I don't think it needs to be very complicated, even if you want to implement a full battery protection system.
A 1R resistor should limit the current to a safe level and if you're worried about it being plugged in for too long then you could use a CD4060 oscillator/counter IC as a timer to cut off the power, if it's left connected for too long.
If you're worried about over-discharge a comparator and voltage reference could cut the power when the voltage drops below 10.5V, or higher. If you're clever then you could design it to reset the timer when the voltage drops below 12.5V (or even higher) so it doesn't have to go through a full cycle. You'll need some time delay for the comparators as you don't want current surges to trip the protection but that's as simply as adding a capacitor to a potential divider to form an RC circuit. A PTC resistor (polyfuse) could be used as over current protection rather than a fuse which needs replacing every time it's tripped.