They're not cheap, but Antigravity batteries (1) implement that. It monitors battery voltage and disconnects when it reaches a given level. To start the car y

They're not cheap, but Antigravity batteries (1) implement that. It monitors battery voltage and disconnects when it reaches a given level. To start the car you just press a button on a fob to internally connect to the reserve, and off you go. Doubly useful for Lithium batteries since they really don't like being over-discharged. 1: https://antigravitybatteries.com/ https://antigravitybatteries.com/