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Old May 31st, 2010, 12:35   #15
coach
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Join Date: Mar 2008
Location: T dot
Quote:
Originally Posted by m102404 View Post
A more competent battery guy can chime in to expand on this, but it is my understanding that running two batteries in parallel is not a good idea. Theoretically, if all the cells are perfectly matched and behave exactly the same, it should work just like that...but in practice they never are.

My understanding is such that if the cell(s) in one pack either discharge faster or are not at the same capacity as the other pack, when you run down the packs you run the risk of driving the "weaker" cell(s) down below their tolerance threshold. Once driven too low they may suffer permanent damage and "kill" that pack.

I suppose that you could hedge your bets by:
- buying two packs with the same matched lot of cells
- being very meticulous with the wiring/connections
- being cautious not to run the packs down too much
- charging them both up the same amount each time
- monitoring any differential in charge between the packs and adjust accordingly

...all in all doesn't seem worth it to me. Even in frenzied all day long firefights, I'm hard pressed to drain a good quality large battery pack if it was fully charged up the night before....and if a battery dies, just chuck in another one.
you bring up some valid points of concern but I have been unable to find anything that says using 2 packs in parallel would be bad or dangerous. caveat here being that it's NiMH or NiCad packs we're talking about.

a weaker sell in the pack won't affect voltage but rather decrease run time. I am sort of agreeing that if one cell of one pack fails prematurely, that it will render the one pack useless but it would likely happen if it was used as a single pack. over driving any pack would probably cause eventual damage. I'll take back what I said earlier and suggest if anyone wants to do this, to match packs. brand, voltage and capacity. when charging packs independently, use a smart charger and not a wall wart. additionally, keep track of how long they charge for to ensure they receive identical charge times.

I'll have to think a bit more about this but I don't think it's a problem to do such.
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