Most e-kits do not come with batteries , 250w CONTINUOUS rating is the UK legal limit / 16.5 mph ( 15mph+- 10% )
The spec is "bent by some of the e-bike suppliers " since july the new bike specification is pedelec only without a throttle ..
BMS in china is one of the places I have used and Panda which is in in the UK and advertise on e-bay , Buy direct saves you a few pounds( as the prices add commission on
ebay )
Panda sell batteries packs either rack mount or drinking bottle mounts . On the e-trikes we manufacture our own mounts to run along side the main boom .
generally the batteries are supplied with a short pair of cables to allow you to match the controller cables to the battery with eith a supplied socket of one of your choice , several units use "kettle lead " connectors whilst others use a more refined plug and socket .
There are several controllers for sale with LCD displays ( Panda sell one ) that allow you to bypass the ON-ROAD limit for off road use by software .
D8VEH on the pedelec site is one of the experts on e-power and has converted many bikes including raleigh 20's up to a 3000 w hill climb bike with 2 wheel drive .I have converted several bikes and e-trikes to e-power and after 4 years I have gained enough experience to play with them .
E-power is not a black art but does need looking into to get the best performance and range . We use both LifePo4 and Lithium Ion batteries and have dabbled with Lipo's as we use these in RC electric power model aircraft .
THE MOST IMPORTANT PART IS TO USE THE CORRECT CHARGER , some of these batteries can supply hundreds of amps of current for a short while and do go BANG and catch fire Life Po4 and Lithium ion are the safer types to use .
you are looking about £220 - £300 for a fair battery and a little less for a Kit -
Also a crank drive will give you a better range of power than a hub motor
regards emma