You could try something like this:
simulate this circuit – Schematic created using CircuitLab
You'd need to operate the switch to allow current to flow to your load device. When the switch is open, no current will flow to the load, when the switch is closed, the PMOS will allow current flow to the load. Notice the high value for the pull-up resistor (1M\$\Omega\$)—it really could be something lower (eg. 47k\$\Omega\$, 100k\$\Omega\$, etc), but since you're using a battery pack, you probably want to be more efficient.
The PMOS has to be capable of handling the whole load current (>12A). This MOSFET, as an example, can handle continuous current up to 18.7A. You may still have to add a heatsink to avoid overheating it.
EDIT: As suggested by @RussellMcMahon, better transistor options are available (lower RDS(on) but the approach still holds.