A Simple Solar Charger for Nickel/Metal Hydride and Alkaline AA Cells

This simple device can be placed on a window sill or hung in a window facing outwards and will charge up nickel/metal hydride cells slowly in the background. When fully charged the cells can be left in situ receiving a very gentle float charge and be available for use when required.

It will also allow the user to experiment with the recharging of alkaline AA cells but please note that on 13th May I edited the text in Step 5 to reduce the possibility of such cells leaking--a very messy event!

The circuit diagram is shown above. We have four AA cells and each cell is connected to the 6 Volt 100mA solar cell positive terminal via a resistor and rectifier diode. The resistor values depends on the circumstances--470 Ohm is a good all round start particularly for AA alkaline cell recharging experiments. However nickel/metal hydride recharging may be faster with a lower value, even as low as 47 Ohm especially in winter conditions with low overall light levels.

The diodes are vital for isolating one cell from another and to prevent the charge leaking back through the solar cell when in darkness.

This is a very simple circuit and construction is not at all critical. You will need basic soldering equipment and a soldering iron that has enough capacity--mine is 40 Watt. My simple construction method has the advantage that the soldering is on the top surface and the bottom surface is left smooth and the device can be placed on a windowsill with no danger of damage by scratching, otherwise Vero board or equivalent will be fine if you have it.

Components:

1 X 6 Volt 100mA solar Cell--Easily available around the world, mine came from www.cpssolar.co.uk

4 X resistors--low wattage 47 Ohm to 470 Ohm according to chosen design.

4 X rectifier diodes--e.g. IN4xxx series

4 X AA Battery boxes--If you follow my method of construction use the ones with solder tags.

If you use Vero board or equivalent get PCB mounting devices.

If you follow my method of construction you will need a piece of single sided printed circuit board of sufficient size to accommodate the components--mine was 100 X 130 mm. Also saw squares of printed circuit board 15mm X 15mm and one 100mm X 15mm as shown in the picture above.

I glued the components to the copper side of my printed circuit board using MS polymer glue as shown in the picture above but see below. The solar panel overlaps the printed circuit board as shown in the second picture above.

Note that the battery boxes are made from polythene and I found out the hard way that MS polymer glue does not take to it. In the picture in the next stage you will see that I had to back track and use double sided adhesive pads just for these items. Note that the battery boxes have their curly spring negative connections at the bottom of the picture.

Solder the solar panel negative connection to the printed circuit board copper layer and the solar panel positive connection to the wide copper strip.

Solder the diodes with the negative side to the wide copper strip and the positive banded end to each respective square of printed circuit board.

Solder your chosen value of resistor from each respective square of printed circuit board to the positive end of the respective battery box.

Solder each negative battery box connection to the printed circuit board copper layer.

If in any doubt note that the construction physically mimics the circuit diagram exactly.

The construction is now complete.

The picture above shows the charger loaded with two nickel/metal hydride cells and two alkaline ones.

The charge current for a given cell can be ascertained by measuring the voltage across the series resistor and using the Ohm's Law formula:

Current = Measured Voltage/Resistance

How quickly nickel/metal hydride cells charge with this device will depend on a number of factors. Using 470 Ohm series resistors I found that two 800 mA hour cells charged fully while I was away for ten days. This was during late spring in the UK and with the device placed on a conservatory window sill.

Using the device on ordinary alkaline AA cells should be regarded as experimental. I was encouraged by previous experience gained using an ordinary alkaline AA cell as the storage medium for a solar powered battery electric clock, see:

https://www.instructables.com/id/How-to-Run-a-Batte...

However, the clock in question ran for some six weeks and the battery consistently read over 1.8 Volts whilst receiving a float charge of up to 3 mA during daylight. Then disaster struck and the battery leaked badly so using the edit facility I will alter my instructions and advise the removal of alkaline AA cells from the charger once they have reached 1.5 Volts and do not leave them receiving a float charge. There is no way that an alkaline AA cell could be regarded as being restored to the 1700 to 3000 milliamp hours capacity of a new cell and high current applications such as in cameras may be discounted. However there are many low current applications such as clocks and remote controls where AA cells recharged in this way may see useful service. This is one for the experimenters.

My construction is very simple, possibly crude and it is open to the constructor to put the device in a case with the solar cell on the outside. I have been concerned that the batteries can get rather warm when the charger is placed in direct sunlight and in practice I have shielded them with a piece of cardboard to mitigate this. My simple construction would certainly allow for the charger to be hung up against the window glass facing outwards although the simple expedient of mounting the solar cell the other way round whilst the batteries face inwards would be beneficial.

This is somewhat of a niche item. Most enthusiasts today will be searching for a device that will rapidly charge mobile phones etc. and this will not be of much help for that purpose although, that said, four nickel/metal hydride cells charged using this circuit should be able to charge a mobile phone using a suitable controller.
Nickel/metal hydride rechargeable cells are now very cheap and our local 'Pound' store has them at two for a pound, albeit only 800 mA hour capacity, so there could be a regime whereby devices around the house that use AA cells over a couple of weeks or more could have discharged cells replaced immediately from the charger.

As for recharging alkaline cells I have to concede that the more cynical may say "What's the point" and indeed I have just purchased a 24 pack of alkaline AA cells for just four pounds Sterling so point taken. However, I think that it's good fun and may be laying in experience that might just be useful in the future