There are a couple of important things to consider when purchasing an e-Bike and batteries are one of them. Batteries are an expensive component and consequently, it is important to make sure you get this right.
Firstly It is important to get a bike that has quality cells in the battery. Budget permitting we normally try to get something that has Samsung, Panasonic, LG or another common brand name cell.
The next thing to consider is the watt-hours (Wh) of the battery. This is worked about times the volt (V) of the battery by the amp hours (Ah). i.e:
36V x 10Ah = 360 Wh or 48V x 10Ah = 480Wh
Watt hours is a good pessimistic way to estimate the range of a bike in the highest level assist or using the maximum power. As a guide, we typically divided watt-hours by 10. This typically gives a good guide on how many kilometres you will get out of the battery. i.e
500Wh will give you 50km. 600Wh will give you 60km.
Ultimately the more you help the bike and the lower the assist the further you you will get.