The popping sound made when plugging the battery pack in is normal and nothing to worry about. The sound is made from a spark between the terminals when they are first connected.
The most likely cause of the problem is a worn out battery pack. The Rhino 800 batteries are only good for 200-300 discharge and recharge cycles and after a year they might be getting pretty close to that. Some batteries wear out faster than others so it i not unusual for a replacement to be necessary after a year.
There should be a battery level indicator on the throttle which shows the Voltage level of the battery pack. If the indicator drops down when the scooter is going uphill that is an indication that the battery pack is dropping Voltage under load which is sign of a worn out battery pack.
If the battery level indicator is not dropping down when going uphill then the problem might be something else however it can get more complicated to diagnose at that point so lets hope it is just a worn out battery pack at this point.
If you have a multimeter a more advanced load test on the battery pack can be performed using the battery pack load testing guide which is available on this page: https://electricscooterparts.com/loadtest/index.html
However the battery level indicator on the throttle is in essence a Voltmeter and will work to load test the battery pack almost as well as a multimeter would.
Please let us know how it goes, or if you have any questions.
