I see three risks of connecting the jumper battery via jumper cables to another battery.Guest wrote:Sleep_quest, I think AA answered most of your questions so I would like to address the 2nd AA who appears to have contradicted himself.
The jump starter will provide a far larger current when jumping a car than it would to (connected the very same way) charge (with car already running) from the car to the jumper. So they are more than capable of handling any charge current.AlabamaAl wrote:I would not attempt to charge a Jump Starter using the Jump Starter Cables. First, the Jump starter cables are not designed to work that way, they are set up to provide a large immediate amp output to Start Cars, not charge its own internal Batteries. I would suspect that the electronics actually are set up to prevent rapid charging in this manner.
1) Too rapid charge. If the other battery has a full charge and the jumper battery is run down, it might provide too high a current into the jumper battery. While the jumper battery is designed to provide a high current for a short time, it might overheat if you hook it up to a larger battery that can provide a really heavy current for a long time. It might not be able to dissipate the large amount of heat generated quickly enough.
2) Too high or low of a voltage. The standard lead acid battery may not have the right voltage to charge the jumper battery. I think the voltage on a "resting" lead acid battery may not be the right voltage to fully charge the gel cell. The likely result is to not put a good charge on the jumper.
Both of the above don't worry me too much.
3) Shorted cell in the jumper battery. Sometimes when a jumper battery dies, it will die with one of the cells shorted. You now have a 10V battery instead of a 12V battery. The car battery might deliver a really high current into this partially shorted battery and cause a fire or other damage. Charging circuits are usually designed to limit charging current. You don't have this limit with a direct battery to battery connection.