fechter wrote:When running multiple supplies in parallel, each one will run at its limit until the pack reaches near full charge, at which point they may not share evenly. I don't think this is a problem. Most of the time they will be in CC mode and sharing according to their individual limits. Once a supply hits CV, the pack is essentially done charging.
I understand and the uneven current sharing is not my concern, although this setup requires that every paralleled supply must have its finishing voltage as well as current limit set properly. What I mean is that if any of the paralleled supplies has their supply voltage set too high then you don't notice it during the time that this supply is in current limit, until the moment that all other supplies stop delivering current, then the voltage will rise until the level set at this supply. For lead-acid batteries this is an easy trick to give them a controlled over-charge for balancing purpose (I have actually used this setup in the past) but for Lithium this can spell disaster. When all supplies are forced to share then they will stay at the voltage set for the lowest supply.
Another issue is the turn-off (float) point. If two independent supplies are set for approximately the same voltage then usually one will fall below the threshold while the other is still pumping max Amps and so the first supply will turn off (go to float) and the second will continue charging until that one falls below its threshold and also switch to float, so each supply should be set to the current at which you want the charge to terminate, for example 1A. However, at times it can be that the voltages of both supplies are identical and they both continue to charge until the charge current is 2A and they both switch to float. This is a slim chance, but it can happen and if you parallel more supplies, the effects become more significant: For 4 supplies, they could stop charging at 4A, 3A, 2A or 1A depending on how equal their outputs are. With a current sharing control, this will not happen as all supplies should be regulated to the lowest configured one, with 4 supplies you set the threshold for float to 0.25A on one and they all go to float as soon as the charge current drops to 1A (4 x 0.25)
fechter wrote:Most of the issues with the 3 pot board were with the setup and testing instructions. There were also some issues with the parts fitting the holes on the board, both of which are easily corrected. It is still a bit tricky to set the cutoff current as you need some way to get the current at the cutoff point (around 100mA?) to make the adjustment. If everyone wanted the same cutoff current, I could use fixed resistors and eliminate the adjustment, but as cor points out, the error in the amplifiers is significant at the low end of the range so some adjustability is nice.
Correct, you need adjustability even if you always want to set a fixed current due to component variations, but with an option to share you actually want to select a lower float current, because it is multiplied by the number of parallel units.
fechter wrote:The simple limiter board was laid out for a particular potentiometer from Mouser that has a tendency to go out of stock, so maybe I can have two sets of holes to fit more than one style of pot. I think I can also add a second set of holes for the shunt resistor to allow currents over 20A.
In the schematic you say that the shunt is 5W (I do not know which temperature it will reach at this power level) while I am guessing it can take only 2W because 20A times 0.005 Ohms is 100mV, times 20A is 2W.
I hear you on the component placement/ holes stuffing. It would be good to be able to use different style pots.
fechter wrote:I'm still on the road now and don't have the files with me. When I get home I'll look at revising them for the lastest changes.
I will first try a current sharing schematic that I found in a paper, after I designed my own. In essence it will add two opamps plus some passives, so it should work with a single quad opamp or two dual. I have a couple reels of opamps, so I will try to use those in my experiments. Feel free to go ahead with your boards, I think it will take some time to work out the kinks in current sharing setup. As I said, I have a couple S-240-48 that are perfect to test the current sharing while charging my pack.