KT-S02 — 36V Seatpost Battery, 21700-Compatible

The KT-S02 is the KT-S01 with the constraint removed. Where the KT-S01 is 18650-only in two lengths, the KT-S02 accepts a 21700 holder as well and runs in three post lengths — 350mm, 480mm and 610mm — which puts five configurations on one platform and takes the capacity ceiling from 7.5Ah to 15Ah.

The full set: 350mm gives 36V 5Ah on 20 18650 cells. 480mm gives 7.5Ah on 30 18650 cells, or 10Ah on 20 21700 cells. 610mm gives 10Ah on 40 18650 cells, or 15Ah on 30 21700 cells. Note that 10Ah is reachable two ways — a long 18650 post or a shorter 21700 one — which is genuinely useful when the seat tube is the binding constraint rather than the bill of materials.

Cell format is a tooling decision here

Each cell format uses its own holder, so unlike the down-tube platforms — where an 18650 and a 21700 block drop into the same case — the KT-S02 commits at tooling. Choose the format alongside the capacity target and the tube depth, not afterwards. Our 18650 vs 21700 guide covers the cost and cycle-life side of that decision.

What 540Wh in a seat tube is for

Fifteen amp-hours at 36V is roughly 540Wh — double the KT-S01 and enough that a concealed-battery city bike becomes a genuine commuter rather than a short-hop bike. It is still a fraction of what an external down-tube pack carries, and it should be. The reason to specify this format is that the bike does not look electric and the frame stays clean; range is what you spend to get that.

Options and documentation

Integrated tail light and bottom-discharge outlet are both available, and bottom discharge is the one that matters most on concealed builds — it routes the harness out of the lower end and into the frame instead of past the saddle clamp. Clamps are available plain or lockable. CE, UN 38.3 and IEC 62133 files are issued against the exact length and cell format ordered.