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Make a simple solar charger for 4 AA rechargeable batteries

In this article you will find out how to make a very simple solar battery charger for 4 AA rechargeable batteries using a small 6 Volt solar panel. A labeled photograph of the completed charger is provided at the end of the article.

Here is the parts list:

• 1 of 4 x 2700mah AA Rechargeable Batteries.
• 1 of 6V 250ma Solar Panel.
• 2 of 2 AA Battery Holder with Flying Leads.
• 1 of Blocking Diode.
  

The Solar Battery Charger Specifications
The long life rechargeable batteries used have a capacity of 2,700mah - therefore a charging current of 10% of this (i.e. 270ma) is safe. The nominal voltage of each battery is 1.2V, so four in series is 4.8V with a fully charged voltage of around 5.2V being normal. Therefore our 6 Volt 250ma solar panel is perfectly rated to be used as a charger for these batteries.

In order to prevent stored power in the batteries being released through the solar panel during the night, a blocking diode is used. Placed in the postive line from the solar panel this only allows electricity to flow from the solar panel to the batteries and not from the batteries to the solar panel.

Up to 0.7 Volts from the solar panel are lost as heat in the diode as electricity flows from the panel to the battery leaving us with a perfect charging voltage of around 5.30 Volts for the batteries.

Putting the Solar Battery Charger Together
To keep things simple no soldering is required to built this solar battery charger. Instead terminal strip is used to make the connections - only a small flat head screwdriver is required to secure each wire/component in place.

First of all the four batteries are put into the two battery holders and then the holders are wired together in series. To do this the positive lead from one battery holder is connected to the negative from the other. The remaining free wires - one positive and one negative - are our charging inputs.

The ringed end of the blocking diode (pictured above) is then connected to the positive battery input (red), and the other end to the positive output from the solar panel (red). The negative output from the solar panel (blue) is connected directly to the negative battery input (black) .

Pictured above is the completed Solar Battery Charger. The voltage measured across the points labeled B and C is the voltage coming in from the solar panel.

The voltage measured across the points labeled B and A is the voltage of the batteries. (Note that during charging the battery voltage measured will be higher than the true voltage of the batteries. Cover the solar panel to measure the true battery voltage.)

A suitable multimeter is required to make these measurements.

A Solar Charge Controller (or Regulator) is a device which protects the batteries in a solar electric system from being overcharged or being over-discharged.

Overcharged batteries have a much shorter life time than well cared for batteries since the electolyte is boiled off as gas and lost. Overly discharged (i.e. flat) lead acid batteries become permanently damaged, so a solar charge controller is used to disconnect any load when the battery is discharged down to a safe cut off voltage.

LEDs and (in the more expensive regulators) LCD displays are used to show the current state of the charge of the battery, the state of the current entering the batteries from the solar panel(s), and the current being drawn from the battery by loads.

Do You Need a Solar Charge Controller?
A charge controller is not always required if the maximum current going into the battery is not more than 10% of the batteries Ah capacity (C/10) - i.e. if the battery is 60Ah then a charge controller is certainly required if the current from the solar panel is likely to exceed 6 Amps - as long as you make sure you don't over-discharge the battery.
Some suggest a charge controller is required if the maximum current from the solar panel is just 1% of the Ah capacity (C/100) of the battery bank.

PWM - Pulse Width Modulated Battery Charging

Good quality solar charge controllers are up to 30% more efficient than basic 'on-off' regulators as they have provide pulse width modulated (PWM) battery charging. When the voltage of the battery bank reaches the regulation voltage (14.1V), instead of stopping charging altogether, the patented PWM algorithm reduces the charging current and continues charging slow enough to prevent the battery overheating or gassing.


PWM helps to get the batteries charged up to well over 90% capacity, extends the life of the battery, and means that more of the power generated by the solar panels is stored. Since the batteries store more energy on average, a smaller battery (or less batteries in a battery bank) can be used reducing overall system costs.

Solar Charge Controller Manufacturers
Well known manufacturers of solar charge controllers include: MorningStar, BZ Products, Iota, Outback, RV Power Products, ProStar, Solar Converters, Statpower/Xantrex,Trace/Xantrex.

Buy a Solar Charge Controller in the UK
Prices range from around £15 to many hundreds of pounds depending on the size of the solar power system to be regulated and the quality and features of the controller.

With solar charge controllers you really get what you pay for. It is well worth spending a few pounds extra to buy a good quality PWM controller from the start since it will pay for itself by getting more energy into your batteries and by protecting the batteries properly.