I'm using a 48V 1500W BLDC motor in a solar car.

One of the rounds I'm taking part in requires the removal of batteries and running the motor solely on solar power. But due to space restrictions I can only use a 300W 48V solar panel.

The peak current from the panel is approximately 7 amperes which is not sufficient to generate the current required to start the motor. I calculate that I need at least 40-50 amperes to start the motor and a 30 ampere continuous current to run it. Is it possible to generate current spikes of higher values than 7 amperes at 48 volts so that the motor can run?

I am considering Maxwell ultracapacitors of 116F 32V (two 58f 16v in series with corresponding bleed resistors) through a closed loop buck converter to keep the 48V constant. Will connecting it in parallel with the solar panel be enough? Or is there any other alternative solution to run the motor?

  • Very interesting question! Can you tell us more? How long does the motor need to run for? Can you start with charged capacitors? Does the motor need to run continuously?
    – dlu
    Commented Sep 17, 2016 at 16:04
  • Just curious why you wouldn't add another 16V worth of ultracapacitor to provide a complete 48V ... seems it would work better in the long run, plus provide you more capacity. The Maxwell Ultracaps are awesome little devices. I had asked a question about using them in lieu of a lead-acid battery in motor vehicles on here before. Commented Sep 17, 2016 at 17:20
  • @dlu in the round without batteries the motor needs make the car move for 40m minimum to qualify for that round. But it's better to make it sufficient enough to run just from the solar panel. Yes we can start with charged capacitors and yes the motors need to run continously. I somehow need to make the motor run from a 300w 48v solar panel where the current requirement for the starting of the motor is at least 40A.
    – user22483
    Commented Sep 19, 2016 at 20:30
  • @paulster2 the car basically will run on either lead acid or Li-ion. Weighing more on Li-ion because of its varied benefits and less weight but insanely more expensive than lead acid batteries. The second round however I was thinking if I could just generate pulses using the ultracapacitors to make the motor move continously because my input is from a 300 watt solar panel(at peak conditions) . I had read somewhere that if you can control the discharge and charging of the capacitors you can generate high current pulses and keep the motor moving with its own inertia during the off period.
    – user22483
    Commented Sep 19, 2016 at 20:34

1 Answer 1



The motor

has 1500W, which means it draws 31A at 48V at full load. The inrush current can be higher, that's your 40-50A. However, this doesn't mean the motor needs 40-50A to start and 31A to run, it's just the absolute maximum. At lower voltage, it will sink a lower current and have lower power. Remember, a combustion has its maximum power near the red line, but it's rarely driven at that RPM, even not when accelerating.

The solar panel

has an maximum output of 300W. But only under best conditions, which means no clouds, no fog, the sun as high as possible in the sky, and the panel pointing exactly to the sun.
In a few days, we have September 21th, where the sun is exactly above the equator, and the latitude of your location gives the angle between the sun and a vertical line at noon. New york for example is at 40°N, so the angle will be 40° (or: The sun will be 90°-40°=50° above the horizon). If you place the panel on the ground, it will only give cos(40°)*300W=0.77*300W=230W. And this doesn't take into account that the light had a longer way through the atmosphere and has been attenuated more than at the equator. (And it's high noon. Not morning, not afternoon)

Further more, a solar panel is a constant current source. With a load of 7.7Ohm, it will deliver 48V and 6.25A (=300W). With a higher load, the current will still be 6.25V, but the voltage will break down, which means the power delivered will break down, too. If you apply a smaller load, the voltage will rise, but the current will fall. Again, the power decreases. So, this 48V, 6.25A are the maximum power the panel gives with the right load connected. And the 48V are about 80% of the voltage when no load is connected, which is about 60V.

The capacitors

First, if you put two caps in series, the voltage rating of both can be added, but the capacity is

Ctot = 1/  (1/C1 + 1/C2)

which means the combination of two 58F caps gives a 29F cap. If you connect three caps in series as Paulster2 suggested, you get just 19.3F. But this doesn't really matter in terms of stored energy. Each cap can store

E=1/2 CU²

and a combination of three caps in series will provide just 1/3 of capacity, but 3x the max voltage. It turns out three caps will store three times the energy a single cap can store, as expected.

The max energy of a single cap is 7.4kJ (for comparison: a 40Ah car battery has 1728kJ) and it will take roughly 7400J/300W=25s to charge it at the max. power of the solar panel. More caps need more time to charge.

Now, let's say the motor pulls 1500W, whereof 300W are supplied by the panel. This makes 1200W for the caps, and a single cap will last for 7400J/1200W=6s.

A few warnings: Your caps (this ones?) are rated for 16V, and absolute maximum voltage is 17V, which isn't much more. You should never exceed that voltage, otherwise the cap could be damaged or even burn/explode.
If you connect three caps in series to get 48V and directly connect them to the panel without protection, they will be charged to the open circuit voltage of 60V and BAAANG!
Next, large caps have a lousy precision in capacity. Your caps have a capacity of 58-70F. If you connect three in series to 48V, two having 70F and one 58, the big ones will charge to 15V and the small one to 18V - and BAAANG!

So I would recommend to either not put the caps in series - though this results in high currents - or not charge them to the theoretically rated voltage.

The electronics

So it's clear you shouldn't connect the panel to three caps in series, and it's also clear that you shouldn't connect them to the motor directly - their voltage will decrease and they will not provide max. power to the motor.

You will definitely need some DC-DC converters, and my calculations assume this, too. Converters always have some inefficiency, which heavily depends on input/output voltages and currents. High currents are always a little problematic, and when the caps are almost empty, they must provide a giant current to provide the requested power. I'm not able to give any numbers, but in principle you have to take into account that whenever energy is transferred from the panel to the caps and from the caps to the motor, a fraction is wasted by the converter.

(And finally, you want to control the power of the motor, too)


Though your motor has a max. power of 1500W, you do not need to provide 31A at 48V unless you really want the entire 1500W. Typically, you will need much less power.

The max power of the panel is 300W which already is a bit low, but you should expect to get waaaay less out of it.

This caps can for sure help to buffer energy and boost the motor when more power is demanded or some clouds show up , but they must be charged, first. I guess they could count as battery in your ... contest unless they are not charged for the start.


Your comment is a little confusing, I'm still not sure what exactly you are planning to do. You also mention a battery, though the question says you have to remove the battery...

Anyway: You will need a DC-DC converter to transfer energy from the caps to the motor. On the other side, you need to charge the caps with the excess of power from the panel. And when you need max. power and the caps are empty, you want the entire power from the panel flow into the motor, and not into the caps.

Further more, as I wrote, when you try to get more power from a panel than it can deliver, it actually delivers less power.

Unfortunately, I don't know any electronics you can directly use, though it for sure exists.

  • I guess that was a mistake on my part. I meant to say that I plan to add the capacitors in parallel to the panel across the Motor with appropriate protection circuit as you suggested (which I am not sure what kind though). This ultracapacitor would then be two maxwell 16.2 58f caps in series connected across the motor in parallel to the battery. And yes I was thnkng alng the lines of a converter as well to keep the voltage at 48 volts while the cap discharges. Is it possible to regulates the charging and discharging of the cap so that I can generate current pulses enough to run the motor?
    – user22483
    Commented Sep 19, 2016 at 20:52

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .