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wlfk -> Simple balance charger. (9/23/2007 6:26:15 AM)
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I've been pondering about the cheapest way to charge some A123 cells. I'm currently thinking of doing regular high-current charging using a power supply with current/voltage limiting. E.g. 10A max; 10.8V max for a 3s A123 pack (using 3.6V as the cuttoff per cell).
I understand that A123s don't need to be balanced every charge, but that it's nice to do it on a regular basis. So for balancing the cells I've been considering connecting each cell in parallel with a 3.6v zener diode bridge + series resistor between each cell + the junctions between the diodes. The idea is that you could leave the cells on this circuit all the time that they're not being used / fast charged. It would consume about a watt per pack so would not be overly wasteful.
As all I can buy off the shelf are 0.5W zeners, I would use 50mA constant current flowing through the circuit, with a series resistor value of 22 ohms.
If the cells are 1v under-balanced this would cause a 50mA restoring current to flow into the cells.
If the cells are 1V over-voltage, this would cause a 100mA current to flow through the diodes, which is within spec for the zener power dissipation.
Any comments on this idea? One flaw I see is that as the cells approach balance, the process gets slower. E.g. when they're only 0.1V out of balance, the current will be only 5mA.
2300mAh capacity / 5 would mean it would take 460 hours to fully charge a cell at this current. Almost a month. But looking at the A123 charge curves, only about 1/10 the cell's capacity lies between the 3.5 and 3.6V mark (probably less - this is from eyeballing a graph with poor axes). So balancing at this point on the curve could take place over a matter of days. Provided the cells aren't going out of balance too quickly (and some people seem to use them happily without balancing at all) then simply leaving the cells on 'slow charge' every night should keep them reasonably within balance.
An alternative would be to build an active balancer using op-amps. I've seen circuits for these, but would prefer to go with this, if it's a reasonable idea.
Has anybody tried anything similar, or can anyone see a flaw in my plan?
K
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