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If low cost is the goal, consider a voltage measurement device. ICE engines have electrical systems that run ear 13V when the engine is on, and ~12.5-12.8V when the engine is off

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That would require plugging into the wiring. At that point you no longer need a battery and can just use the car's power.

I'd be wary of draining the battery while the car is off. You don't want to prevent the car from starting

The ~10 or 20mA or so one of these things draws would take months to do that.

Untrue. Imagine a cute convertible car or sportbike in the snow country.

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