With that in mind I'm gonna crib your proof of concept and the battery control module and prototype a larger-scale pack.
The 20A 3-cell BCM you selected you can technically double the number of cells from three to six by running two sets of three in parallel. My major caution here is that this then means you cannot (and really you shouldn't ever be) mixing and matching lithium cells.
I'm pulling cells from a batch which after a charge and sitting for a week the best and closest reading cells are being used.
If you have ever wondered, six 18650 cells will fit *perfectly* inside a GRiD pack, with room on one end and space above, so we're on the right track!
I'm placing some foam underneath the cells and at the rear end so the pack is firmly held in palce. For this prototype I don't have access to a spot welder or ribbon so I had to make do with wire. First thing of note is you MUST have ribbon spot welded to the ends. There's simply not enough space to be attaching wires, much less soldering to pre-welded tabs. I'm also using cardboard to layer the wiring which adds thickness but ultimately everything does fit inside the pack and the two halves can be glued together without a fight beyond making sure they are pressed together.


Personally, after modifying a pack like this I'd put a caution sticker somewhere indicating this is now a Lithium-ion device instead of Ni-Mh/Ni-Cd. The energy density of this pack is a
LOT higher than GRiD could of ever imagined and you're in for a bad day if something about the pack now fails or the terminals are shorted. I would seriously consider at least one safety fuse somewhere.
On the table with more or less charged cells I'm seeing 12.5vdc at the terminals which verifies 0xDEADBEEF's experiment and I guess should be fine.
Now be aware I have NOT tested this yet. I don't see why this will not work but I'd really like to test and see how it handles discharging and charging, presumably without bursting into flames. Unrelated work is going to keep me away from my desk for the next week or so.