This may be the battery power is basically gone, if you put the battery for a long time is very likely, the universal charge is not large output current, the mobile phone battery power is gone resulting in too much charging current will lead to the light is not bright, there may be some reason the charger output current is smaller, this needs to be tested to confirm, Or you use a battery to try if you can, that is the first case, use the table to check the output voltage, normal should be 4.2 volts, if the voltage is wrong is the charger problem