Battery Home Charger
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Need help getting the right charger for my Ni-cd battery pack! Home built Project!?
I have put together a 4.8v battery pack for a project I am working on. it consists of 20 1.2v 1200mah cells in 5 groups of 4. if I have done my math correctly, the total current output on this should be 24,000 mah or 24 amp hours. since the voltage is 4.8v and when fully charged the volt meter reads 5.2v across the pos and neg terminals. I need to come up with a charger for this battery pack.
I figured on basing it on a 8 hour charge schedule having the charger put out a total of 6400 mah or 6.4 amps. my problem is that I don't know how much higher to have the voltage output on the charger. I read somewhere that for each cell that is 1.2v the charger should put out 1.5v to .1.6v is this true or is there another approach?
Uh... a few things wrong here.
If you connect four 1.2V, 1200 mAh cells in series that stack will give you 4.8 volts, 1200 mAh.
Five of the above connected in parallel will give you 4.8 volts, 6000 mAh. Not 24,000. You do not get to multiply the mAh rating by the total number of cells in the entire pack unless they are all simply connected in parallel - and then there is no voltage increase.
If that is confusing, think about it this way. One of your cells provides 1.44 watt-hours (1.2V x 1.2 Ah) of energy. 20 of them can therefore provide 20 x 1.44 = 28.8 Wh. Using my figures above, 4.8V x 6 Ah indeed equals 28.8 Wh. Using your figures you'd have 4.8 x 24 = 115.2 Wh. That would be a conservation of energy violation. It can't happen.
To charge your 6 Ah pack at the 8-hour rate would then need a charger that can supply a little less than an amp. HOWEVER - I should warn you that series-parallel connections like this are a bad idea. The cells are always different enough from each other that if you try to charge the sets in parallel, some will be overcharged and some undercharged. If you need 4.8V at 6 Ah, you should get some 6 Ah cells, connect 4 of them in series, and be done with it.
Failing that you should isolate the series sets and charge them individually.
Using standard old NiCd "dumb charger" techniques, yes, 1.5V/cell is about right. Be sure to take them off charge when they are fully charged or you will overcharge them, likely causing voltage depression - often mistaken for NiCd "memory".