Charging a marine 12 volt battery from home made electricity

Thread Starter


Joined Sep 13, 2007
I have some small motors (from my sons RC cars) spinning from mini windmills and hydro power. They are generating about 2.1 volts at about 1 amp*.

Is this sufficient to "trickle" charge a 12 volt battery? Or does the voltage HAVE to be higher than 12 volts so the battery doesn't try to reverse spin my motors? If so, can I "add" the volts of the motors/generators up to make them over 12? I have read a Battery University that 14.40 is recommended for a 12 volt but I can't find anywhere that says lower than 12 will not work. IF it just takes longer, fine.



*(I say about 1 amp because I plugged my 2.1 DC volts and 1.4 ohms into a ohms law calculator on a website) Do I just attach my multimeter to the 2 leads of the motor while it's not runing to get the resistance)


Joined Apr 20, 2004
Charging a battery requires a higher voltage than the battery produces. You might be able to do some charging with several of the motors in series, but you would at least need a diode to isolate the motor string if the wind drops and the voltage out falls below that of the battery - in that case, the battery starts to supply current to the motors. A lead-acid battery consists of 6 cells, each producing 2.2 volts at full charge. The 12 volts is only a nominal figure - 6 time 2.2 is 13.2 volts. To charge the battery, a source of above 13.2 volts is necessary.


Joined Jun 13, 2007
You cannot determine the current rating using the method you describe. You need some sort of dummy load that will allow you to increase the current demand whilst monitoring the voltage.


Joined Sep 10, 2007
It sounds like you'd do better for experimentation using a Tamiya gearbox
to increase speed with less torque, but then in the long run maintenance would become an issue.