nimh questions??

toby dawg · Post by **toby dawg** » Sun Mar 12, 2006 11:36 am

Do nimh AA batteries need to be fully empty of power before recharging?

Turbo Tax 1.0 · Post by **Turbo Tax 1.0** » Sun Mar 12, 2006 11:39 am

no they can charge whenever and have no memory effect

Post by **Sparkfist** » Sun Mar 12, 2006 11:41 am

No Ni-MH don't have any memory effect. You can charge them when ever you feel you need to, or feel like it.

Post by **Skyone** » Sun Mar 12, 2006 11:45 am

Since I don't want to overpopulate the 42, I'll ask my question here:

If you overcharge NiMH, will they explode?

If so, is there a protection circuit?

daguuy · Post by **daguuy** » Sun Mar 12, 2006 12:02 pm

NiMH chargers stop charging when the battery's fully charged. rechargable batteries also last longer if you charge them before they go out. the only kinds that i know of that literally blow up when overcharged are lithium batteries

timmeh87 · Post by **timmeh87** » Sun Mar 12, 2006 2:50 pm

i still dont think its good for them. they start to heat up. this might actually cause them to explode eventually (warning: do not put in hot places or fires, may explode, blah blah blah)

Post by **Sparkfist** » Sun Mar 12, 2006 9:51 pm

How Ni-MH and Lithium ion batteries work is this; they speed charge them, keeping track of the temperature. Once the batteries reach a point, maybe 80-85% they will slow the charging to be sure they don't explode. Also keep in mind that a charger is really forcing electricity into the battery, that will generate heat no matter what you do.

timmeh87 · Post by **timmeh87** » Sun Mar 12, 2006 10:37 pm

as far as i know, the heat produced in charging a battery is not signifigant until they reach a full charge. until then they are absorbing almost all of the energy you give them. i have felt many-a charging li-poly cells, and they never feel warm.

this site seems to confirm that (a decent read if you want to know about charging your batteries)
http://www.batterieswholesale.com/charg ... d_nimh.htm

therefore i dont think lithium batteries can use the "delta t" method you described... i think they explode before that happens. when we are charghing single cells (solar car team), we hook them up to a power supply, and give them 4.2v or less. the current slowly tapers off until the battery reaches 4.2v (the maximum safe voltage) and no current flows anymore.

nimh charges can also detect a small voltage spike that indicates that the battery is full ("delta v" method). this is technically a better way to do it than temperature sensing, because heating up means they are already overcharged. its harder to do though, sice its a pretty small voltage spike, (20mV IIRC) and you need more complicated circuitry. (edit - looked it up, its ~2.5mV)

in either case, the charger does usually kick into "trickle charge" mode