low battery won't charge while connected to car, will charge disconnected

Advice needed, keep in mind I'm a total electrical idiot.

Had blinking dash/clicking that seems to indicate low battery, put it on the charger and it's too low like 11a, and the charger won't put out a current when the battery leads are attached but when I disconnect them it will take a charge. Like I say, I'm an idiot so I don't know what that means. Someone tell me!

2 things come to mind. Some chargers won't put out a charge if the battery is too flat. This can be gotten around by connecting another battery in line with it till it charges some. Some chargers won't charge if things are shorted. Make sure they aren't any where.

Battery chargers, unless they are smart, will not charge if the battery is stone dead because it needs a "connection" to let the charger know its hooked up. If a battery is that dead, I have to turn on the headlights to force amperage thru the battery to get it started.

Or maybe the top of the cables you're clipping the charger to are so dirty and corroded they need to be cleaned so a good connection can be made through them to the battery.

+1 on the bench battery charger needing to "see" voltage before engaging.

A good working order battery CONNECTED IN PARALLELL with the dead battery and the bench charger will make this trigger voltage visible to the bench charger.

As long as the dead battery is not too sulfated it may recover.


Good luck

yup. you probably need to introduce a second battery in the system.. preferrably one thats not stone dead.

im also an electrical idiot... parallel and series can get confusing...

hook the positive of your charger to the positive of a good battery, then with a second jumper cable, hook the positive to the good battery to the positive of your dead battery.


same thing with the negatives. and it should trick your charger and give the dead battery a charge.

A lot of the smart chargers will pulse until the voltage comes up and then kick in . . . I can't tell you which, but on mine, if I wait it out, that can work also. That, or just go get Dad's old "dumb as a boot" charger, and bring it up to the point the other will kick in.

As far as not charging in the car, that's simple: The battery is so dead that even the nominal loads of the non-running car pull it down. With no load, it can float high enough to let the charger start. Charge for a while with the cables off (if this still let's the charger start) and then reconnect, and you should be good to go.

(And I suspect that the goal here is to not be able to have the charger start a 12V charge on, say, a 6V battery and perhaps have a fire . . . )

- Tim

Was the car sitting for a while? remember if you leave a battery sitting for a few weeks or months it will self discharge and sulphate. Which in turn decreases the batteries health if don't repeatedly. That's why you always but batteries on a charger over the winter months if unused.

If your really not that good with electrical, you might want to bring the car to someone that knows these vehicles for assistance. Or it might be cheaper to just buy a new battery and go from there.

Have any friends that are good with a volt meter and google?