Simple nickel-metal hydride solar charger

February 6, 2016

Nickel-metal hydride rechargeable batteries can be recharged with a simple trickle charger. I have made a one AA solar charger for my headlamp and a two AA solar charger for other devices.


Solar trickle charger wiring diagram. The blocking diode may not be required if reverse current is very small.

Caution: The simple charger circuit described here is not for charging cells in parallel. When charging cells in parallel, I think there should be some technology to distribute currents and balance charge among the cells.

Charging current

Self-catalysis can recombine gas formed when NiMH cells are overcharged if the charging current is less than 0.1 × capacity.

The cells I want to charge are 1900 mAh AA Eneloops:

Safe trickle charging current = 0.1 × 1900 = 190 mA.

The same charging rate would apply for these cells in series (where each cell experiences the same charging current).

Caution: The simple charger circuit described here is not for charging mismatched cells in series (e.g. cells with different capacities, different ages or different states of discharge). Matched cells should be used and assigned to specific devices. This will maximise cycle life (i.e. avoids one bad cell affecting the good cells) and allow safe series charging.

Charging voltage

Cell voltage increases towards full-charge and maximum charging voltage for a NiMH cell is 1.4 to 1.6 V.

Solar panels are over-rated at standard testing conditions. Voltage decreases by about 25% at typical operating temperatures and solar irradiance:

Maximum charging voltage = 1.5 V
Solar panel (maximum power) voltage = 1.5 ÷ (1 − 0.25) = 2 V (at standard testing conditions).

The minimum solar panel is about 2 V and 200 mA = 2 V × 0.2 A = 0.4 W. I have some 2 V solar cells and they are capable of charging NiMH cells.

Voltage upgrade

Low voltage circuits are sensitive to voltage losses. A higher solar panel voltage is recommended to overcome contact resistances at the battery terminals and to allow a blocking diode in case of reverse-current leakage.

Schottky diodes have low forward voltage drop of 0.15 to 0.45 V:

Schottky diode forward voltage = 0.3 V.
Solar panel voltage = 2 + 0.3 = 2.3 V.

From the solar panels available to me, I selected a 3 V 260 mA (= 0.8 W panel). These ratings are greater than the 1.4 to 1.6 V maximum charging voltage and 190 mA safe trickle charging current for AA Eneloops. However, self-catalysis handles gases produced by over-charging and I have not observed over-voltage or high cell temperatures using these panels.

My one AA solar charger


My solar trickle charger for one AA NiMH battery

My one AA solar charger is assembled from the following components:

  • 3 V 260 mA solar panel
  • 1N5819 Schottky diode (rated for 40 V, 1 A)
  • Twin-core cable
  • 12 mm, 2-pin “aviation plug”
  • Small project box
  • AA battery holder

The diode forward current and reverse voltage ratings should be greater than the maximum charging current and battery voltage respectively. I soldered the positive leg of the diode to the panel. I covered the solar panel contacts with acrylic sealant. Acrylic sealant is opaque, dries harder than RTV silicon and is easier to peel and scrape off if necessary.

The solar panel has a two metre lead, so that the battery can be kept out of the sun and rain. The detachable lead is for convenience in packing away the charger.

The project box provides a mount for the aviation plug and protects the cheap and flimsy AA battery holder. I inserted rubber spacers to help support the ends of the battery holder. A rigid battery holder with stronger spring contacts would be an improvement.

There is no fuse because voltage is low and contact resistances limit the the short-circuit current. I measured about 2.5 A when shorting the battery holder leads. Heating of the battery holder contacts and leads was insufficient to result in a fire.

A battery tester is helpful to check the state of charge before and after charging. An ammeter would be useful to check charging rates, but is relatively expensive.


Battery tester bought on ebay.

In strong sunlight and with the panel facing the sun, my charger delivers about 260 mA. In weak, afternoon sunlight the current is about 65 mA. Charging current does not vary with cell voltage because the solar cell is operating in its plateau region.


Solar panel operating parameters. Current changes very little for voltages less than maximum power voltage.

A one-day charge resulted in 1460 mAh of capacity (77% of 1900 mAh rated capacity). A full-charge could be achieved with 340 mA panels or by moving the panel to track the sun or with an extra 2 hours of charging.

My two AA solar charger


My solar trickle charger for two AA NiMH batteries.

My two AA solar charger uses two 3 V 260 mA panels in series (6V 260 mA) and a two AA battery holder (charging voltage 2.8 to 3.2 V).

Compared to spare batteries

Weight and cost are relevant criteria for choosing between a solar charger and carrying spare batteries.

The cost of my one AA solar charger was AUD 4.59 without the battery tester or AUD 6.57 with the tester; approximately equivalent to the cost of two Eneloops. The solar charger is cheaper than four Eneloops.

The mass of my one AA solar charger was 121 g with the lead; approximately equivalent to the mass of four Eneloops. For short trips, it is more convenient to bring spare batteries and no charger.