essentially the 5th capacitor is connected in parallel across each of the main capacitors, so it spends 1/4 of its time across each one (speed could be quite slow, probably determined by the size of the shuttle capacitor and charge current - but thinking perhaps 1KHz).

Apart from the consumption of the drive circuit, any losses would stem from capacitor ESR and switch ESR?

Cost - would require 8 switches (2 per main capacitor), and each switch might need to be made of two MOSFET's (Nchan/Pchan) perhaps FDS8958A or similar - drive waveforms could be generated from something as simple as a 4017, but realistically a cheap micro with 4/8 outputs would be used.

Haven't thought too much about the drive voltages required, not complex though - with enough thought you could also create a small charge pump to generate the over and under rails required for the gate-drivers, perhaps even use the main capacitor above/below to supply the gate drive rails.

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Offline Mr Evil

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Re: Supercapacitor balancer
« Reply #11 on: January 18, 2020, 05:47:19 pm »
What if you had a 5th smaller capacitor which 'shuttled' any excess charge between the 4 main capacitors?
It's a good idea. I think it's how the one I linked to above works, if I'm interpreting the Chinglish correctly, which say
Quote
The basic principle of the active equalization technology is to use the ultra-pole capacitor as a temporary energy storage medium, charge the battery with the highest voltage to the ultra-pole capacitor, and then release the energy from the ultra-pole capacitor to the battery with the lowest voltage.
I wonder how they limit the current when connecting an initially empty capacitor in parallel?

Offline fcb

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Re: Supercapacitor balancer
« Reply #12 on: January 18, 2020, 09:18:54 pm »
What if you had a 5th smaller capacitor which 'shuttled' any excess charge between the 4 main capacitors?
It's a good idea. I think it's how the one I linked to above works, if I'm interpreting the Chinglish correctly, which say
Quote
The basic principle of the active equalization technology is to use the ultra-pole capacitor as a temporary energy storage medium, charge the battery with the highest voltage to the ultra-pole capacitor, and then release the energy from the ultra-pole capacitor to the battery with the lowest voltage.
I wonder how they limit the current when connecting an initially empty capacitor in parallel?
That’ll teach me to read posts - there’s nothing new under the sun. I didn’t consider the surge issue if some of the capacitors have charge when the circuit starts, i was only envisioning a much smaller shuttle capacitor.
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