for example, lets say i have a battery pack that is starting to get weak, it will no longer hold a charge for a long time and when it is dead it only takes 10-15 min to charge or the charger light turns green almost instantly when i try to charge it. this is a "weak" or "dead" battery pack and it needs to be replaced. so if i were to add a brand new fully charged battery pack to the old pack the new battery pack would have to work really hard to make the old batteries perform better than they could, thus damaging the new battery and severly shortening its life.
you now have a brand new batteries that act, perform and charge like the old battery pack.
if you want to do this it is recomended that you add a new battery when the scooter is new or you have to replace all the batteries in your scooter so you can have all the batteries the same with the same usage and age in the pack. this will make your battery pack last a long time ( about 2-4 years depending on usage, charging practices, and various other things that batteries needs. they really are a fickle bunch)
Speed, Distance, Torque... Pick any 2