The LT3650 circuit is a monolithic two-cell Li-Ion/ Polymer battery charger that operates from 9V to 32V input voltage range (11.5V minimum start-up voltage). The LT3650 si produced by Linear Technology and provides a constant-current/constant-voltage charge characteristic, with maximum charge current externally programmable up to 2A, set using an external current sense resistor.

So a battery is charging during daylight from a solar panel and can discharge through solar panels during night. This problem can be prevented with a diode but this has a disadvantage so we use this switch.
This solar panels switch circuit replaces the diode and connects the panel to the battery through a relay contact. This is powered by the panel. When the supply voltage is low, the relay wont act so the battery is not connected to the solar panel.

This circuit is an alternative to expensive battery chargers, the trick to lower the cost is to buy a cheap battery charger and expand it with an automatic charge breaker circuit such as the one presented here.
The heart of this automatic battery charger is the comparator which compares the NiCad battery’s voltage with a referance voltage. When the NiCad battery’s voltage exceeds a certain presettable maximum voltage level, the circuit breaks the charging via the relay and when the NiCad battery’s voltage sinks below the preset minimum level, the circuit closes the relay and the charging resumes.