How to make a super capacitor jump starter that doesn't empty into battery?
My previous answer spawned quite a discussion afterwards, so I decided to add another answer here after a number of clarifications.
If modifications to existing vehicle circuitry are allowed, it seems the goal can be accomplished by putting a high-current SPDT &quot;selector&quot; before the car starter, like



simulate this circuit &ndash; Schematic created using CircuitLab
where R-(misc) is just the usual load when the car is off, often comprised of things like the clock, or radio, or even the &quot;ECU&quot; in some cars (which also get &quot;reset&quot; when the battery is disconnected). Since R-(misc) is always connected in the above circuit, resetting said components should never be required.
So, in the above circuit, once the capacitor jump-starter is charged, it's connected to the &quot;free&quot; (or unused) throw of the selector and the system ground, and now the vehicle may be started once the ignition is pressed -- without having the jump starter discharge anything into the depleted battery.