The circuit board contains the charging and battery protection circuits. At the heart of these is IC1, a 6 pin IC type DWP01-P. This has 3 functions: halting the charge when the battery voltage reaches 4.25V, preventing over-discharge if the battery voltage falls below 2.4V, and over-current protection.

A failure to operate or to charge (assuming the battery is good) may be due to this IC or any of the components around it, becoming faulty.

NPN transistors Q1, Q2 and Q3 can be tested with a multimeter on the diode range. First, disconnect the battery. With the positive probe on B and the negative lead on either E or C, it should read around 0.7V.

Q4 is a PNP type which can be tested in exactly the same way except with the negative probe on B. Q5 is a N-channel MOSFET and cannot be tested so easily.