DerbyDad03 writes:

As many have already replied, these devices will not charge a battery.

They will partially recharge a pretty flat battery.

I once left some lights on in my car. When I went to start it, it
wouldn't turn over at all. I didn't have a commercial "jump start"
battery pack, but I *did* have a charged 12 V 10 Ah gell cell sitting
around. I took that out to the car, connected it to the car battery,
and waited 10 minutes or so. At the end of that, the car started.

Now, there's no way the little 10 Ah battery could deliver enough
starting current on its own, particularly since its connection to the
car was via some rather wimpy cables. But over 10 minutes, it
transferred enough energy to the car's own battery to start the car.

You're never going to transfer all the energy in the gell cell this way,
because its voltage drops while the car battery voltage rises, and
eventually they'll reach equilibrium with a substantial amount of energy
still left in the gell cell. But what did transfer was enough.

(I have seen special charging cords that seem to contain a voltage
booster for charging one 12 V battery from another. But I just used
plain wires.)

Of course, this only works if the car's own battery will still take a
charge. If it has died, not merely been discharged, you need a second
battery big enough to start a car on its own, and jumper cables heavy
enough to carry the current needed to do that.

Dave