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24v NiMH Battery Charger Using a 12v Solar Panel

September 7, 2011         A while back I received an email from a guy in the UK.  He wanted to use a small solar panel, designed to charge 12v lead acid batteries, but use it to charge a 24v battery pack.  His 24v battery pack used 20 AA size NiMH cells.  He went on to explain that his 20 cell NiMH battery pack was used very infrequently.  He used the pack to open and close window blinds powered by a brush motor and controlled by a rocker type toggle switch.  He said that the motor would draw about 2 Amps from the 24v pack for 20 seconds for each operation.  He also said that typically the blinds were operated only 4 times each day; two times to open and two to close.  He wanted to know how to charge the higher voltage battery using the lower voltage solar panel.

 12v 1.5 watt Solar Panel    10 AA Cell Battery Holder
There are lots of low power 12v solar panels available but fewer 24v panels.  A typical example of such a panel is shown above.  These devices are often mounted on the inside of a carís wind shield and with the aid of a cigarette lighter adapter, can keep the carís 12v battery charged.  This is handy when the car is parked at say an airport lot for weeks or even months at a time.  Many car manufactures also use these things to keep the battery charged when the car might be at a loading dock for long periods.
The typical panel has 32 solar cells.  This kind of panel typically cranks out 1.5 watts to 5 watts with an open voltage of about 18v and about 15v under load.  In the typical mode, the panel pumps a constant current into the battery.  Normally, a diode is placed between the panel output and the 12v battery to prevent current from leaking back into the panel from the battery at night.  The low current from the solar panel during daylight hours is not enough to harm the massive car battery, which usually has an Amp-hour capacity about a hundred times higher the charge current from the solar panel.  Solar panels are expensive so using just one panel with a voltage boost circuit would cost a lot less than the cost of two panels. 
A battery pack consisting of 20 AA size NiMH cells will produce a nice 24 volts.  But, most NiMH cells require about 1.36 volts to achieve full charge.  That means a voltage of about 27 volts would be needed to charge the pack of 20 cells.  This voltage is much higher than available from the 12v solar panel.  Fortunately there are lots of ways to boost the voltage.  A switch mode type DC to DC converter could be used to take the lower voltage from the solar panel and pump current into the battery at a higher voltage.  A voltage doubler circuit could also be used.  The 100ma from the solar panel would be transformed into about 50ma into the battery pack.  Figuring about 6 hours of sunlight each day, the circuit should be able to pump up to 9 watt-hours of energy into the battery.
The guy in the UK said that the motor load required 2 Amps for 20 seconds, 4 times each day.  Take 24 volts times 2 Amps and you get 48 watts.  Multiply that by 80 seconds and you get 3840 watt-seconds.  Divide this figure by 3600 and you get 1.07 watt-hours.  So, the 1.5 watt solar panel, even if it received only 42 minutes of sunlight per day, would keep up with the energy demands of the window blind motor.
I decided the voltage doubler circuit below would do the job.  A 555 timer is configured as an oscillator, powered by the voltage from the solar panel.  The square wave output of the oscillator is connected to a two transistor push-pull driver circuit.  The output of the transistor driver is connected to a voltage doubler circuit.  The 1uF capacitor C3 is charged and discharged at the rate of 40KHz.  This pumps current into the 24v battery.   The 50ma of average current is about 50 times less than the 2500ma-hour capacity of a typical AA NiMH cell, so the battery should be able to handle the excess current, even after the battery is fully charged.  This circuit might also work well if a 6v solar panel were used to charge a 12v battery pack.

12V to 24v doubler designed by David Johnson, P.E.


Voltage Doubler Circuit

Click on Drawing Above to view PDF version of Schematic


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