I want to charge a 12v lead acid battery with a dc motor used on the Power Core E100 rated at 24v 100w. I’m spinning the motor with a bike so the output voltage fluctuates which I assume isn’t good for charging lead-acid batteries. I've seen elsewhere that I also need to limit the current to 10-30% of the capacity of the battery, so what components would I need in between the motor and the battery to safely charge it?

  • 2
    \$\begingroup\$ Fluctuating voltage is actually okay. The battery will stabilize the voltage and the current will fluctuate instead, which is fine. To limit the current, just measure the current, and if it gets too high, pedal slower. \$\endgroup\$ Commented Jul 11, 2023 at 5:24
  • \$\begingroup\$ There are small circuits designed to take the low output of pv panels or small wind turbines that "collect" the low output and store it until there is sufficient to send to the battery to charge it. You can search for it. \$\endgroup\$
    – Solar Mike
    Commented Jul 11, 2023 at 8:24
  • \$\begingroup\$ 10 months old. Clear requirement. 4 answers. 2 close votes. Not closing. \$\endgroup\$
    – Russell McMahon
    Commented May 11 at 1:56

4 Answers 4


There many ways how to limit the current.

  1. Simplest is to use a series resistor, but it is not efficient because it will waist the power at motor low speed when the power can be used for charging.

  2. The second option is transistor/linear current limiter. Certainly better than 1) but only sometime better than 3)
    (when the motor voltage is too low for charging, not your case)
    (or when the motor voltage is far away from maximum power peak, can be your case)

  3. The most efficient is buck-boost converter with current limiting feature. There are many modules of these a can be buy for few dollars.

Btw, when the motor generates 4A and you want to charge with 2A the another 2A are waisted, so the converter does not provide the advantage. It is the same efficient as transistor limiter.
Is you want to make it perfect efficient you must to deal with MPPT regulator with current limiter together.

  • \$\begingroup\$ I plan on going with number three, I would use a buck converter since my battery is lower voltage than my power source right? I also need to get a buck converter with a correct wattage rating, since the charging could draw a lot of power, right? \$\endgroup\$ Commented Jul 11, 2023 at 1:58
  • \$\begingroup\$ You are right. Motor is putting approx. 18-26V on the output that is too far away from 12V on battery. Get the converter with amperage according to max battery charging current. \$\endgroup\$ Commented Jul 11, 2023 at 2:11

Battery charging is much more complex than that

A battery charger is a thing you should never homebrew - that's the surest way of destroying a battery.

Several billion dollars of engineering have gone into determining how exactly to charge a lead-acid battery successfully. Proper, modern 3-stage charge controllers are built into lots of things. You are better off getting such a controller and put it in front of your generator. A solar charge controller might be a good choice.

Another option is to not hack that motor at all, and go with a different thing - an automotive alternator, which is specifically designed for that One Job and has the correct battery charge controller built-in. A "one wire" alternator would be ideal but any GM or Ford alternator isn't particularly difficult to adapt. Your only real design challenge is getting it spinning at an appropriate speed.

  • \$\begingroup\$ I considered using an alternator, but I want to try using what I have. \$\endgroup\$ Commented Jul 11, 2023 at 19:49

Fluctuating voltage is okay. You should have a diode in the circuit to prevent the battery from making the motor spin when you don't pedal. When the voltage is below the battery voltage, no current flows. When it's above, the battery drags it down to the battery voltage. Components in parallel have the same voltage - it's not possible to have for example 14V across the battery and 24V across the motor at the same time.

In your battery's data sheet there should be a chart showing the current and voltage in a typical charge cycle, like this one. You notice two phases: first the current is limited and the voltage increases, then the voltage hits a limit and we keep it there and let the current decrease. So basically, both the current and the voltage have to be at their limits or below. Whichever one is at the limit, the battery chooses the other one.

So you just watch the voltage and current, and if either one goes too high, you pedal slower. Eventually you stop pedalling and they are fully charged.

If you have a multimeter, you don't need a fancy circuit for that. Watch the current mostly, and every now and again, check the voltage. When the voltage is high enough, watch the voltage all the time. It sounds a bit tedious, so if you want to build a circuit, you could build one that turns on an LED when either the voltage or the current is too high.

If your battery is big enough, then you don't need to measure the current at all because you know that if you pedal as hard as you can you're still not overloading it on current.

Lead-acid batteries are quite resilient - nothing like those lithium-polymer batteries that like to swell up and then explode in jets of fire. Under almost all conditions the worst thing that's likely to happen is you reduce the battery's lifespan or capacity (in extreme cases, all the way to zero).

You will see discussion about the best ways to charge them. This is all about maximizing the lifetime so you get 3 years of working life instead of 1. If you don't really care about the lifetime so much, you can go nuts.

There is one danger to be aware of: charging (especially overcharging) can generate hydrogen gas, which has a risk of explosion if it builds up to a certain concentration in the air. Therefore, you should charge your battery in a ventilated area where any hydrogen will disperse instead of building up. These hydrogen explosions are rare, but they have happened before.

Apart from that, enjoy your experiment.

  • \$\begingroup\$ If I put a buck-boost converter in between the generator and battery to set a voltage and limit current, will I still need the diode? \$\endgroup\$ Commented Jul 15, 2023 at 1:26
  • \$\begingroup\$ @AnonymousHamster the converter won't work in both directions, unless it's a special kind that does. Some converters might break if there is power at the output and no power at the input; if yours is like this, and there's no way to really know without trying it and seeing whether it breaks, then you want a diode after the converter. \$\endgroup\$ Commented Jul 15, 2023 at 16:38

Just because the motor is designed for DC input doesn't mean that if you run it as a dynamo you'll necessarily get DC output. Unless it's a brushed motor, you may need to rectify the output (with a diode), smooth it (with a capacitor) and limit the voltage to an appropriate voltage for charging your battery (lead acids should not usually be charged at more than 2.3v per cell when charging at standard temperature, and your 12v battery will have 6 cells, so you should not provide it more than 13.8v). This is the most complicated part of your circuit, but you could potentially use an off-the-shelf voltage regulator IC for it. Limiting the current can be achieved with a suitably calculated resistor, assuming you don't mind wasting a little power.

Alternatively, you could look into ICs specific to the application, e.g. a MAX17702.

  • 1
    \$\begingroup\$ The OP doesn't specify whether the motor is brushed or brushless. However, a brushed DC motor will output a DC voltage if the rotor is turned in a single direction. \$\endgroup\$ Commented Jul 10, 2023 at 21:11
  • \$\begingroup\$ It is a brushed dc motor, and I connected a multimeter and got 24 volts output, but that is with pedaling very hard, so I pedal moderately to get about 15 or so volts. \$\endgroup\$ Commented Jul 10, 2023 at 21:53
  • \$\begingroup\$ 12 V lead-acid batteries can definitely be charged at more than 13.8 V. Most batteries specify a voltage of around 14.2 - 14.4 V for "bulk" (high speed) charging, tapering of to 13.8 V when the battery is nearly full. \$\endgroup\$
    – StarCat
    Commented May 11 at 8:18
  • \$\begingroup\$ @StarCat - fair enough, but knowing when you can and can't do that requires even more complex circuitry, and realistically is only possible if you're using an IC designed for the purpose. \$\endgroup\$
    – occipita
    Commented May 12 at 22:39

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