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How to power a 12V motor with a pair of 250W solar panels? Answered

I am on a team in a solar vehicle competition that doesn't allow for the solar panels to be connected to the circuit while running the 12V motor in a solar power race.
The solar panels are 24V @ 8.6 amps each. We have 2 of them, and are going to connect them in parallel for a total of 24v 16 ish amps.
The panel's over voltage is 30.4V. The maximum current through the decades old (but free to us) brushed 12V electric motor is 35 amps.

We are planning to run a DC to DC step down / voltage regulator between the Array and the motor, and a small charge controller to a battery for in between races.

We are looking into charge controllers that allow a small battery and a load to be connected to them at the same time for charging and between races. The battery size is limited by the race organizers to 180Watt hours. 
1st ?> Are charge controllers powered off of their Input sources? I.E. Will the charge controller even turn on with out a battery present?

2nd ?>   What would happen if my array is connected to the charge controller and the battery isn't connected to the charge controller? Wouldn't that be harmful to the charge controller or motor?

3rd ?> If charge Charge controller to motor is a viable option, then would a DC to DC step down still be necessary?

Thank you in advance,
Tony S.


Most charge controllers are powered by the source.

A good charge controller should know the battery is not there and automatically shut down.

Since you are not going to be using the solar panels or the charging circuit during the race I assume you will have a switch or relay of some kind.

I would use two 12 volt batteries totaling 180 watt hours.

Charge them in series with a 24 volt charger, I know this works.

Then I would run the batteries in parallel 12 volts in the race.

No DC to DC converter needed just a switch to take the batteries from series for charging to parallel for racing.

In the race I am concerned about, all the batteries must be obviously disconnected from the circuit or disqualification criteria is met. This leaves the solar array the motor and what ever we need to allow the panels to run the motor from the panels. So since the Charge controller will shut down when the battery cutoff switch is thrown, we must use some other interesting device to manage he voltage through the motor.

im not sure why charging 2 12V @ 7.5amp hour batteries in parrellel would be a benifit over charging a single 12V @15 amphour battery. The amperage is the same and the voltage still the same. What's the advantage?

I think you missed it.

Charge the two 12 volt batteries in series, 24 volts and no DC to DC converter needed to charge the batteries. In fact you may not even need a charging circuit just clip the solar panels to the batteries in series. A charging circuit does prevent over charging.

Then run the 12 volt motor with the 12 volt batteries in parallel making 12 volts.

Only kicker might be you can only have one battery, not two. Don't know if that's in the rules.

We are allowed any combination of batteries so long as the total watt hours of the battery bank doesn't exceed 180 Wh.

The problem is that we can't use a battery for certain races.
Over charging will be an issue and we plan to get a PWM charge controller to prevent over charging our battery, which is only used for 2 out of 4 races.

I forgot to mention charging the batteries in series at 24 volts cuts the charge time in half.

I understand now, and that is still a possibility. I will speak to my team about what we can afford and see if this is a cost efficient solution to our battery charging situation. We are fairly confident that the 500W array will charge any arrangement of 180Wh battery bank fairly quickly, and have all decided to get a charge controller of sorts already. We are actually concerned that we might charge it TOO fast actually.

Yea to fast or over charging can kill a battery so a 24 volt charging circuit is best.

If you use a solar regulator you will need a battery or it will be damaged, what you need is a solar maximiser, Ive use them and they all most magical how well they work.

have a look here.


you can buy one here


I would also consider running a 24volt motor which would better match your panels, something like this would be perfect.


If your running bike wheels you can also get a gear motor that you can stick the chain straight onto, and if you use the gears and derailed from the bike it will also help a lot. like this one. Good luck with your project


This is just a little over my head... I feel like I'm so close to understanding the circuit details behind this. The capacitor has been talked about before and I was reluctant to try that route because the concept is just beyond my comprehension. Perhaps I can speak to my circuits professor about it.

This mini maximizer looks like exactly what we need, i'll talk to my team & advisors about it this week.

One problem you might face is that SLAs really don't like rapid charging. You will significantly reduce the number of charge cycles/capacity if you try and charge at a C rate and not C/2 or more. The battery capacity is only 15Ah by my reckoning, so 16A, (in full, perpendicular, noon sun) is not going to be good for a flat battery, at all.

Rapid charging cause hydrogen production on the plates, and then they will vent.

If you only need 15Ah, then Hawker Cyclon batteries may be the answer - SLAs that are about the most tolerant you can get.

I'd look for a cheap brushless motor if I were you. Or make one from a car alternator.

Do you have to carry the panels on the charged vehicle ?

What battery tech is permitted ? Li-ion or just SLA ?

There are a few different races at the event. 2 of which are solar power only. We will have a battery for only 2 races, and in those race the panels are disconnected and removed from the vessel. There is no race that permits using solar and batteries at the same time in this event. We will only be allowed to charge our battery in between races, and that must be done via the on board solar array.

The panels will be mounted on the vessel during the majority of the day aside from 2 races where they will be removed because they aren't to be used for that race and for weight savings purposes.

The limitations of the battery are that the total energy of the battery array mustn't exceed 180 Watt hours, and the batteries must be seal marine grade batteries. We have bought a LiFePhO battery , but I think we will designate that as a backup battery for the cloudy weather battery that is required if it's a rainy day. We are looking at getting a new SLA battery.


2 years ago

Decades old brushed 35 amp 12V electric motor, is it a PM, Shunt or Compound machine ? These older machines easily run at over 120% voltage for 10 minutes.

And peak motor efficiency is not at full motor power.

Remember batteries are less then 89% efficient, with lead-acid at only a 50% return to the motor on charge energy going into the battery.

Depends on the charge controller, do you have a circuit or manual ?

A step down from 48v is more effective then 24v because a transistor forward drop is the same at both voltages... The lower voltage a 1v drop is 1/24 and the current is higher, while a 1v drop at 48 volts is 1/48 and the current is less.

Honest answer to the questions about the motor: I have no Idea. The motor is an old Minn Kota 65 turbo 12V trolling motor. ( yes this vehicle will float [insert technical joke about grounds here]). The only reason we are using this motor is because it was gifted to the project and we are trying not to go over budget, which is small. The panels , vessel, motor, a lot of the circuit relays, breakers and fuses will all be donated by local supporters and families of the students involved.

It has certainly come up that we should have a 24v motor before, but cost is a prohibiting factor.

Our next working meeting will be testing the panels and motor for maximum efficiency values.

So a voltage drop from our parrellel panels would be 1/24 * (delta V) = 1/24*(12-24)=-1/2 V ? That doesn't seem like a lot.