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A Boosted Solar Powered Battery Charger
Using a low voltage solar cell to charge an NiMh battery.

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Jules May spotted the White LED With 1.5 Volt power supply page and realized that this blocking oscillator flyback converter approach would be ideal for charging the battery in one of his projects from a solar cell. I'll let an excerpt from Mr. May's email explain it from here.

Figure 1. The few discreet components fit nicely on this hand-wired circuit board.

Thing about NiMH is that they're generally safe to overcharge, so long as the charge current is low enough.  But you don't want to overdo it!  So what this does is keep a trickle-charge going when thereís sunlight available, but shuts off the oscillator if the battery is showing signs of overcharge.

I found that driving the oscillator from a solar panel had its problems - the panel produces power which is variable both in current and voltage! A driver which worked fine at 1v (from a bench power supply) could get itself locked-up and unable to oscillate when driven from the solar panel, even when the panel had returned to a clean 1v.  For that reason, I found Bob Parrot's base capacitor essential.

This circuit works from a photo cell voltage from about 0.7v upwards, and Iíve had it drive batteries up to 12v (though this is just driving 3v).  When the photo cell voltage is above the battery (+ one diode drop) the cell charges the battery directly - the driver only starts when the battery voltage is above the photo cell voltage. 

Figure 2. Feedback through the Zener diode limits the charging voltage.
Both transistorsa re ZTX300 but most small signal NPN transistors should work fine.
The Zener diode (to the base of the lower transistor) is 4.3 volts, again small signal type.
The rectifier diode is an 1N914 or similar.

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First posted in September, 2017

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