First that you need to know that you should beware of cheap dumb battery chargers (God save the Queen from buying dumb chargers!) I’ve had three (!) dumb chargers before I found out that it eventually would cost me more to buy a new pack of rechargeable batteries every each time they start to fail charging then buying ordinary batteries. I was buying different brands of rechargeable batteries hoping that the next pack will be better in terms of lifetime. Once I realized that I’m constantly literally throwing money away.

What is the problem?

The problem is that a cheap charger never knows if an individual battery is full or not and continues to charge them all together, will leads to either undercharging or what is worse to overcharging a battery that shortens it’s lifetime greatly. Also, never buy ultrafast chargers–they will kill your batteries very fast too!

On the contrary to dumb chargers, smart chargers:

• stop charging right after the batteries are full, so they do not overcharge and live much longer;
• charge each battery individually giving it exactly the power it needs;
• allow you to mix bad and good batteries in one charger;
• allow you to mix different sizes (like AA with AAA);
• have other smart functions, like discharging, testing and slow charging.

How to know if a charger is smart?

Basically if any charger’s description doesn’t contain the word ‘smart’, than you can presume that it’s a dumb charger.

Here's Bluejay's Law of Battery Chargers:

Any charger that's not described as being a smart charger, is definitely a dumb charger. Conversely, any charger that's indeed a smart charger, will definitely be described as such.

La Crosse Technology BC-500

The charger I’m using currently is La Crosse Technology BC-500 (depicted below). It’s not produced anymore and lacks some functionality of his bigger brothers, but you still can find it around as cheap as $28 with free shipping and it’s very handy charging the batteries. Bought just to prove the concept it helped me to resurrect 3 packs of 4 AA batteries, which made me extremely happy given the price of a single battery pack.

How to resurrect a dead battery?

Even if you fancy smart charger tells you that the battery is dead there is a chance to resurrect it. The simplest method I use: find a metal surface and stand the batteries (a dead battery and a good fresh one) on it, then connect positive terminals with some metal thing, like a screwdriver or a paperclip. If you don’t have a metal surface around simply stand both on a metal paperclip and then connect the positive terminals of the both batteries for a second (see the picture below). Try charging your bad battery with your smart charger after the procedure, repeat if this didn’t work. | Disclaimer: I haven’t heard of any problems, like explosions or whatever, but use this method at your own risk!

Needed assistance while making a shot, hence that fancy manicure above

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Update from the comments to this post:

@dgiors57
I have a couple of LaCrosse chargers. A year ago I got an Opus BT-C2000 charger and it's a keeper. It rejects fewer batteries in their deep discharge state.
BTW I keep one of my old "dumb" chargers around for refreshing a "dead" battery. The one I have charges a pair of batteries. 5 minutes in there won't risk over charging but gives enough juice they can be transferred to one of the smart chargers.
Thank you @dgiors57 for your valuable comment!

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Useful links:

Best battery chargers
Battery Charging FAQ
What does mAh in a battery mean
A simple step to revive a dead Ni-MH battery
La Crosse Technology BC-500
La Crosse Technology BC-700
La Crosse Technology BC-1000
Why we shall not throw batteries into a trash can

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Thanks for reading!

Sincerely yours,
@richman