Compact size, perhaps 'ziplock bag' batteries would be better. Think they are called Prismatic.
Be easier to parallel and series connect them.
I will see what I can find, OSN Power has some I remember. I see 3.3V 20Ah packs, but I dont see any dimensions.
BMS Battery - https://bmsbattery.com/ebike-battery/640-high-c-rate-10ah-li-nicomn-li-ion-cells-battery.html
3.7V 10AH
Dimension: 6.4x60x260mm
6.4mm High
60mm Wide
260mm Long
Stack 10S high to make 36V(nom) 64mm high then 60Wx260L
That is workable.
13S is then 48V(nom) and 83mm high. Which would be noticable on that bike.
You could most likely put 2 length wise (520mm L)for a decent 20Ah.
Whats the average length of a down tube for 26" bike?
????Front-Center????? 650mm(Small) to 725mm (XXL)
https://www.specialized.com/us/en/bikes/mountain/trail/rockhopper/rockhopper-29/106525
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So if BMS Battery is selling a 8.4mm x 60mm x 260mm pouch rated at 10Ah and 3.7Vmax 3.3Vnom
I just eyeballed my Makita's 5S2P 3Ah (1.6Ah cans), but you get 3.5Ah cans thats 7Ah total, can fit 6 of those in the same space as 12 pouches, netting double the Ah's.
I can only assume OSN Power would be about the same size. Feel safer with cans anyhow, in a metal box.
3.5Ah cans from NKON will be my next purchase.