Hi all,
I am new here and this is my build:
bike: specilized stumperjumper fsr 2007, al frame
motor: bbs02 750W
battery: 48V 13Ah, 65cells LG M26
battery is in large shark case, it is in backpack for now as it cant fit on downtube. I ordered smaller shark casing 52cells, will go without cell holders and place 56 cells 14s4p, so it will be 52V 10.6Ah and it will fit on downtube.

last picture shows how I solve power cut off, basiclly it is power on demand, this is momentary switch taken from brake lever that came with kit, this is working great as I ride this bike mostly on trails. I wil eventually go for palmrest handles and integrate this switch better in handle or change switch for some smaller one. This works simple, when holding switch pressed you have power, as soon as you release it you have power cutoff.

Update: 3.9.2017.
I finally received smaller shark case (52 cells), as I had case with 65 cells. I swap batteries this weekend into smaller casing (without cells holders, so I fit 56 cells 14s4p) and install 52v battery on down tube. it fit perfectly, here is picture.

I had to make modification on cradle to bolt it onto downtube, but nothing major.

Finally I made saddle bag battery 14s2p using samsung 30q cells. Battery is just great, it fits in topeak medium bag.