The battery brain is a very simple device and can likely save you from having to whore yourself out for a jumpstart somewhere in the ghetto. You know, because cars only refuse to start that one time a year that you have to make a late night trip into the shady part of town.
The Battery Brain stays attached to the positive terminal of your battery. It will continuely monitor the voltage of the battery and if it happens to drop below the level required for the engine to start the Battery Brain will kick off the battery and save the juice to get the car started again. Prices for this unit begins at $40 and goes upwards of $70 for the RV unit.