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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?enter image description here

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.)enter image description here

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

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  • \$\begingroup\$ You still need to limit the voltage. \$\endgroup\$ – winny Jun 20 '17 at 21:04
  • \$\begingroup\$ 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 \$\endgroup\$ – Kyle Hunter Jun 20 '17 at 22:09
  • \$\begingroup\$ 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. \$\endgroup\$ – David Schwartz Jun 20 '17 at 22:15
  • \$\begingroup\$ @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? \$\endgroup\$ – Kyle Hunter Jun 20 '17 at 22:19
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    \$\begingroup\$ 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. \$\endgroup\$ – Bruce Abbott Jun 21 '17 at 2:55
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The panel will automatically limit the current to a safe 'trickle' charge, so all you have to do is connect it directly to the battery.

If the panel has high dark current then a small Schottky diode in series will block the reverse current. Your panel probably has pretty low dark current so the diode may not be necessary, though it could be a good safety feature if the panel is wired remotely (a short in the panel leads won't short out the battery).

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  • \$\begingroup\$ So is the voltage fine? Gotten mixed views and opinions on it \$\endgroup\$ – Kyle Hunter Jun 21 '17 at 2:39
  • \$\begingroup\$ Yes, the voltage is fine. In fact the battery will never get 12V because the maximum voltage on trickle charge is about 1.45V per cell. The panel just has to be able to produce at least that much (+0.4V for the Schottky diode) at full current. 12V is higher than 9.1V so it's fine. The panel is running in current limiting mode so the 'extra' voltage is 'lost' inside the panel. \$\endgroup\$ – Bruce Abbott Jun 21 '17 at 2:47
  • \$\begingroup\$ Note that the battery is the only thing that keeps the voltage below 9V. If your regulator can't handle 12V then you could put a 9.6V Zener diode across the panel to protect the regulator in case the battery becomes disconnected. \$\endgroup\$ – Bruce Abbott Jun 21 '17 at 2:50
  • \$\begingroup\$ @BruceAbbott "12 V" solar panel =~14 Vpp > 6xAA = 1.46*6=8.76 V. How can you say the voltage is fine? \$\endgroup\$ – winny Jun 21 '17 at 9:16
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    \$\begingroup\$ If the batteries are fully charged, they won't clamp to ~8.7 V anymore and the voltage will just rise until they puke out their guts. Get yourself an MPPT/MPPC (tge new from Linear is fantastic) and buck, boost or buck/boost yourself to a suitable level. Theoretically you could run straight against the battery bank and program your voltage to 8.6 V, but better would be a charge controller from your intermediate rail. Or skip both and build yourself a linear pass regulator set to 8.6 V. \$\endgroup\$ – winny Jun 21 '17 at 15:26

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