The most common batteries on the Standard Chinese Ebikes over the last 14 years are the Silver Fish type that are attached to the seat downtube , they slide down a rail and are located with a plunger . Replacements are reasonably priced and plentiful compared to the scores of oddly shaped ones in the Video posted on here . For instance I purchased a YOSE one on Ebay UK for my Fat Tyre to extend it`s range about 5 years ago and I see the 36V 15Ah is still £190 on their site . This is considerably cheaper than replacements for the Named make Ebikes , that`s even if they are still obtainable .
I just googled Ebay to see if the Silver Fish type are fitted as standard to larger wheeled Ebikes and came across the ONESPORT OT 18 for £499 which appears to be a bit of a bargain . There are numerous 20 in. Folders with these batteries including the OT 16 from the same Manufacturer with a 48V 17Ah Battery for £599 . There are numerous others by Colourway ,Farfrees , Samebike, Hitway etc. with combinations of battery capacity , fat tyres and frame material .
So Steeve you don`t need to spend a fortune on a named make with the standard Chinese fare so plentiful on Ebay at reasonable prices .