# Charging 6 NiMH batteries in series via solar

I'm currently working on a project in which a Atmega328 is powered by 6AA NiMH batteries in series (7.5v, 2000MaH). It is in sleep mode most of the time, except to turn a servo a couple times a day, for a couple of seconds (Hence the need for the high voltage).

I have a solar panel at 12V and 1.8W, so maximum of 150mA. After doing a lot of research, and looking at this post: Charging NiMH batteries with 1W solar panel, charge controller needed? . The maximum output for my solar panel is within the c/10 -> c/40 range that is safe for the NiMH batteries to be charged at.

My question is, none of the resources I could find explicitly say if what I am attempting is OK for multiple cells like I have.. Due to the remote nature of my device, charging each cell individually, or using a LiPo, is not an option.

Thank you very much.

EDIT: Below is a PCB design for this circuit. According to @BruceAbbott The voltage of the panel would be fine, but since my servo can only handle a max of 8v, would the batteries still be able to lower to voltage to not damage my servo?

I believe the reason it is safe is due to how the current and voltage is controlled from a solar panel, for example, this IV curve (NOT the panel i'm using, just an example.)

This should ensure that the voltage never exceeds the dangerous limit?

• You still need to limit the voltage. Commented Jun 20, 2017 at 21:04
• What would that voltage be? Since my voltage and current source won't be guaranteed to be constant or known @winny. I was thinking of using a lm317 to limit current, since voltage doesn't seem to be hugely important from what I've read, but since the max current is low enough I was thinking I didn't need it Commented Jun 20, 2017 at 22:09
• You need just need to limit the voltage (to about 8.6 volts) and prevent the batteries from discharging through the panel. It could be done with a zener diode, a rectifier diode, and a resistor. Commented Jun 20, 2017 at 22:15
• @DavidSchwartz OK. So I'll just use a low drop off regulator for that I suppose. Was hoping to avoid that, since now the panel will have to have even more sun to ensure the voltage is high enough.. How did you get the 8.6 volts number? Commented Jun 20, 2017 at 22:19
• Trickle charging is not 'ideal' but in this case it's the only viable option because there isn't enough power for fast charging. The cost of a sophisticated charger probably isn't worth the potential saving in battery life. Commented Jun 21, 2017 at 2:55