The post below will help diagnose it if these simplified ideas do not get there.
First is to measure charging voltage. With the battery charged to full (overnight with a 2 amp charger, or smart charger that tapers the charge, measure the resting battery voltage 6.3 volts if a 6v battery 12.6 if a 12v battery. If lower, than 6.15, or 12.3, the battery is suspect.
Next start and run at 3/4 throttle, no lights. it should be at 7.1 to 7.5 volts if 6v, or 14.2 to 14.8 if 12v.
If still battery voltage, it is not charging.
(a needle style volt meter may be needed to accurately diagnose the voltage (digital meters tend to be wild on these noisy systems)
If it is wild and variable with a good meter, the VR might be going. Jim