This issue is usually caused by the wrong charger which does not match or malfunctioning. Please check the label of the charger to see if the voltage matches the battery. When the charger is connected to a power source, the indicator of the charger is green. The indicator is red while charging. If not, please replace the charger.
The actual voltage of the battery is lower than protected voltage. Please check the battery voltage by using a voltmeter. The standard voltage for a 36V battery is 32V-42V, for a 48V battery is 42V-52V.