This is a mini UPS based on TI’s BQ24650 Li-Ion battery charger and Liner’s LTC4353 load switch. The BQ24650 is essentially a multi-chemistry battery charger specifically meant for solar panels. It can charge upto 4 Li-Ion batteries. Other chemistries are also supported. This specific instance is configured to charge 4 18650 Li-Ion batteries.
BQ24650 features the MPPT (maximum power point tracking) technology. Solar panels typically have a maximum power point curve. This curve is defined by the available light that falls on the panel. This defines its output voltage and current (the V-I curve). Li-Ion batteries are extremely finicky about maintaining a constant current and voltage for their charging. Which is why a solar panel (even with a regulated output) cannot be directly connected to the charger circuitry. The BQ24650 tracks the solar panel’s V-I curve to maximize the power available for charging while maintaining the battery’s charging profile.
I needed an always-ON system. This meant that the battery source and another mains power source needed to be switched to achieve UPS like functionality. This is achieved by the LTC4353 which is an ideal diode-OR. The battery charging schematic is based on the reference design provided by TI. If you need just the charger, you can omit the LTC4353 section all together. Separate outputs are provided for both sections are provided.
Note: This design is untested.
PCB layouts are coming soon.