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Will i damage small solar panels on a car battery? Answered

I've got a couple tiny solar panels and i wanted to have them charge a car battery, The panels in series put out 18v at 50ma but the leads are just those tabs they spot weld to batteries. Is it possible to fry the solar panel or leads? 

I am not sure about amperage of the battery. It's similar to one i bought a long time ago with 650 Cold cranking amps(not sure how that translates to total amperage but i'm guessing that is how much you can safely draw from it at one time) 

i understand that it wouldn't be proper to attach 18 volts to a 12 volt car battery, it charges at around 15 volts like most car batteries. 
I understand that 50ma will take an extremely long time to charge something like a car battery.

But in a hypothetical emergency situation, would this work temporarily without destroying the solar panels.

7 Replies

icengBest Answer (author)2016-01-22

You only need a single diode to prevent reverse current to flow from your car battery through the solar cells when a cloud blocks the sunlight... A 1N4001 diode is all you need, it is a 50 volt 1 amp one way electric valve..

A solar cell turns into a resistor when the combined series connection drops below the battery voltage and in your case there would be damage to the tiny cells if you left out the six cent diode.

As usual click on the picture to see the the full image.

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Downunder35m (author)iceng2016-01-23

For multiple solar cell there should be a diode between the paneles as well !??
AFAIK a cell partially shaded would otherwise be acting as a resistor and the system loses power, not to mention that with seriously sized cells the shaded cell can burn out.
But I guess at just 50mA there is nothing to fear from this end LOL

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iceng (author)Downunder35m2016-01-23

The single diode is just for preventing the battery discharge.

Iv made an early battery charger panel out of 28 series 3.5" dia 0.5v 1/2amp solar cells and one diode to maintain a camper battery charged in a forest campground ground which worked very well all summer... Obviously the tree branches would reduce some peak cell output but current cannot flow backward even in a totally dark cell.

I think Only parallel cells need individual diodes..



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Josehf Murchison (author)2016-01-22

As long as the panels have a diode in them or you add one your panels are safe.

As to charging your battery.

If you parked the car and left the lights on and when you discovered the lights on you did not have enough power to start the car. Between the battery coming up a little on its own and a trickle charge for an hour can make the difference between starting and not. This happen to me and it works.

If you have a seasonal vehicle parked for months on end it can keep the battery from dying while parked. Many of the people where I live do this for their lawn tractor in the winter.

Cold crank amps is different than amp hours, 650 cold crank amps means 650 amps the second you turn the key. Amp hours means so many amps over so many hours. The two batteries charge differently so in the case of the car battery it takes less to charge, but at a point of discharge it would be useless.

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Downunder35m (author)2016-01-22

Do you want to charge the battery or just pretend to charge it?
50mA might be enough for some stacked AA solar batteries but if you are taking about something like a real car battery....
Your 50mA could be enough to keep the battery fit during the summer when not in the car but it is not enough for a charge.
You might not need a charge controller but you do need protection diodes.

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Tomahawk92 (author)Downunder35m2016-01-22

That's reasonable

The diodes would be to protect the solar panels?
Do you think attaching them directly would fry the panels?
And would it literally not charge? or just seem pointless because it is charging so slowly?

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steveastrouk (author)Tomahawk922016-01-22

It wouldn't harm the panels, but as DU35 says, it WILL charge at a vanishingly low rate. To charge a 100aH battery, you'd usually charge it at a 10 hour rate, which would be 10A. Your cells push out 200X less, and would charge it in around 3 months, in noon sun, with the panels perpendicular to the sun.

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