Can I use a 9w solar panel and 24v permanent magnet motor to charge 12v battery?

1. Jul 20, 2010

Clive_

Hi,

Can I use a 9 watt solar panel to charge a 12v battery directly? Do I need a rectifier or is the loss back in to the solar panel too small to worry about?

Also, can I use a 24v electric motor (permanent magnet generator) to charge a 12v battery - or is it likely to cause damage to the battery?

Thanks,

Clive.

2. Jul 20, 2010

Staff: Mentor

What is the output voltage range for your solar panel?

3. Jul 20, 2010

Clive_

Hi,

The solar panel is 12v 9w rating. Though the meter shows 20v in good conditions (and between 2-6amps)

Clive.

4. Jul 21, 2010

Staff: Mentor

So the answer is no, you shouldn't use the output of the solar panel to directly try to recharge a 12V battery. You should put a circuit between the solar panel and the battery to ensure that the charging voltage is the best possible (and safe) for the battery.

A boost-buck or Sepic DC-DC converter circuit would be a good fit for this job. You can learn more about them on wikipedia.org, or from most switching power supply books.

5. Jul 21, 2010

vk6kro

The open circuit voltage gets up to 20 volts in bright sunlight, so it is capable of charging a 12 volt battery. The voltage will drop to near the battery voltage when the two are connected.

You will need a diode between the two (with the cathode connected to the battery and the anode connected to the solar panel) so that the battery does not discharge into the solar panel at night.

Then, you will need to time the charging to ensure the battery is not overcharged. Put a current meter in series with the diode to check the charging current. Then divide the Amp-hour rating of the battery by this charging current to get the time it would take to fully charge this battery if it is not being used. If it is being used, there may be no need for this.

Also, can I use a 24v electric motor (permanent magnet generator) to charge a 12v battery - or is it likely to cause damage to the battery?

If you have a cheap way of rotating the motor, you can use it as a generator and it will give a bit less than 24 volts out at the rated RPM. You would need to limit the current with a suitable resistor. A 24 volt auto lamp between the generator and the battery would be OK for this.
And, as above, you would need a diode from the motor/generator to the battery. This is so that the battery doesn't try to drive the motor as a motor when the rotation is removed.