PV Gap

Helen Said:

How do you use a solar cell to charge a battery or capacitor?

We Answered:

The voltage of the solar cell has to be a few volts higher than the battery. A 12 volt lead-acid battery, for example, requires 14-15 volts to charge.

This is complicated by the fact that the voltage out of a solar cell depends on the intensity of the sunlight, and the spec is usually at the brightest noontime sun. So typical output could be well less than the spec.

Re current. If you have a 10 amp hour battery, it requires 10 amps for one hour to charge it, or 1 amp for 10 hours, or 0.5 amp for 20 hours. In theory. In practice, it may not be that linear.

You should put a diode between the array and the battery, otherwise when the panel voltage drops below the battery voltage, the battery will discharge through the panel. This adds about 0.6 volts to the panel voltage needed.


For a cap, it will charge to the voltage of the solar panel. For a 3 volt panel, it will charge any cap to 3 volts. How long it takes depends on the current output of the panel. The math is complicated and depends on the internal resistance of the panel and how it varies with sunlight. An estimate can be obtained by CV = It
or t = CV/i
time = cap in farads, x voltage divided by current.

Everything has a polarity, you have to match them.