I was on here last year asking about what bike to buy. I ended up buying a 5303 hub motor and controller from ebikes.ca. I'm very happy with it. Let me say that if you do buy a 5303, make sure you buy a big enough battery for it because it's kind of hard to pedal. Other motors are not quite as hard to pedal as the 5303 but the 5303 has way more acceleration than other motors. Basically, if you don't like to pedal much, the 5303 is great for that.
I just bought a lifepo4 battery from elitepowersolutions.com. I'm very happy with it. The cells are very balanced. I'm not even using a BMS with it. I took a ride on it today. I came back and checked all the cells and they were all at 3.36v. I only went 3.5 miles on it, though because I'm still conditioning the battery. If you only use about 50 % of the battery, you don't need a BMS.
My suggestion for anyone who wants to build a bad ass bike would be to buy a good motor like a 5303 or 5304. Then go to elitepowersolutions and buy 3 or 4 of the 12v40ah packs. Or go to evcomponents.com and buy the Thundersky 40ah cells. That way you got a lot of capcacity that you won't ever run out of it. These days, you can buy so much capacity for cheap that you dont' even need a BMS. Get a black and decker 12v charger, one that tapers like mine does and it will charge all the cells evenly. I got a black and decker 2/10/15 amp automatic charger and it tapers the amps as it closer to filling the cells so that even the weak cells get charged too.
So you'll have a 36v40ah or 48v40ah Thundersky lifepo4 pack. You'll never run out of capacity. You can get a 36v40ah Thundersky lifepo4 for the same price as a pingbattery 36v20ah pack. You can get twice as much with Thundersky cells. It just doesn't make sense to buy from Ping anymore in my opinion. It's up to you. Either you can use 90 % of your ping and kill it in 2 years or use 50 % of a 36v40ah Thundersky and have it last 5 yrs or more. It's up to you.