Updated: Jun 4, 2019
This was the second choice I had to make pretty early on in the process. I had selected the motor - a 3Kw BLDC unit operating at a maximum of 72V. I now had to decide upon the battery requirements. My logic went like this :
 the higher the pack voltage, the lower the resistive losses and lower the current draw will be for any particular power requirement. Lowering the expected current draw reduces the inter-connecting cable diameters and, therefore, the costs of, pretty much, all additional components.
 battery pack voltage relates to top speed - higher voltage -> higher top / maximum speed (due to motor higher possible motor rpm).
 the IEC low voltage rating is 50V; voltages become more dangerous the higher they are. however, in reality, any voltage is 'safe' IF it generates less that lethal current. Having said that - any current over 10mA will produce a nasty shock and any current over 100mA is considered LETHAL. The 'problem' is that as voltage increases so too does its ability to pass or 'push' electrons through the human body - creating the inevitable 'electric shock experience'. I mused over this issue in the battery section of this site and determined that 72V, given reasonable handling precautions, should cause the average person no real threat. I am not pooh-poohing or trying to diminish the precautions one should take when handling ANY high capacity electrical source and, make no mistake, Li-Ion batteries are more that willing to give up their stored energy in fractions of a second given the right conditions.
BE VERY, VERY CAREFUL HANDLING LITHIUM ION BATTERY PACKS.
IF IN DOUBT SPEAK WITH AND EXPERT OR, BETTER STILL, LEAVE THE JOB TO THEM !
YOU HAVE BEEN WARNED - A 72V LITHIUM ION BATTERY PACK, UNDER THE WRONG CONDITIONS, WILL HAVE NO PROBLEM HOSPITALISING AND EVEN KILLING YOU !
The power requirement, in this particular case, is 3Kw or 3000Watts. The highest (nominal) voltage my selected motor / controller is designed for is 72V and, at this voltage, the nominal current drain will be circa 40Amps.
As I pointed out above, I covered my battery pack build, including chemistry selection (LiFePo4), interconnection methodology, final output protection and packaging in the Battery section of the site. If you're interested and have some time to spend, go read it - it's pointless my trying to duplicate it here.
The end-result for this blog post though, is that the battery pack I have opted to use for this build is :
 Lithium Ferrous Phosphate (LiFePo4) : not quite as energy dense as cobalt or aluminium based chemistries but FAR safer !
 3340mAHr 18650 cells from PANASONIC / SONY (3V6 nom : 2V8 / 4V2 cut-off)
 20S4P configuration : that is 20 cells in series and 4 cells in parallel (see the blog post image)
 individually fused - both cathode and anode
 spot-welded connections
 fully integrated / onboard split-port Battery Management System (BMS)
 thermal circuit breakered output (HV) protection with separate charge port / socket
 Anderson connectors on the output (HV) side
thus providing a nominal 1Kw pack : 13AHr at 72V nominal output voltage (fully charged @ 84V, fully discharged @ 56V)