How to test Ni-Mh batteries used in wall phones?

[Rolomoto]

Is there any way to test this type of battery with a multimeter?
Thanks!

[Signal32]

What do you want to test ? If it's completely dead, if it still has the rated capacity, etc ?
A basic test would be to measure the DC voltage on the terminals. If it's 2.8V you know it's probably good.

[Rolomoto]

It read .82V and it won't charge, does that mean it's dead?

[jeroen79]

NiMH will be 1.2V/cell between 80-20% charge so 0.41V/cell would be very very discharged and/or dead.

Does the charger break off charging right away or will it not keep the charge even after a long period of charging?
In the first case you could try running a constant current through it for a long period without a 'smart' charger deciding that it is unchargeable. Just as a last straw.

[Rolomoto]

It charges when the phone is on the base but it doesn't even start charging. I guess it's dead.

[Audioguru]

It is Chinese. When the Chinese Ni-MH battery in my phone failed in 6 months I replaced it with a much better Philips battery that has lasted for years.

[Chalcogenide]

I just had a similar pack fail on a cordless phone. I ended up ripping apart a 2*AAA holder and using its contacts to make a AAA holder in the cordless itself, so that in the future I will be able to just replace the batteries in a minute instead of buying an expensive custom made battery pack. I have no pics to show the - crude - hack but I may be able to take a pic in the weekend if you want to see how I did it.

[SeanB]

Typically if it is under 2V after a 12 hour charge, it is dead. If it is over 2V after charging, then load it with a current equal to the capacity, in this case 400ma, using a resistor of around 4R7, and see if it will still hold over 2V for more than 5 minutes. If it does pass then it is good, but most will fail. the reason they fail is both that they are poorly made, and that they are overcharged for long periods.

Take the phone off the charger, and leave it off until the battery goes low, then charge it only for the recommended charge time ( normally 12 hours) and take off again. Leaving it on for long periods damages the batteries as most chargers will always pass current through the cells and the long term overcharge will damage the cells.

However a third party pack is cheap and will work. They tend to come with 2 individual pin connectors though, so do not plug in backwards as you will kill the handset.

[Rolomoto]

Thanks for all the replies!