Unorthodox LiCad battery charging

[anythingyoucanimagine]

I don't know a darn thing about NiCad batteries. Sticker on the back says NiCad 1000mAh, 7.4V, 7.4Wh. It's from a marine VHF radio. This is important to me, I f'd up and forgot the wall-wart & charging cradle at home and need to charge this in a hotel tonight.

I have car with cigarette lighter, factory 12V plug & cord. I can use a paper clip and tape to figure out the wiring (battery is marked +/- on charging terminals) and I understand that 12V is same 12V whether 300 amps or .00003 amps. Just not sure how long to charge the battery without ruining it. (replacement battery is $20 so all I need it two days' worth of use from it and I'll buy new if needed).

Thanks.

How long do I apply 12V to bring a NiCad battery up to full/medium charge? Thanks.

 

[MikeF2316]

You need some way to limit current. You have 1 Ah, and 7.4Volts, therefore 7.4 watt-hours. What else you need to know is how long it takes to charge it normally. For example, let's say 3 hours. So if you charged at 7.4 volts and 1/3 amp for 3 hours, your battery will be fully charged. But your supply is 12 volts, you will need a resistor to drop the 4.6 extra volts. R=V/I = 4.6/(1/3) = 13.8 ohms. Power = V * I = 4.6*(1/3) = 1.53 Watts.

So you could put a 15 ohm 2 watt resistor in series with your battery on a 12 volt source. What you could use for a resistor is an 1156 automotive bulb, that would be in the ballpark.

I can't stress how silly this is. Watch the battery, it shouldn't get warm until it's nearing full charge. Seriously, don't leave the car while it's being charged. And don't even consider doing it without a voltage dropping resistor.

 

[anythingyoucanimagine]

You need some way to limit current. You have 1 Ah, and 7.4Volts, therefore 7.4 watt-hours. What else you need to know is how long it takes to charge it normally. For example, let's say 3 hours. So if you charged at 7.4 volts and 1/3 amp for 3 hours, your battery will be fully charged. But your supply is 12 volts, you will need a resistor to drop the 4.6 extra volts. R=V/I = 4.6/(1/3) = 13.8 ohms. Power = V * I = 4.6*(1/3) = 1.53 Watts.

So you could put a 15 ohm 2 watt resistor in series with your battery on a 12 volt source. What you could use for a resistor is an 1156 automotive bulb, that would be in the ballpark.

I can't stress how silly this is. Watch the battery, it shouldn't get warm until it's nearing full charge. Seriously, don't leave the car while it's being charged. And don't even consider doing it without a voltage dropping resistor.

Thank you kind sir. If you ever make it anywhere near Boston or NYC beers are on me. I appreciate your help very much.

edit: wanted to confirm that the auto bulb (from 24-hour Walmart) worked perfectly. Thanks again.

 

[MikeF2316]

I'm glad I could and did help. Also nice to hear the feedback, especially when it's positive!

I've charged the occasional battery this way, and let me tell you, if you forget about it, there's a lump of melted goo when you get back.

 

[PhysicsDude]

Using an automotive bulb as a current limiting resistor is very clever. Very cool to see it all worked out.