That is for 6 volt charger use 6.1 volt Zener and for 9 volt charger it should be 9.1 volt Zener. The circuit is based on the switching of two NPN transistors (BC547) to drive the corresponding LED.

One advantage of this little circuit is that we can use any ordinary 5VDC mobile phone charger ( or a 5V solar panel ... for charging the battery. Transistor T1 (BC547) is wired as an electronic ...

The following automatic battery-charger design is created with a circuit that could qualify as the simplest window comparator ever built around a single transistor (see the figure). It starts ...