PV Gap

Annie Said:

180ma solar panel kill my 9v rechargerable battery ?

We Answered:

The typical rechargeable 9V battery can be anywhere from 250mah to 300mah. And you don't want to charge it too fast because you run the risk of overcharging. So it's common to trickle charge batteries at C/10. So you take the current capacity of the battery and divide by 10 to get the safe slow-charging, trickle current.

For example, a 250mah would trickle charge at 25ma. The 300mah would charge at a rate of 30ma. This way, you can leave it charging indefinitely without risking overcharging because you are slow-charging.

I have the higher capacity 300mah 9V battery and use my 12V panel to charge it. And I limit the current to about 28ma just to give me some extra room for safety. Never had any problem leaving it hooked up charging for days!

The only way you'd be able to avoid limiting the current is if you won't be getting current beyond C/10. For example, on one side of my house, it's very shady all the time. So if I put my panel in that area, then the max current is so low that it's easily within the C/30 to C/20 range.

Because the current I get from this area is at most 15-20ma, that's below the C/10 trickle charging for indefinite charging, it's totally safe for me to charge it all day, day in and day out without the need to limit the current. The normal shade I get all the time limits the current to levels that won't overcharge the battery.

But since I use the panel in a sunnier location, it's necessary to limit the current to prevent overcharrging. And as I said earlier, I limit a bit more than C/10 just to play it extra safe. So it's common for people to use anywhere from C/20 to C/10.