Aha the batteries for m/bikes are not very big in ampH,
so I'm not supprised they are running down, If you need to buy batteries of that size, try a Disability shop, they very often have high amp ones as used on mobility scooters and sell second hand ones for about a fiver, failing that you can buy leisure ones from halfords local to yourself.
I don't have an online link, but mind the physical size of them, make sure it will fit your trolley system, the higher the ampH the better.