Did you know that you can recharge alkaline batteries? Well, they're not perfect (don't have very good recharge capacity, few recharge cycles, etc) but you can recharge them to an extent. Many people have developed fancy charging circuits, but I was able to get charge into some Alkaline batteries with just an old cellphone plug.

The adapter says 3.7V, 300mA. With that sort of current, you shouldn't leave the batteries connected very long at a time. The voltage on the batteries will go up quickly, but the actual amount of charge will not. Because of this, you have to disconnect the batteries frequently to let them "cool down" (though they usually don't even get warm, the voltage will quickly drop down to around 1.5 V after disconnecting). After they have leveled off (usually around 1.5V) put it back on the adapter for another minute or so. Repeat this a bunch of times to get the charge up. I've just started experimenting, but I was able to get enough charge in two completely dead batteries to start up my power-hungry Canon digital camera and take a few pictures (with screen and flash) and turn it off without it dying on me (died the next time I turned it on). With a better circuit, you could automate the on/off cycles to charge them up.

Step 1: Get Your Power Source!

You'll want to find an adapter plug from something you don't care much about. Backwards powering batteries can lead to current backflow (battery power going through adapter) which can ruin some adapters, though usually at 1.5v you're safe (burned out a cheap 5v plug trying to recharge a 9v though). I found a Nokia phone charger works good.

You also don't want really high amperage. Since high current charging leads to leaking batteries, try to find as low current plug as possible (look at the mA rating). A good charger has only 65 mA or so going through the battery, but mine has 300 so that's why I have to keep cycling the power to the battery.

Step 2: Take a Stand!

You'll need something to put your charging battery on. If, by chance, it should leak, you don't want to get battery acid on anything nice. I used a LEGO baseplate to protect the floor and then used an old LEGO motor power pack (took 6 AA's) to hold the charging battery. To connect the battery to the adapter, use alligator clip wires.

Step 3: Connect!

Remember, when charging, always go + to + and - to -. Normally, a series circuit would go + to - because current flow will stay the same (+ to - to + to -, same direction through both batteries). However, since charging is the opposite of draining a battery, you obviously hook it up backwards (+ on transformer to + on battery, - on transformer to - on battery).

Most transformers are "Plus-tip" meaning the inner circle (the hole in the center of the plug) is positive and the outer ring is negative...but test it first, some are Minus-tip (opposite).

I use a small length of wire with a bent end to stick into the center hole, that way I can clip wires to both connections on the transformer plug.

Step 4: Charge!

Just wait. Yeah, that's right, stare at it. It's a battery, it's hooked up to some wires, and it's charging. Charging. Yes, just what the label says NOT to do. Well you don't care, you're staring at it...boring, isn't it? If you want, you can test the voltage going across it with an electrical tester, mine came out around 2V. Yeah, just keep staring. Touch it every once in a while, make sure it doesn't get hot. After a while (not more than a few minutes, to be safe) take it out of the charger and test the voltage it puts out using the multimeter. If the voltage is high (about 1.65 or greater) let it "cool down" (let the voltage drop back to 1.50 - 1.60V) and then put it back in for a while more...Repeat this step...and then keep repeating it.

Step 5: Try It Out!

Once you've repeated step 4 a bunch of times, take your batteries and stuff them in something useful. I used my digital camera (Canon PowerShot A520). Turn it on, and watch as it amazingly WORKS on batteries you thought were DEAD. Take a few pictures, try the zoom out. Eventually it will probably stop working, probably before the low battery light comes on. Why? Well, the batteries have high voltage but low charge. The camera usually measures voltage to determine when the battery is dead...so it probably will die without warning. This can be fixed by repeating step 4 a ridiculous huge amount of times.

Step 6: Enjoy!

Charge up some Alkalines and reuse them. Think of all the landfills you're helping to keep battery-acid-free and all the money you save from the big companies. Think about the price difference between those fancy NiMH 15 minute rechargeables and your $4 Alkalines that can be recharged maybe 25 times. If nothing else, think of your accomplishment as an experiment and a proof of concept.