I have a 12 V DC with a gearbox motor that I want to use as a generator to charge a 12 V battery, how do I do this?

If I connect the motor directly to the battery, the motor starts to spin, how do I prevent this from happening?

The battery is a 12 V Li-ion battery pack with a BMS attached. The idea is to connect the motor in a gas powered motor that I am putting on a kart. The battery is for powering lights and other low power stuff. The BMS does not have a input power, only two terminals a positive and a negative.

  • 4
    \$\begingroup\$ It depends. What kind of "12 V motor"? Is it a permanent magnet motor (with a commutator)? Or a BLDC motor? Or some other kind of motor? \$\endgroup\$ Jan 6 at 15:29
  • 3
    \$\begingroup\$ Does the BMS have a charge input? That would seem like a good place to start. \$\endgroup\$
    – TimWescott
    Jan 6 at 15:33
  • \$\begingroup\$ why do you need lights when the gas motor is stopped? \$\endgroup\$
    – jsotola
    Jan 6 at 16:31

2 Answers 2


Lithium batteries are sensitive to being overcharged, so it's best to have a proper charging circuit. Your motor will generate a voltage in proportion to the engine speed, so you'd benefit from a charging circuit that can handle a wide range of input voltages or a switching voltage regulator to convert the motor voltage to the appropriate range for the charger.

The traditional way to charge a car battery from the engine is to use a dynamo that has a field winding, and modulate the current to the field winding in response to the dynamo voltage and current. This was done entirely with relays and a power resistor (search Lucas mechanical regulator), but lead acid car batteries are more tolerant to that kind of thing. Later came alternators, which generate alternating current which is rectified and regulated electronically.

  • \$\begingroup\$ Do you have a specific charging circuit in mind? I'm kind of new in the area, if tou could post a link or something I would appreciate. Thank you. \$\endgroup\$ Jan 6 at 16:02
  • \$\begingroup\$ You can design a charger around a lithium battery charging IC, but you probably want a pre-built module. I don't have any particular recommendations. \$\endgroup\$
    – William
    Jan 6 at 16:19
  • \$\begingroup\$ @LorenzoLoriniKalil you can Google lithium-ion battery chargers and find many circuit boards which fulfil this function. The purpose is to prevent charging too much or too fast; although it is possible to watch the voltage and current yourself with your multimeter, one day you'll screw up and hurt the battery, and it's also quite boring. Much better to make a circuit do it. \$\endgroup\$
    – user253751
    Jan 6 at 18:40

Assuming that you are using a DC motor... Use a diode oriented in such a way as to prevent the battery from powering the motor, but which will allow current to flow from the motor/generator to the battery.

That is to say, the cathode should be closest to (the positive terminal of) the BMS, and the anode on the generator side:


(Image source: Proto-PIC - Diode Symbol)

This is prevent current from the battery from going to the motor, whilst allowing current from the generator to pass to the battery.

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    \$\begingroup\$ OP, DO NOT DO THIS! It will harm the Li-ion battery and may damage the motor. There MUST be some form of current limiting between a voltage source and a Li-ion battery. \$\endgroup\$ Jan 6 at 15:31
  • \$\begingroup\$ The OP states that there is a BMS attached to the battery - I've corrected my wording - that should perform the current limiting, should it not? \$\endgroup\$ Jan 6 at 15:39
  • \$\begingroup\$ I was under the impression the BMS balanced the cells but didn't protect against overcharging or charging too quickly. \$\endgroup\$
    – William
    Jan 6 at 15:42
  • \$\begingroup\$ The BMS has voltage protection and regulation, witch diode I should use, can it be any? \$\endgroup\$ Jan 6 at 17:02
  • \$\begingroup\$ The BMS voltage protection/regulation is good but does it also have current limiting capabilities? If not, then extra components may be required. Any rectifier diode would suffice, e.g. IN4001 through to IN4007 should be ok. Does the BMS have a charge input? If so, then use that, and the diode may not be required in that case - depending upon its internal circuitry. Please can you edit your question and update it with specific details about the motor (what type is it?) and the BMS (which connections does it have?). Otherwise it is rather difficult to give you are precise and safe solution. \$\endgroup\$ Jan 6 at 17:07

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