There are couple of important things to consider when purchasing a e-Bike and batteries are one of them. Batteries are a 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. I 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 general way to estimate the range of bike in the highest level assist or using the maximum power. As a guide I typically divided watt hours by 10. This typically gives a good guide of how many kilometers 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