Lead-Acid Batteries Charger with Solar Panel

This Batteries Charger is intended for charging sealed lead-acid batteries with a solar panel in small and portable applications. The customary diode that prevents the battery from discharging through the solar panel has been replaced by a FET-comparator combination.


The batteries charger will stop charging once a pre-set voltage (temperature compensated) has been reached, and recommence charging when the voltage has dropped off sufficiently. The load is disconnected when the battery voltage drops below 11V and reconnected when it gets back to 12.5V.
Solar Batteries Charger Schematic

The batteries charger circuit has the following features:

• Charges until Vbat = 13.8V (adjustable), then float charges;
• Shuts down load when Vbat < 11V (adjustable), resets at 12.5V; 
• Temperature compensation; 
• Will work with cheap and readily available components like LM393 comparators and BUZ11 FETs; 
• Uses less than 0.5mA when using TLC393 comparators; 
• Burns less than 20mW in FETs when charging at 0,5A. (More expensive FETs with a lower RDSON will yield even better results). Note that the charging current is limited only by the solar panel used.

Solar Batteries Charger PCB

 
Note that I added three DC/DC-converters (for 9, 6 and 3V) on the PCB, the actual charger is less than half of the PCB. I don’t have a silk screen, so if you want to build this, you’ll have to figure out the PCB for yourself.
With all the components into place, some additional electronics to get the DC/DC-converters in and out of standby, two small SLA-batteries (2.2Ah each), some odd parts and wiring, a fuse, a front plate and housing.

 
A final word: you build everything at your own risk, no functionality is implied, the resulting apparatus might not work, be unfit for drying pet dogs, and contain small parts that could suffocate children when swallowed or inhaled.