Solar charging

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jaycool1995

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Hi,
I am trying to make a usb charger. There are going to be two solar panels @3v 20mah in series, they will be trickle charging two 3v AAA batteries. What i am not sure about is how to stop the batterys from overcharging. I thought i could use zener. Help would be much appreciated, and possibly schematics. Any info extra infomation i have missed just say.
Thanks,
 
Hi. You do not say the type of cells. The cells should be in parallel during the charge, also connected in parallel to a zener diode to limit voltage if the cells chemistry asks for constant voltage charge. I use 1N5335B in parallel to Li-Ion's and works superb.

For constant current charge chemistry cells, a low-drop current regulator should work and cells should be charged individually.

Miguel
 

If you're trickle charging you don't need to do anything, the entire point of trickle charging is that you can leave them charging permanently and they will never over charge.

You're also not going to overcharge anything with those puny cells!
 
As Nigel says, 20ma is not going to overcharge the cells. However, they won't fully charge them in a day either, so don't forget to put a diode in series with the solar cells so they don't discharge the batteries when the sun goes down.
 
Last edited:
kchriste said:
As Nigel says, 20ma is not going to overcharge the cells. However, they won't fully charge them in a day either, so don't forget to put a diode in series with the solar cells so they don't discharge the batteries when the sun goes down.
Click to expand...

Schottky diode to keep the losses to a minimum

Robert-Jan
 
AAA Ni-MH cells are 850mah. They are usually charged at 85mA for 14 hours or trickle-charged at 21mA.
The solar cell is rated on the equator of earth at noon with it pointing directly at the sun.
You might get 15mA at noon then 10mA for a couple of hours berfore and after noon.
Then it will take 2 weeks of sunny days to charge dead cells.
 
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