Trickle charging 10 NiMh in series with ~1 W solar panel

Question

I have been put onto a project that needs to be powered by a small (1-2 W) solar panel trickle charging a battery pack. The product will be located on rooftops.

We have reviewed similar products and have found them to be using NiMH batteries so are planning on following suit. I have reviewed many similar posts, particularly the below, but still have some unanswered questions, apologies if I am repeating the questions of others.

• Charging 6 NiMH batteries in series via solar
• Charging NiMH batteries with 1W solar panel, charge controller needed?

The battery needs to supply power to a 12 VDC motor and a 24 VDC actuator (planning on boosting 12 V to 24 V) but these will only be active for a small amount of time each day, so overall energy requirement is quite low.

At this stage we are planning on using 10xAA NiMH 3600 mAh in series to get a 12 V output. I plan on using a Zener diode between the battery and panel to prevent back discharge at night.

The solar panel will be something like the 12 V model at the bottom of this page: https://www.futurlec.com/Solar_Cell.shtml

I understand that trickle charging NiMH batteries is not ideal, but it seems to be commonly done and NiMH seems like the ideal chemistry for the application.

My questions are:

• Is a 12 V panel the best choice for trickle charging the 10 batteries in series?
• The panel linked outputs a peak of 80mA, 80/3600 = 1/45C, my understanding is that this is well within the safe limits for solar charging of NiMH, is there a benefit in regulating this to a constant current (with LM317 for example)?
• Should we be considering including an IC such as the DS2715 to monitor battery level and prevent overcharge, or is this completely unnecessary given the low output of the panel?

Happy to provide any more information that would be useful.

Answer 1

I have a panel of this current level and the reverse current is less than 1 mA so I don't use a blocking diode. The panel will need an open circuit voltage of 18 to 20 V and should be able to supply a reasonable current at 16V. The panel is a current source so adding an external one is redundant. Determining end of charge is very difficult here because the battery voltage will vary with current which will vary with sun intensity. An undervoltage lockout might be desirable. 3.3V zeners have a very high conductance at lower voltage and will be a significant drain. When NiMH were first introduced they could not tolerate a trickle charge but they seem to make ones that can now.

Answer 2

Look at the datasheet of an AA Ni-MH battery cell to see that it is charged to 1.4V to 1.5V and is 1.2V when it is almost dead.

Answer 3

You need to know the short circuit current and open circuit voltage of your solar panel to be able to tell for sure, but NiMH is a good option here since they self-limit any overcharge well given that the current is low enough, and at 80 mA, it should be. NiCd is even better in this regard, but they are terrible otherwise. If your open circuit voltage is much higher than 1.6 V/cell, I would at least think about it and test it first.

Perhaps you could put a bit less stress on the batteries by using an LDO like your suggested LM317 to limit the peak voltage. If it's a one-off, I would not bother. If it's mass production, you need to find and test the worst case scenario in this regard and check the datasheet of your batteries.

Normal NiMH charging is done with either negative delta V detection or a temperature sensor to terminate the charge. The former is possible in your case but the current is too low for the latter.

If you want to keep it simple still and limit the voltage and balance the cells, something like this would be a safe bet. 1.5 V Zeners don't exist, hence two cells in series per Zener (~3.1 V):

^{simulate this circuit – Schematic created using CircuitLab}