First, the watch still did not start. It turned out that the ground between the battery ground lead connection point and the watch chassis is broken. I fixed it with a jumper wire.
Second, the watch warmed up a lot when the charger was connected. The USB connector's surge protector was short-circuited. Its type seems to be Semtech SMF05. It can be found near the vibration motor. I don't know what had happened. Now everything seems to be fine.
