Revival/ charging of rechargeable batteries (AA/AAA)

[Pale Rider]

I had a similar problem with a dead flat battery on my car.

The smart charger I bought would not work because it reads the voltage of the battery which in this case was too low for the charger to kick in.

The dumb charger I bought next did the job.

I'm bound to observe that if smart chargers were truly smart, they would be able to switch to a dumb mode when attached to a fully discharged battery.

 

[palinurus]

There's a (dumb) charger at work that we use for the digital camera batteries, now if I discharge mine too deeply I give them a headstart in that one.

 

[ufkacbln]

If you have a "Smart charger" and use the dumb charger trick it is worth using the reconditioning cycle when you put them back in the smart charger

 

[andrew_s]

I use a Maha MH-C9000 charger that's got a break in mode for new or very flat batteries - you tell it that it's meant to be a 2500 mAh cell, and it puts 250 mA in for 15 hours (or something like that).

Having said that, batteries aren't always recoverable when they die.
Sometimes the internal resistance goes right up so you need 2.5V+ to put anything in, rather than the more normal 1.4. Most chargers, including mine, will give up and show High, or flash a red light at you, and refuse at that point.
Also, sometimes you'll get batteries that superficially charge OK, but then self-discharge themselves back to flat in only 2 or 3 days.

 

[Wobblers]

I recently bought a new smart charger because it had a better display and I wanted to see what capacity was left in some of my batteries, but it is very sniffy about ones that have been discharged heavily in a low-drain device like a clock. My older tri-mode NiCd/NiMH/Alkaline charger manages to recharge them and doesn't seem to cause leaks or anything, but it does mean I still don't know what capacity is left. A review is http://www.geekalerts.com/battery-wizard-alkaline-recharger/ but I don't know if it or a later model is on sale anywhere.

Most smart chargers actually start by testing the call's impedance when you put it in the charger: this is to ensure that it doesn't try to charge non-rechargeable battery if that's put in by mistake. Which is quite a good idea actually, because alkaline cells have been known to explode if charged... And the potassium hydroxide electrolyte isn't vey nice to boot. The problem is that a very flat cell won't have enough charge left to pass the impedance test, so will look like a non-rechargeable battery to the charger. The trick, as has already been said, is to charge it for a few minutes (you don't need long) on a dumb charger, or with a fully charged battery of the same voltage - just make sure to connect positive to positive and negative to negative.

 

[mjr]

this is to ensure that it doesn't try to charge non-rechargeable battery if that's put in by mistake. Which is quite a good idea actually, because alkaline cells have been known to explode if charged... And the potassium hydroxide electrolyte isn't vey nice to boot.

Which is presumably why the Battery Wizard still charges them despite being a smart charger, as it can charge alkalines too. I've not had any explode (does that really happen unless you're beastly with them?) but I have had enough leak an hour or two after charging to stop bothering with them and switch mainly to eneloops.

I did try one of the "fail" batteries in the newer smart charger again after a few minutes in the wizard. It registered a total capacity of about 140mAh, compared to the 900mAh on its label. So I guess that's pretty much past it anyway!