Try replacing the bad battery with a capacitor and resistor in series of just enough resistance to allow the motor to turn, In parallel to the resistor you want to put a diode blocking the direction of flow of current but allowing the back-spikes to bypass the resistor. The diode need to be very low forward voltage and fast switching. The output load wants to be matched so that the voltage drop on the source is not too much. an additional diode opposing the current in parallel to this load will help too.

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