Updated: Jun 8, 2019
I started off trying to work out where to site the batteries I would be making. Once the site was chosen I'd have a better idea as to final available / required dimensions. I had already played about with weights etc and set any pre-requirements I had :
 they must be removable for security, off-scoot charging and protection against extreme temperatures.
 being removable they need to be pretty light - around older laptop weights; 1Kg (plus or minus) would be great. For me 'lighter' not only means more easily carried about but less likely to be thumped around when placing them down etc and more manoeuvrable.
 two (or more) smaller packs would tend to suit the previous requirements whilst benefitting from more spaces on board the scoot (there seems to me to be a lot of 'small' spaces available but few large ones on board the scoot) and also lends them more readily to be re-tasked between projects.
 maintain as-low-as-possible a centre of gravity; a top heavy scoot is horrible to handle both in traffic and whilst re-positioning it by hand
 if at all possible, maintain the helmet area below the seat. Having said that, on this scoot any helmet would have to be a pretty small open face or half helmet if it were to fit at all !
Anyways, I had already selected 72V : 13-14AHr per pack equating to a 20S4P (1Kw) minimum pack power / size. Each pack was to be thermally protected and be able to be individually and / or group charged.
Initially, I had hoped to build two 'long and slim' (20 cells long, 4 cells wide) packs and site them under the riders' feet, within the fairing area. Unfortunately, without massive frame mods, there simply wasn't enough space for this to be feasible.
Next I looked at using the space vacated by the recently removed fuel tank (although, in reality, too
high - from a centre of gravity point of view.). The main (and pretty much insurmountable) problem here though, was that the seat lock bracket was in the way; The two packs would fit but they'd not be removable. That coupled with the height 'issue' meant option 2 was, again without major grinding and welding work, a dead end too !!
Then I looked at designing and building two exhaust-style 'cans' - one for each of the battery packs and either side of the scoot... a wee bit drastic but definitely worth investigating. Maybe at a later date.
In the end I decided to fit them directly above the motor; in the area below the seat, at the very bottom of the seat 'box'. It would mean removing much of the base of the seat-box - so that they could be taken out (for in-house / external charging) and put back in again - but this was the most feasible of the options I'd looked at...
Once I had the location sorted it came time to make a bracket/shelf arrangement for the two battery packs to sit in above the motor. Initially, I made a mockup in cardboard then in some left-over 10mm EPA foam that was lying around. Once that seemed to be OK, I re-made the bracket so that the original would fit inside it, removed the original and fibreglassed up a shelf unit complete with side and fixing points to locate it securely on the scoot framework whilst, at the same time ensuring the sufficient room for the motor to be rotated for chain tensioning... A bit for sanding and painting and...
With the battery mock-ups now fitting snugly in their new 'home' on board the scoot I gingerly cut, re-cut and cut again (and again - little by little) the base of the under-seat helmet / storage box to allow their insertion and removal. That all done, it was time to move on to making the battery packs. I have discussed the 'whys' and 'wherefores' in the previous project section (if you're interested in why I made the battery manufacturing decisions that I did make - for good and bad ! - they're available in a previous project section, "LiPo Battery Packs", dedicated to just this. It's a long read but... )
Anyways, after gathering together the parts :
NCR18650GA batteries, SplitPort BMS, Thermal CB, Charge Plug/Socket & Anderson connector
... I began, with the help of the Spot Welder I built (in the previous project section 'e-Weld'), SketchUp and my 3D Printer to build the battery packs.
I ended up :
...Building the packs - 20S4P configuration...
I decided to spot-weld (not solder) and fuse the cells (top and bottom / anode and cathode). I also decided to go with a split-port BMS and a thermal, 50A circuit breaker for output protection.
I designed in 2 x thermistors and placed them 1/3 of the way / 6 cells rows in from each end-cap. The cables were routed to the CB end-cap and left insulated for a future date add-on project. The idea being to enable battery cooling / heating during charging and battery cooling when running and too hot. I haven't gotten round to this just yet but, when I need a break, I am programming and testing the AVR code.
...Designing, printing out, populating and connecting up the end caps...
(I designed the end caps with three ridges - for the heat shrink to grip onto)
...and putting them all together before applying 3mm foam protective sheet and heatshrinking the lot...
The final items :
NCR18650GA cells are 'C' rated at 3340mAHr. 'C' effectively equates to capacity (measured in AmpHours).
In the packs (designed & built above) there are 4 x NCR18650GA cells in parallel. That produces a total of (just over) 13AHr. They are rated at 3C continuous (10C max) discharge and 0.7C designed charge current which equates to a 9A charge current PER battery pack, therefore, a 10AHr charger would be fine (seeing as a 9Ahr charger isn't readily available and 1 additional AHr shouldn't cause an issue) for these packs.
Furthermore, whilst each pack has a 'C' rating of (just over) 9AHr, the PAIR of packs onboard the scoot will have a 'C' rating of (just over) 18AHr, so a 20A charger COULD be used IF the battery packs were to be charged as a pair (with a similar caution as per the 10AHr charger for a single pack).
I purchased (quite a while ago) a 72v 5AHr charger, however, should I wish to install an onboard (on the scoot) charger than to minimise charge time a 20AHr unit would quadruple the available charge current and (a little simplistically) speed up the re-charge time accordingly - if the room is available at the end of the project.
Either way, I don't foresee an immediate need to have 'high speed' charging and intend (at this point in time) to use the 5AHr charger for both onboard (dual / paired pack)and in-house (single pack) charging. As as 'aside' the 5A charger I purchased was $60 (USD) whereas a 20A version would be around $160 (USD) - not a huge increase for a far quicker re-charge - if required. The real 'downside' is :
the 5A charger comes in at around 1Kg and has dimensions around 200mm x 110mm x 60mm
the 20A version comes in at around 3+Kg and a ballpark size of 330mm x 175mm x 115mm.
Finding enough room for the 20A charger onboard the scoot would be much more difficult - in my case ! At some point though I'll do a more in-depth study and comparison of 5A, 10A, 15A and 20A chargers (in regard to size, weight and cost)... but not today... I'm still on a mission to finish this scoot !