Failed NIMH Batteries?

[smp]

Hello all,

I went to use my HP-41 today and the display did not come on. Hmmm, I've had my NIMH rechargeable batteries in since the summer, so maybe they're run down? I never got a battery warning, that I can remember, but maybe they avalanche quickly.

I put in my normal backup alkaline batteries, and the calculator came on as normal.

The fun starts when I go to recharge the NIMH batteries. My BC-700 shows two of the batteries charging and two of them show up as NULL.

The manual says this will occur if I install damaged batteries into the BC-700.

Hey! I just purchased these batteries back in August! I charged them up, and they've been in the calculator since then. How can they be damaged?!

So, I am currently charging my two batteries that show up as OK. Once they're charged, what then? Has anyone had brand new NIMH batteries simply fail like this? Can I put all 4 batteries back into the calculator and hope to get a bit of life back into the 2 dead ones, and then maybe they'll take a charge? Am I doomed to purchase more batteries?

Any advice would be appreciated.

Thanks,
smp

[everettr]

The "failed" batteries are probably just so deeply discharged that their voltage is below the threshold for detection by the charger. One thing you can do is to very briefly (~20 seconds) connect a failed battery to a charged one, with positive (+) to positive, and negative to negative, so that the good battery charges the failed battery. Immediately afterward, place the failed battery into your charger, and it should be detected and start charging.
I don't know how this procedure affects the condition of either battery, but I have done this with batteries in a 6 cell LED closet light for at least two years, and the batteries still seem to be effective.
Be careful not to go on too long with the situation that resulted in the deeply discharged batteries. At some point, they will get a reverse polarity, and that is definitely not good for a battery.

[hth]

I had the same problem with a BC-1000 charger and NiMH N cells, just as you describe. The charger would not detect them, 2 were OK, the others dead.

I put them into my slow N cell charger (wired for lower capacity NiCd cells) for 15 hours at 15mA (charger shuts off at that time), then put them back in the BC-1000 and then they came back to life!

So they are probably just too deeply discharged and giving them some juice will bring them up again.

Now I am kicking my butt because I have thrown some probably good NiMH AA cells that had the same syndrome, believing they were gone. Apparently, they might have been good ones... argh!

Stupid stupid charger! I have never had a charger behave like this in the past.

Håkan

[smp]

Hi guys,

Thanks very much for your experience and ideas.

I dug out an old Radio Shack 15 Minute I-C3 Charger (23-039) and to my surprise I found that it is capable of charging NiMH batteries (size AA or AAA) "overnight".

The 2 batteries that showed up as NULL in my BC-700 are now charging in the Radio Shack charger. I am checking for any fault, or rising temperature of the batteries every 10 minutes, or so.

If I can bring these batteries back from the voltage being too low, then I'll be able to check them out in the BC-700 to make certain that all is well with them.

I'll post back with results when I have some.

smp

[smp]

Well, that didn't take too long. 15 minutes in the Radio Shack charger, and now the batteries are back in the BC-700 and charging successfully.

I figure that those 2 cells dropping below 0.9V is what caused the calculator to not work. IIRC, the other 2 batteries showed something like 1.3V when they went into the charger. I still cannot figure out why 2 batteries would fall off like that, while the other 2 stayed healthy.

Maybe after a couple of charges, things will even out? I don't know.

Thanks for listening.

smp