can 8.4v charger charge 7.2v battery?

daguuy · Post by **daguuy** » Mon Apr 24, 2006 3:47 pm

unfortunatly my good 8.4v 4200mah nimh dvd player battery pack is dead

it has a built in charger that would be really nifty to have in my portable so i want to get some high capacity nimh C cells to replace the dead ones and hook them up to the charger. it would certainly work if i used 7 of them to get the same voltage as the original cells but it would be more convenient to have 6. can a charger that was used for an 8.4v battery pack charge a 7.2v battery? i know my alkaline charger can charge anything from 1.5v-9v but i don't know how picky NiMH battries are. i know i could just use any 7.2v nimh charger, but 2 things i'm going for with my N64p are battery life and niftyness

XFactor · Post by **XFactor** » Mon Apr 24, 2006 4:03 pm

Yes, that's fine to use 8.4v. My batteries are Lithpolys and they are rated at 7.2v and my charger charges them at 8.4v. To my knowledge, although nihms are of a different chemical composition, they still follow the same principals of charging as an alkaline, a lithium polymer, or a lead acid battery.

All I would tell you is that you have to watch out for overcharging. If you wire multiple batteries together in a parallel configuration, get a protection circuit to make sure they don't explode (although that'd be cool as hell to watch, granted your not in the explosion and limbs fly everywhere, which would be cool too because it reminds me of that scene in Kill Bill where she slashes all the people's arms off and blood goes everywhere and then that chick with the mace comes and starts flailing it around and all the dudes are like "WTF mates?" and then they all die and then she does the five finger move which is funny because my hand has five fingers).

So yeah, 8.4v is alright.

daguuy · Post by **daguuy** » Mon Apr 24, 2006 4:09 pm

awsome, thanks

my charger stops charging when the batteries are full so overcharging shouldn't be a problem

superdeformed · Post by **superdeformed** » Mon Apr 24, 2006 6:04 pm

If the charger was meant for 8.4V batteries then it probably charges a volt or two above that, which may or may not be an issue with a 7.2V battery. I'm not sure what the usual charge voltage range is for a 7.2V NiMH pack, but it may be worth looking in to. Another thing is that if the charger monitors the voltage to determine when to shut off (and was made to monitor 8.4V batteries only) you're going to seriously overcharge your battery, which leads to the usual extremes of overheating, venting, exploding, etc...

XFactor · Post by **XFactor** » Mon Apr 24, 2006 9:31 pm

"Another thing is that if the charger monitors the voltage to determine when to shut off (and was made to monitor 8.4V batteries only) you're going to seriously overcharge your battery, which leads to the usual extremes of overheating, venting, exploding, etc..."

...hence why i said a protection circuit is needed on the battery..

timmeh87 · Post by **timmeh87** » Tue Apr 25, 2006 12:57 pm

1) no matter what voltage you give a battery, the voltage will drop down to the voltage of the battery. the higher the difference in voltage, the more current will rush into the batteries. but if you put some sort of current limiter on it (like those diodes and transistor), its safe to put in any voltage that wont break down the semiconductors. because the voltage will drop to the voltage of the battery, and the current will be limited to a safe value.

this is sort of a theory i came up with based on watching batteries getting charged on a power supply... perhaps someone just knows how it works and could tell us.

2) i 'think' "delta v" chargers only look for a delta-v, not a specific voltage. i cant quote any source on that though.