Battery charging from a 1V or 2V solar panel

For battery-powered applications, such as remote data logging, it is often necessary use solar power for battery recharging.

Unfortunately, solar panels are expensive, currently retailing at around $50 to $90 for a typical 12V 2W panel. Many such applications only require a fraction of this power and this could be provided by much cheaper low voltage solar modules of 1V or 2V.

However, the low voltage outputs of these modules are too low for directly recharging batteries.

What is needed is a cheap circuit that will boost these low voltages to more useful levels. The circuits shown here will do just that.

These two circuits can also run off one or two dry cells, for example. They will operate from voltages down to about 0.6V but also up to about 6V. In both circuits transistor Q1 and transformer T1, together with associated components, form a flyback converter.

Feedback from the output of the converter back to Q2 regulates the output voltage. Fig.1 is the "bare-bones" circuit which works quite well.

The circuit of Fig.2 is essentially the same but op amp IC1 and a voltage reference diode have been added to improve the voltage regulation.

The efficiency of both circuits varies typically from 50% to 70%, depending on the load current. They can deliver up to 70mW of output power with 1V input and the output voltage can be adjusted with trimpot VR1 to as much as 12V, at light load currents.

If used to charge a battery, either of these circuits will draw some current from the battery during periods when there is no input power.

If this is a problem, then connect a diode between the output of the circuit and the battery to prevent reverse battery current.

H. Nacinovich,
Gulgong, NSW. ($60)