I have a project planned to convert my cargo bike into an eBike. I've settled on a TSDZ2 mid drive motor with torque sensor and can source this for a very reasonable price.
I also need to source a battery and was looking for recommendations of where to buy. I don't mind local bricks and mortar or online, just needs to be good quality and preferably at good price.
Best on-line source I've found (by far) has been EM3ev.
Can't speak for local suppliers but maybe Mike at MiCycles?
I've found Paul at EM3ev to be very helpful, but shipping batteries from China is now a major drama. Basically you can only ship them in shipping containers so it takes a long time.
You could also build the battery yourself, sized in accordance with your requirements. (Distance, load, speed, hills.) The analogy is "fuel tank size". (Budget for no more than 80% of watt-hour rating in service.)
LiFePO4 cells; (Safer that Li ion or Li polymer, but slight lower voltage. Also increased charge/discharge cycle life.)
Battery management system - essential. (Balances cells, prevents over-discharge, controls overload.)
I've recently built myself another e-bike and settled for a 10Ah "Dehawk" bottle shaped battery. It's main claim to fame is its ability to be easily demounted for charging away from the bike. One flick of the key and one hand to flip it out of the carrier. The contacts are large and robust. They are available in 36V 10Ah and 14Ah, from ebay or Leon Cycle, online. I intend to strip out its guts one day and replace the Chinese cheapie cells with top grade Panasonic cells, should nearly double the capacity for no extra weight. That'll be an expensive exercise but useful if I want to travel further than around 50km assisted per charge. I've done around 1000km on it so far and it's not showing any signs of deterioration yet.
The battery came with a nice simple mount and charger, so foolproof to use. I've experimented with another battery that's on a slide on mount, it's extra fiddly to get it to slide on as the mount is flimsy. I wouldn't go for LiFePO4 any more as they are quite a lot heavier and probably harder to get in a quick fit format.
Oh and I fitted an inline power meter (ebay again) so I can see exactly what I've taken out of the battery, much more accurate than some voltage based level gauge. David's suggestion of 80% discharge matches the point at which my battery starts to sag in output, so that's far enough for me.