Have read a lot of the posts, good information.
I have a 500W front kit from GM and it is powered by two 20V 6 ah batteries purchased from Canadian Tire for 129.99 each and 39.99 for a charger. Charging takes about 4 hours per battery. These batteries come with a one year warranty and an over the counter exchange policy. I could find no technical info about the batteries but they work!
The batteries weight 2.6 pounds each and are 4 by 2.5 by 2.75, very compact.
So far there seems to be no issue with the 40 volts on the controller as expected based on other posts.
Speed usually in the 25-32 Kph, range is 17-19 KM with mild peddling. I only put this thing together last weekend and have ridden just over 60KM.
The Kit was easy to install expect the tire is not fully round and the tube was replaced because I have no idea how to fill the tire with the strange nipple.
Total Cost to build, 360.00 for the kit, 99.99 for bike, 299.00 for batteries and charger, 10 for various electrical connections, tie wrap and such.
Thus under 800 and it provides cheap transportation.
Calgary Alberta