I do not know how you measure the charging current is negative, if it is measured with a meter, it may be the watch hand is put backwards. If you do have a negative number, you can only do it in the following two situations:
1, the battery voltage is higher than the charger voltage, this situation is basically impossible, because even so, the charger circuit will not allow backflow;
2, the charger output or short circuit in the line, if so, your battery should be very hot, soon to the problem. If it is not the watch hand detection problem, it is recommended to take the charger to the repair shop to check.