Using a 12v charger on a 6v battery?

[jipjob]

Sent: 4/22/2004

I have a simple question? What will happen to a battery if you have a six volt battery (car) and you charge it with a 12 volt? Need a short answer. Jon

[denisondc]

Sent: 4/22/2004

Are you talking about charging a single 6V automobile battery, or two of them connected in series?
Unless your charger has a 6V-12V switch, the most likely thing is that you would fry the charger, using it to charge a single 6V battery. Look for smoke.
If its a small charger and isn't putting in too much current, perhaps 20 amps or less, it wont hurt the battery at first. BUT when the 6V battery was brought up to full charge, the charger would then go on trying to overcharge it. Not good.
If its a heavy duty charger, it would either blow a fuse, burn up inside, or it could overheat the battery and cause it to go bad.
You could wire a headlamp bulb in series with the battery, this would limit the current to a safe level, but you would need to leave it on the charger longer. Overnight maybe if the headlamp you used was from a 12V vehicle.

[jipjob]

Sent: 4/22/2004

No the reason I'm asking this question is that not only do I own a 74 Winne but I happen to be the proud owner of a 1942 Dodge. But my only battery charger is a 12 volt and dumb old me didn't get one with a switch to run it on 6 volts. I have a trickle charger 6 volts but that takes forever to get that old battery to charge up. You said I could maybe get away with using the 12  volt charger for a short time? It does have a feature that only uses 2 amps but still thats at 12 volts. I hate to spend another 20 or to dollars on another charger just to get use, what is simple lay-man term can I do? This sound kind of redundant to your answer but I just want to make sure.

[Oz]

Sent: 4/22/2004

I did the same thing, bro.  Didn't think I'd have any need for a charger w/6v at the time.  I broke down and bought one.  Better spending the extra money on the right stuff than likely needing to spend more later to replace a messed up component because I didn't. - Sob

[denisondc]

Sent: 4/22/2004

You could put a low resistance in series with the 6 volt battery when you connect it to the charger.
A typical 12v headlamp bulb would probably have about 3 ohms of resistance when lit - less when the filament is cold. If you connect the positive lead of your 12 volt charger to one side of the headlamp bulb, and connect the other lead of that headlamp to the positive side of the battery, then connect the negative lead of the charger to the negative terminal on the battery - you would giving the 6 volt battery a gentle charge. Perhaps 4 to 6 amps, depending on the rating of your charger. This would take overnight to charge up the 6 Volt battery, and if it were well discharged, up to a day I suppose. It would not hurt the charger, or the battery. If you want to cut the time to charge up the battery in half, you would connect 2 headlamp bulbs in parallel. The headlamps bulbs might not illuminate very bright, but they would be plenty hot. This is the scheme I used to charge up the battery in my 6 volt vehicles.

Assuming you had a 6 volt battery and a 12 volt battery the same size, and both were discharged halfway, then putting 8 amps into the 12 volt battery will charge it up in the same time that putting 16 amps into the 6 volt battery would do. It takes twice as many amps to put the equal number of watt hours into a 6 volt battery, compared with a 12 volt.

I think a fully charged 6 volt battery should read about 6.4 resting volts, I'm not sure.
Connecting the 12 volt charger onto the 6 volt battery might quickly create a need for a completely new battery charger.