I'm trying to understand which voltage I need to charge my NiMH batteries so I can purchase the components to do this.

Actually I have 2x AA Ni-MH 1.2 V 1900 mAh and 4x Solar Panel 2 V 220 mA (0.44 W).

I read Solar Cell - Preventing overcharge of an NiMH battery and Voltage input for charging NiMH Batteries but I'm still confused as I'm not an expert in this field.

I gathered this information:

  • Batteries should not drop under 1 V or they will be damage also not go over 1.78 V;
  • Constant voltage doesn't work to recharge them (not sure).

I checked out some components to stabilize the current and I found a LM317 in order to stabilize the current to (C/10) 200 mAh and use a MAX471 to check out these values.

But the problem will be the voltage that during the day will drop or go up due to weather so the voltage will never be stable, it is a positive thing?

In the case I need a stable voltage I also found these components (M2578A, TL497A).

I hope to find somebody that can help me. I'm free to other solutions if this one doesn't fit my purpose.

  • \$\begingroup\$ NiMH are very resistant to low voltage, but not at all to reverse voltage. I suggest charging them individually. batteryuniversity.com/learn/article/… has lot of good info. \$\endgroup\$
    – Indraneel
    Oct 17, 2018 at 1:20
  • \$\begingroup\$ You're welcome. You can also upvote comments by clicking the up arrow! \$\endgroup\$
    – Indraneel
    Oct 20, 2018 at 22:44
  • \$\begingroup\$ I can't because less then 15 rep. I press it anyway the upvote :D \$\endgroup\$ Oct 21, 2018 at 16:27

2 Answers 2


You must have at minimum a battery charge controller because you need a constant current mode. You also need something to make sure you don't overcharge the battery and start a fire. To adequately use a circuit like this you will also need something to stabilize the voltage from the solar panel called an MPPT tracker to make sure the voltage is constant.

enter image description here Source: https://www.mpoweruk.com/chargers.htm

  • \$\begingroup\$ Thank you for reply! Im working on software that check this delta V so I can stop charge process :D And also looking for a battery charge controller. I found the LM317 that could be fit my need along with a XL6009 to stabilize the Voltage from Solar panel \$\endgroup\$ Oct 20, 2018 at 18:54

Quickly charging Ni-MH from a solar panel is very hard to do. Unlike lithium ion or lead acid, you can't reliably depend on the cell Voltage to determine when to stop charging, and depending on the brand, Ni-MH can be damaged by over charging much more so than Ni-MH.

The easiest way is to just set the charge current at the safe value of 0.03C for all types and just let it trickle charge. With NiCd you can go up to .1C and it'll handle that for a couple of years just like a cordless phone battery does with continuous charging.

Since the solar panel doesn't produce electricity continuously, full charge detection methods like negative Delta V or temperature is difficult or impossible. Do you know of a battery charger that can reliably charge Ni-HM cells if it is unplugged every 10 minutes?

One method that can work is to use a type of counter logic to track the total discharge and charge going in and out of the cells. Take in to account that Ni-MH charges with 2/3 efficiency and you have a good idea of the state of charge. Trickle charging can ensure full charge is reached eventually.

MPPT is just a method of adjusting current being taken from solar panels in order to keep the solar panel Voltage at its optimal level for given temperature and sun conditions. MPPT is a good investment if the cost of the solar panels exceeds the value of adding MPPT by a couple of times. It has nothing to do with battery charging, even though MPPT devices are often associated with battery chargers.


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