Just a mention:

If the primary concern is being stuck with a dead battery in a single vehicle, one can add a second battery under the hood (best to have a bracket welded in place with a hold-down clamp). The second battery can be connected to the electrical system with a single diode (Digikey has the MBR4045WT-ND 40 amp diode with only a .4V drop for about 2 bucks, it will need some heatsinking). When the primary battery dies, you've got a "hot spare" for roughly the cost of the battery, bracket, and diode.

No light, no airpump, no inverter, and you can't move it between vehicles though.