Can a jump starter charge a battery?

[Sodo]

The jump starter has a built-in 12v battery and its own built-in battery charger. So, why wouldn't the built-in charger also charge an external battery when its all connected and plugged into the mains power?

You can test this if your car battery is below 12.5v (slightly rundown). 12.4v is a good number, that's about 20% down, and probably won't cause problems.

1) Charge up the jumpstarter, so it's "full".
2) Plug jumpstarter into the 120v wall outlet
3) Attach the jumpstarter clamps to your car battery.
4) Measure the voltage at the car battery. If it's about 1 volt higher, (13.4v) it will charge the car battery. If 0 volt higher there is no charging. If .5v higher, it is charging but durn slow.
5) Leave it hooked up and after awhile, if the car battery gets to 12.7v it's been charging (both) batteries.

It's more complicated than this actually, but to keep it simple this will tell you something. I wouldn't do the test if the subject battery is below 12.4v.

So, temporarily disconnecting the internal battery might make it suitable as a slow charger?

Much better to run the car for awhile, your car probably has an output of 90 amps, which is a serious charger. This jumpstarter, I would be quite surprised if it's charges at more than 3 amps, it's made to charge its internal battery overnite. Your car alternator will recharge a bad battery maybe 20-30 times faster than the jumpstarter. These are pretty wild guesses, but I don't think your jumpstarter will be a "suitable" charger (in comparison to the car's alternator).

 

[PILOON]

Old fashioned chargers that had a set of diodes and put out constant voltage and amperage will charge a dead battery.
New intelligent transistorized 'intelligent' ones just won't as they read zero and don't know what to do. (not all that intelligent)
Clamp 2 batteries in parallel and the hot one will then add some charge to the dead one, disconnect and charge the formerly dead with your 'intelligent' and you will be good to go.

I have an 18 volt transformer with one hefty diode and that revives many dead batteries to then add to an intelligent charger.

That being said I had one battery with the deadest cell that I simply trashed as no matter what I tried I could not revive that one cell.

 

[bf109v7]

Let's you connect it to your tractor battery and than try to strart the tractor, it will burn the electronics in your charger pack.
Alex

 

[herrydas]

When car battery is dead using jump start, we start the car. Jump starter is not a battery charger, A jump starter can be used anywhere only when battery is dead.

 

[joefromga]

A dedicated battery charger tends to be relatively heavy. I assume its because it contains a hefty heat sink. And also it contains the electronics to convert AD to DC.

Conversely the electronics in a jump starter are quite light. Is this because they are very slow chargers and dont require much of a heat sink? And they don't contain an internal AC/DC convertor.

New ones are lighter because they use mosfets and transistors instead of transformers. They are better but more complicated unless your battery is flat dead.

 

[joefromga]

I have two jump starter boxes and neither will arc when you touch the alligator clips together but will if you connect them to a set of jumper cables.

They also will no start my tractor. Just clicks. There must be some circuitry in there that makes them useful/useless.

I should have just bought a battery and an extra set of cables.