What Happens to NiMH Batteries When Left on a Charger?

You produce H2 and O2, pressure builds, and the cell vents.

Net result is the cell dries out, seal is compromised, lifespan reduced.

James Arthur

I guess it would make the charger more expensive if it had a built in timer.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Modern chargers have watchdog circuitry in them. They not only provide a safe charging current, but it falls to zero after the charge is considered finished. Are you sure that the batteries do not have an age problem to start with? You only get a limited number of cycles with them.

Are you saying that if the batteries are near expiry (approaching uselessness), the effect will be warm (or warmer?) batteries on the charger?

I opened up the charger. It's got a tiny transformer, one 3 terminal device (transistor?), some diodes and resistors. That's it. No uC or IC's.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

While building a fast charger (1C and above) I've had these things so hot you couldn't touch them. Now that really does them in...

Regards, Bob Monsen

I would say they are ok.

Cheers

I only measured the temperature after 2 days. I suppose there's a warm up initially and then the batteries settle to a constant dissipation. (For the type of charger I'm using.)

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Yeah.. my place gets to 37C in summer time and I wonder if the charger will still make the batteries dissipate power for a 51C temperature rise.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Yes. You can probably add a taper charging circuit, depending on what's there, or tear out what's there and replace it with a taper charger. A 317 to provide C/10 cc charge, and a TL431 to steal current once the cells reach target voltage. You can even add a green and a red led and have the green one lit while charging at the 14 hour rate, and the red one lit (with green extinguished) while charging at the taper rate. I may still have the artwork for the PCB for that, if you want it.

Ed

oops .. I'm mean 14C rise.. 51C steady temperature..

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

D from BC wrote in news: snipped-for-privacy@4ax.com:

the 3-terminal device could be a current limiter IC. like a LM317.

It's BAD to trickle charge NiMH at more than a few tens of mA (assuming AA type capacity). You'll cook out the electrolyte in short order.

Best regards, Spehro Pefhany

If the cell is heating up (and assuming the charging current has dropped to a sensibly low value) this suggests that the H2 and O2 are recombining , so you will not lose them through venting.

Depends on the overcharge rate--thus the rate of oxygen production-- and whether this exceeds the cathode's capacity to recombine it.

Discussed here at pgs. 5, 7-8, 17:

Recommended maximum trickle charge rate = C/40 (pg. 19)

Duracell's guide--

--recommends 12 hrs. at C/10 as safe, adding:

"Excessive overcharging should be avoided, as it can damage the battery."

Cheers, James Arthur

The critical part above is the "sensibly low value" of the charging current. If it is low enough, it will not produce enough H2 & O2 pressure to cause venting.

Ed

I guess those NiMHs can get warm in the sun and get warm from trickle charging.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

c/40 for a 2500mAh NimH cell is more than the reccomended C/10 for a

you can always stick a meter on it if it worries you.

I find that the 600mAh (labeled) cell when fully charged lasts about

Not meaning to one-ups you, but i replaced an AA NiCD in a solar garden with an AA NiMH cell after changing the silicon diode (solar cell reverse current protection) with a schottky to give more efficent chargeing - - thus have a similar concern.