Most electric scooter and bike battery chargers have a green light and a red light. When the charger is plugged into the vehicle and into the wall the red light indicates that the battery pack is charging, and the green light indicates that the charger is plugged into the wall. When the red light turns off and only the green light is on that indicates that the battery pack is charged and the vehicle is ready to use.
If the charger light stays green and there is no red light when the charger is plugged into the vehicle and into the wall this could be caused by many things. It could be from new batteries that have never been used and have a full charge, worn out batteries with low Voltage that need to be replaced, a loose wire or defective connector between the battery pack and charger port, a defective charger port or battery charger plug, a burned out fuse in the charger, or a defective battery charger.
To figure out the problem first test the Voltage output of the charger. The chargers Voltage should be a little above the Voltage of the vehicle (a 24 Volt vehicles charger should output around 29 Volts, etc.). If the charger output Voltage is correct then the charger and charger plug are usually good.
If the charger has no output Voltage then
check to see if it has a fuse and replace the fuse if necessary, then
retest the charger. If the charger does not have a fuse then it may need
to be repaired or replaced.
Next test the Voltage of the battery pack at the charger port. The battery pack Voltage should be a little above the Voltage of the vehicle (a 24 Volt vehicles battery pack should output around 27 Volts, etc.). If the battery pack Voltage is correct then the battery pack, charger port, and wiring between the battery pack and charger port are good in respect to the battery charger operating properly. However just because the battery pack has the right Voltage at the charger port that does not mean that it is good because its Voltage may drop under load when the vehicle is on causing the vehicle to not operate. The battery pack's Voltage could be tested at the charger port with the vehicle on and throttle engaged to see if it drops Voltage under load.
If the battery packs Voltage is lower than the Voltage of the vehicle then that could cause the battery charger to not charge the batteries and the red light to not turn on. Good battery packs will always bounce back to their normal Voltage range within a few minutes after being used. Battery packs that do not bounce back to their normal Voltage range after being used are defective and most battery chargers will not charge them if they detect this. The reason chargers will not recharge bad battery packs is because charging them could cause them to overheat, swell, and possibly leak which could cause damage to the scooter or its surroundings.