When is a nimh battery dead?

I have a two batteries that the LaCross BC-900 charger claims is dead. One reads 0 volts and the other reads 0.63 volts. However, some batteries that the BC-900 claims is dead can still be charged/revived via my Maha charger. It seems like the BC-900 is very finicky.

Even so, there are other batteries, that when placed in pairs in my Maha charger, no lights come on. It's as if they are not there.

So how can I tell a battery is really dead?

Hello Sygyzy,

Considering that an empty NiMh cell usually rebounds to close to 1.2 volts resting, I think both of your cells are dead.

I consider cells dead if their resting voltage falls below 1.0 volts, and I try to make sure that all of my cells maintain 1.2 volts or higher.

Tom

I use 0.925 as my dead reference.

sygyzy: "Customer Service? My battery charger won't charge two nimh cells. It says they're at a very low voltage. I think the cells are dead. What do I do?"

LaCross CS: "First off, you have to make sure the batteries are dead."

sygyzy interuppts: "OK!"

BLAM! BLAM! BLAM! BLAM!

sygyzy: "Ok, I'm sure they're dead, what now?"




Just a spinoff of a popular joke.

BC-900 displays null it if the cell doesn't have some voltage. You can connect the totally dead cell to not dead cell in parallel for a few sec or charge it in a slow charger for 10 miinutes to bring the voltage high enough for BC-900 to accept it.

Contrary to what SilverFox said, such a battery can be brought to life sometimes. I brought back such NiCd to full capacity after a refresh cycle or two.

Battery is considered dead when;
self-discharge is unusually high
discharge capacity tested is less than 75% the rated capacity (at rated rated mAh/5h, so for 2,000mAh discharge at 400mA for testing)

Hello Handlobraesing,

He was asking about NiMh cells. I also have had some success with bringing back NiCd cells, but it seems that their self discharge rate was high and I am never able to get back to full capacity.

Tom

What do yu mean "dead"?
It occured to me countless amount of times that i left Ni-mh cell in the device ON that has no auto-shutdown capability (flashlights,whatever). Even if cell stays at 0v (at constant load) long time - i still was able to recharge it (i use diy charger, that can charge from any voltage) and battery was pretty usable. Maybe it lost _some_ capacity - but it's not that you can throw it away after a such single case.
Or you guys referring to when voltage drops below 1v by itself in a short time?

I had brand new Ni-MH AA cells still in their packages that I forgot about. Some leaked, some wouldn't charge no matter what tricks were used and what did charge were at a reduced capacity. You can read that "all" you have to do is cycle them a few times, yeah right!

I've got a few that charge to 60-80% when checked on my MBT-1 right off the charger but within 3 days they are 20% or less. I'm guessing that these are gone.

-LT

Thanks for your help guys. Looks like these cells are indeed "dead". I had no idea that cels maintain a voltage even when out of juice. I still am learning the terminology. I wish there was a way to measure the current juice left in a battery. The mAh? Anway, now I need to find a place to dispose of these safely.

Not to derail but what's a safe voltage to discharge NiMHs to?

Shaocaholica said:

Not to derail but what's a safe voltage to discharge NiMHs to?

Click to expand...

0.8V per cell is considered "safe". Below this point, there is a chance for an internal chemical process to begin that will steal as much as 20% of the cell's total capacity. There is also some discussion about cell reversal, which causes permanent damage. This is an issue in multi-cell packs where one cell is signficantly worse than the rest.

Steve Offiler
VP of Engineering
Central Tools, Inc.