Solar powered battery charger using LTC4071
A simple Solar powered battery charger , photovoltaic charger circuit can be designed using LTC4071 Li-Ion/Polymer Shunt Battery Charger System with Low Battery Disconnect .
At low VCC voltage, photovoltaic current flows to both the system at VCC as well as the battery.
When VCC reaches the programmed float voltage (4.1V with ADJ floating) then the LTC4071 shunts excess current not used by the load, limiting VCC to 4.1V and effectively reducing the battery charge current to zero. If the photovoltaic cells stop supplying current, the battery supports the load at VCC through the LTC4071.
Add a diode in series with the photovoltaic cells to prevent reverse leakage of the photovoltaic cells from draining the battery. If the battery discharges to the point where VCC falls below VLBD (3.2V with LBSEL tied to GND) the LTC4071 disconnects the load from the battery to protect the battery from over discharge.
For this photovoltaic cell charger electronic project you can use JAMECO 171061 photovoltaic cells and NTHS0402E3103LT thermistor .