is chargind nicd/nimh in parallel really so bad?

ive been thinking about this a little, ive read that at the end of charge the voltage of a nicd cells drops about 10mV, and a nimh about 2mV. take for example a 36V pack of nicd, suppose the internal resistance of each series string is 1 ohm, just a wild ass guess. and say that just before the drop in voltage each string will be 45V. so one string reaches the drop and the other is at 45V, the one that past the drop in voltage is 45.0 - (0.010 * 30) = 44.7V. if for example the voltage from the charger across the two series strings is 46.0V, then for the string that past the end of charge voltage drop the current through it will be (45.5 - 44.7) / 1.0 = 1.3A, for the one that hasn't dropped in voltage yet the current is (45.5 - 45.0) / 1.0 = 1.0A. thats not a huge difference. and the string that is behind would probably be almost there unless the capacity is much different.

Someone who did just that has a picture posted around here of a battery pack that was burned from having it's NiCd cells explode (violently). Don't try it.

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ok, i guess its just ok at very low rates, like C/10 or less.

The guy I mentioned exploded them with a low-rate charger and even diodes to isolate the pack, IIRC. Lemme see if I can find the thread...

EDIT: Found it. It was Jeremy.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=3241

Schottky isolated, C/10 charged:



Also it caught fire.

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nutsandvolts said:

Lithium batteries do not like to take a charge below freezing. I mean, think about it, would you?

Click to expand...

I'd say I'm pretty addicted to this hobby, but not quite to the point at which I can relate to a battery...:?

Here's a charger already made for this from batteryspace. Good for 15 cells per bank. Probably just 3 independent chargers in one.
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3866