“Handheld part” contains ht45r2w processor, charger contains ht46r005 processor. They communicate together. When this communication fails, the device won't charge. The errors are likely as the data wires are pretty crappy.
I have not tried to reverse-engineer the communication. I have removed the processor and connected my own charger instead.
The wires in the lower joint are pretty weak. The device is soldered together with horrible-quality lead-free solder. OTOH, the device is easy to disassemble and assemble (i.e., no glued parts, everything on easily accessible screws).
After three years, the processor decided to switch the device off immediately after it was turned on. I have therefore removed the entire circuit board and replaced it with a manual switch.