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# Charge capacitor from low voltage solar cell?

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#### magician13134

##### New Member
Hi I'm trying to charge a capacitor to 5V from a small solar cell that only puts out 1.4V. Can this be done? It's a 2.2F capacitor and the cell puts out 20mA in decent sunlight. Can this be done easily? Do I need a boost circuit? If so, what's the best way to do this? Thanks

Use a Joule thief.
Google it or search it within this forum.

It depends on the leakage current of the capacitor.

If it's a memory back-up capacitor then you're probably all right, if if it's a car audio capacitor then it won't work.

You'll need a boost converter to charge it to 5V.

How long have you got to wait?

A rough calculation tells me it'll take roughtly 38 hours to charge to 5V.

Last edited:
It depends on the leakage current of the capacitor.

If it's a memory back-up capacitor then you're probably all right, if if it's a car audio capacitor then it won't work.

You'll need a boost converter to charge it to 5V.

How long have you got to wait?

A rough calculation tells me it'll take roughtly 38 hours to charge to 5V.
Your calculations seem a little too rough.

The capacitor energy at 5V is ½CV² = 27.5 Joules
The power from the solar panel is 20 mA * 1.4V = 28mW or 28mJ/s.
The time to charge for 100% efficiency is 27.5/28e-3 = 982 seconds or 16.4 minutes. Even at only 10% charging efficiency it would still be less than 3 hours.

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