EMF said:
Not to confuse the issue, but Mastech also has a dual output charger that can produce up to 100volts @ 5 amps by configuring the outputs with the panel switches.
It would be slow, but I thought this power supply was pretty cool, due to all the ways it can be configured.
Yes, I've seen these as well, but the big advantage that the 5020 has is that it can charge at a 20A rate. As Steve points out, there's lots of 5A solutions out there for doing a whole pack at once. You still have the balancing issue to deal with as well. A bulk charger only works on the whole pack at once. With a pack tha has one or two blocks that are significantly lower than the rest, they won't receive a full charge, even if the bulk charger is doing the constant voltage mode. You really need to balance and then charge, if you have a pack with mismatched cell voltages.
Another case where individual chargers help is if you have packs that are made with cells that have been stressed, or lost a little capacity, compared to the rest. In my 6p setup, I had a number of cells that were weaker, mainly due to the fact that I had beat the crap out of them in my RC helicopters. What I did was to parallel them up with some stronger cells, in order to "help" them work better. This does lower the "resting" voltage of these blocks a bit, and my RC balancers never do quite get them completely balanced right. If I leave them on there long enough, they will bring the strong blocks down to the level of the weaker ones, but then when I charge them, the "full" voltages will be different again. The problem is that the weaker blocks never get fully charged, if I just use a bulk charger. With the individual cell chargers, each block will get as full as it can. I usually end up with resting voltages, fresh off the charger that range from about 3.72-3.85V, for the strong blocks, and about 3.58-3.65V for the couple that are a bit weaker. Once under a load, for even just a few seconds, all the cells will drop down to somewhere around 3.50-3.55V.
I'm still trying to totally understand just how the a123 cells behave together in large packs Healthy cells seem to have the ability to hold their voltage up, right off the charger, close to whatever the cutoff is set to, and stressed cells seem to alway start dropping to some lower value, like somewhere between 3.40 and 3.45V. Once under load, however, the voltage difference between the weaker cells and the healthy ones seems to be a lot less. They will be very clsoe, actually, and if you check, say, at halfway through the capacity, they will all be extremely close, usually within about .005V.
I've got one setup, on the bike in my avatar, that is made up of 5 strings of 16 cells in series. Even though they are "shrunk" together in two packs, as shown below, they are still basical five 16s1p packs, with all five power leads coming out of the packs
You can see that all five are connected to a 5 position "manifold" that connects them in parallel, in a 16s5p configuration
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I never balance any of these 16s sub-strings, which were all made from cells of varying capacities/resting voltages, and I usually simply use my NG1 to charge all five together at once. Occasionally, I will charge each string individually, but again, the cells inside are not being balanced together. The NG1 has been tweaked to have a CC/CV cutoff at 59V. Usually, most of the five will have a fully charged "resting voltage right around 57.5-58.0V. One pack, which has always been a bit lower, will have a resting voltage, about a half-hour after a charge, of around 56.6V. I'm purposely trying to make this as bad a case as i can, by not balancing the cells within a 16s string, and by not connecting the individual cells in parallel. I'm just going to keep going until I see if one of the strings has a lower voltage than normal, after a charge. So far, it has been pretty consistant, and it hasn't made much difference, even if I just charge them togeher, all at once. Should be an interesting test.
-- Gary
