This is my first question here so if I have done something wrong question-wise iI would like to know.

I am building a mini tractor with a small gasoline engine. I want to use a brushless DC motor from a drill as a generator. I don't have the motor yet, so I don't know the specs.

First I want to know how many volts you can expect to be generated at around 2000rpm. I looked into this and it seems I need to know the Kv (rpm/V I believe). Maybe someone has an indication of what this number is for a 500W drill?

I then have a second question. When I start the engine, the motor will start producing power. If I then connect a buck/boost converter based on the XL4015 and set the constant voltage to 13.8V, would the motor be able to charge a 12V lead-acid battery connected to the buck/boost converter, and how can I connect a second load which has priority over charging the battery to the buck/boost converter?

In the first image attached I show the set-up. In the second image I show how I think the second load should be added.

Is it true in the second picture that if I shut off the engine, the battery will stop charging and will supply power to the Arduino?

enter image description here.

enter image description here

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    \$\begingroup\$ Why not simply use a car alternator? These are specifically designed to charge a 12V battery, with all the electronics built in. You can get new ones <$40 ...and a trip to the wreckers would get one for <$20. \$\endgroup\$ Commented Nov 27, 2021 at 16:09
  • \$\begingroup\$ Thanks for the response! I did look into them, but I couldn't find such a cheap alternator, could you maybe post a link? The ones i found started at 110 euros (130$) and were quite big. Also suppose the first buck converter and brushless dc motor were replaced by an alternator, would the battery charge to the max and would the arduino recieve power? And would the battery take over to power the arduino without trying to supply current to the alternator? \$\endgroup\$ Commented Nov 27, 2021 at 16:24
  • \$\begingroup\$ The small frame alternators are the cheapest here in the US ...YMMV in EU. We should not give shopping answers on this site, but here is a single link that might help you hone your search: amazon.com/… \$\endgroup\$ Commented Nov 27, 2021 at 16:45
  • \$\begingroup\$ Thanks! Do you maybe have the answer to my other question in my first reply? Because you can use an alternator to start an engine right? \$\endgroup\$ Commented Nov 27, 2021 at 16:53
  • \$\begingroup\$ Look at: electronics.stackexchange.com/questions/290871/… \$\endgroup\$
    – user80875
    Commented Nov 27, 2021 at 16:56

2 Answers 2


A drill motor (brushless DC or otherwise) is designed for intermittent use. It will not last very long doing duty as an alternator: its bearings are not compatible with using a pulley due to the high radial load (drills use planetary gears), and it will tend to overheat.

At any rate, besides being mechanically and electrically robust enough for the task, a repurposed permanent-magnet brushless DC motor will also need:

  • a 3-phase rectifier
  • a voltage regulator

to complete the system. That’s a lot of hassle for a solution that isn’t going to last, and if not done properly, will kill your battery.

I can’t stress this enough: a regulator is absolutely essential to prevent overcharging the battery. Lead-acid batteries are very finicky about this. More about lead-acid charging here: https://batteryuniversity.com/article/bu-403-charging-lead-acid

Consider instead repurposing an alternator from a motorcycle, ATV or lawn/garden tractor, and also use the rectifier/regulator that comes with it. This will be a reliable solution that will take care of your battery, sized appropriately for a 9HP (~300cc) engine.

As for the battery connection, most systems wire the battery in parallel with the load. The regulator sets the max charging voltage (13.8V or so for a ‘12V’ battery), above which the regulator will taper off its current output.

Finally, the vast majority of alternators don’t have enough torque to be run as starters when using the pulley ratio they need to work as alternators. While it’s possible to make an alternator work as a motor (and some small engines like scooters do exactly that), the whole thing needs to be designed as a system, and likely needs a compression release for that big of an engine.


In the automotive world, it is common to connect the alternator, battery and loads all in parallel. If the alternator can supply more current than the load requires, the excess current will charge the battery. If the load demands more current than the alternator can supply, the battery will supply some current to make up the deficit.

The system will switch between charging and discharging the battery automatically as the load demand and alternator output vary.

  • \$\begingroup\$ Thanks! I think I can figure the rest out now! I want to tha l everybody for the good and quick answers \$\endgroup\$ Commented Nov 27, 2021 at 17:16
  • \$\begingroup\$ I have one final question, how do you switch between starting the engine with the alternator and using it as a charger? \$\endgroup\$ Commented Nov 27, 2021 at 17:22
  • \$\begingroup\$ I think you will need considerable electronics, and modifications to the alternator, to use it as a motor. \$\endgroup\$ Commented Dec 4, 2021 at 7:32

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