Overcharging 4.8v Nimh batteries?

[TheNameless]

Hey guys. I've been searching for an answer to this, but haven't had any luck. I bought a couple 4.8 volt nimh batteries and the 4.8 volt charger from madvapes. The light on the charger doesn't turn green when the batteries are fully charged, but this is normal for that particular charger. This makes it hard to tell when the battery if fully charged. My question is, will the charger stop putting juice to the batteries once they reach a full charge, or will it over charge them?

 

[bjannr]

I have a hard time keeping mine charged. It is supposed to take 4-5 hours on the charger. I am very rarely in the house that long at any time. I will not leave them unattended on their charger. I have had to take them off early many times because they got extremely hot.

 

[Pamdane]

Mine dont overheat, but I do leave them unattended. I keep the charger and battery in a pyrex bowl, just in case...

 

[BardicDruid]

With Nimh batts the safest thing to do is find a Smart Charger that'll shut off when it's charged, Nimh has a bad habit that if it overheats too much they'll pop. Also every time they get too hot it shortens the life span, after years of having to deal with Nimh in one thing and another the Li-Ion's were a god send.

 

[TheNameless]

Does anybody know where I can get a smart charger for the 4.8's? That seems like the best option. I left mine on the charger overnight to see if the charger would stop charging them, and it didn't. When I woke up, the battery was hot to the touch. I definitely dont want to do that again.

 

[WillyB]

Well many of the cheap 4 cell chargers out there don't do any monitoring.

The cheap Energizer, CHDC-CA, I have just works off a timer. It makes no difference what the individual voltages are, it charges at it's predetermined time.

I don't think you would want to spend good money on a smart 'pack' charger, and even so unless each cell is monitored some will still probably be cooked.

Your best bet would be to use a timer with what you have. Do some testing to determine when your pack is at about ~5.4V and set your timer accordingly.

 

[mdocod]

Series wired NIMH packs are naturally balancing since you are supposed to overcharge them a little bit anyways. All smart chargers technically overcharge the cells some. There is nothing wrong with this. During the slightly overcharging period, out-of-balance cells will catch up to the others that just dissipate the excess as heat. Failing to slightly overcharge the pack once and awhile will result in large crystal formation in the electrolyte and suppressed voltage. The large crystal formations can eventually break through the separators in the cell and cause a faster rate of self-discharge.

NIMH cells can generally handle a 1/10C or slower continuous trickle indefinitely.

The best solution to a nice proper and fast charge is in fact a hobby style pack charger rigged up to make contact with the pack you are using. You can get pretty decent units for $40-60 from places that specialize in radio control devices.

 

[TheNameless]

Thanks guys. Lots of knowledge here. Im going to break out the multimeter and figure out how long it takes to reach a full charge, then hook the charger up to a timer. And overcharge slightly on occasion. Maybe when I have a little more cash to throw around, I'll invest in a better charger.