Pulling power from alternator to drive DC motor

Thread Starter


Joined Oct 17, 2011
Please excuse me if this has been asked before. I wasn't able to find a post on it.

Lets say that my alternator can delivery 12v at 20Amp. Can I connect it directly to my DC motor controller, or do I have to connect the alternator to a battery as well?


Joined Jul 18, 2013
If a standard alternator you will most likely need 12v to initially supply the field, as they do not usually pull up using the residual field as a DC generator normally does.
It could be removed once generating.


Joined Sep 22, 2013
Does the alternator supply power to anything else? Does the prime mover of the alternator vary RPM?

Is an auto type voltage regulator being used?

If you desire stable, long term operation, use a battery. It doesn't need to be large capacity.


Joined Jul 18, 2013
The advantage of an alternator, and one reason it replaced the old DC version, it is capable of generating down to very low rpm's.
Try it by removing the battery after generation started, remember to keep the alternator output connected to the regulator.


Joined Jun 19, 2017
A 12 Volt 20 Amp Alternator is probably NOT an automotive style alternator
What is the Model # and URL Link to the alternator's web page?

Is it a PM - Permanent Magnet - type alternator ?
If it is a PM Alternator then no it does not need to be connected to a battery.

What type of Voltage Regulator are you using?
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Joined Nov 18, 2012
More information needed but in general, you would need a battery. It acts like a stabilizer for the system and keeps the voltage and voltage spikes in check. Without it, alternators can generate very high voltages and produce massive voltage spikes. If you are using a battery and you disconnect it from the system, same deal. Use a battery and all will be good.


Joined Sep 24, 2015
Four days later and we've not heard back from @boonxiong. Are you still with us on this? If this is about powering an alternator from a motor powered by THAT alternator - this is called an Over Unity device; a topic which is not in line with community guidelines, and will not be discussed further, if that's the case. However, if you're more focused on the motor controller being used to power an alternator with the purpose of deriving a different voltage than that of what's driving it - then you're building a dynamo. The only dynamo I'm familiar with used a 12 volt motor and power source to spin an alternator that produced 120 VAC. THAT we can discuss.


Joined Sep 24, 2015
Hello and welcome to @Skipperdan. The way to tag someone is to use the @ symbol and their screen name. Doing so will notify him that you tagged him in a message. As far as contacting him directly, I'll leave that up to him to discuss with you. But it's not difficult.


Joined Jan 23, 2018
A 20 amp alternator could be on a garden tractor or on an outboard motor on a boat. It could even be on a motorcycle. But it would probably not be on a normal car. Possibly on an open-wheel racer, but I think a lot of them use magnetos . A hoist on a fishing boat would be one place that makes perfect sense.