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ko4nrbs

Why Can't I Charge Nimh In Parallel?

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Discussion I found on the web:

 

Two NiMH/NiCad cells in parallel will share charge, sure enough, until they are equal, so long as neither them hits it's peak charge state. And so long as there's no outside source of current (i.e. a charger), balancing is about all that happens.

 

If there IS a charger connected, they will both accept about 1/2 the charge current until ONE of them hits peak. There might be some way to predict which one will peak first, but I don't know it. However, once it DOES peak, it's resistance drops, and it takes more of the charger current than the one that didn't peak, and will maybe even drain the unpeaked one to feed it's habit. As I understand it, this can be an unstable situation - the peaked cell's resistance continues to drop the more current it's fed, which means it demands more current. Kinda like getting hooked on heroin - the more it gets, the more it wants. The charger will be current-limited, which may hold this in check. If it goes to extremes, the greedy cell can burst. This is violent and you don't want it to happen. The unpeaked cell will never peak - it can't get enough of a share of the charger current into it to get it there.

 

You CAN charge Lithium cells in parallel, because the lithium chemistry never "peaks". The more charge goes in, the higher the voltage and the resistance. A low cell will take more than a high one, until they are balanced and eventually they are full.

 

Obligatory disclaimer - while the standard way of "balancing" a NimH/NiCad/Lead-acid series-connected pack is to deliberately overcharge it, (at a moderate rate, so as not to overheat the cells that peaked first), this is DEATH on Lithium packs. Possibly literally, because they can burst and start a serious fire if overcharged. The ONLY ways to balance lithium packs are 1.) to selectively charge low cells until they match the high cells, 2.) selectively drain the high cells until they match the low cells, or 3.) disconnect them from series and re-connect them in parallel so they can share charge.

 

Bottom line is - no you can't charge NiMH or NiCad in parallel. Discharge, yes - charge, no.

 

Simple explanation:

A resounding NO you can not safely charge NiMh or NiCad batteries in parallel. If you do, the one that reaches full charge first drops in internal impedance and starts hogging all the current and will become very hot and self destruct.

 

Bill

Edited by ko4nrbs

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