woosh don't charge as much as you said they do. Check their web page:
http://wooshbikes.co.uk/?cdkit
They recommend only their 36V 15AH battery for use with the BBS01/BBS02 up to 500W because any battery with smaller capacity does not have high enough continuous discharge rate and they don't apparently sell 750W, so you have no competition there.
http://wooshbikes.co.uk/?cdkit
They recommend only their 36V 15AH battery for use with the BBS01/BBS02 up to 500W because any battery with smaller capacity does not have high enough continuous discharge rate and they don't apparently sell 750W, so you have no competition there.