Long range e-bike battery — why a longer case stops helping
Short answer: a longer housing does not always give you more capacity — it depends on the cell format. On one of our shells the 18650 version comes in 4 steps from 40 to 78 cells, while the 21700 version has only 2 steps, 50 and 52, 2 cells apart. Lengthening that housing buys you almost nothing on 21700.
Short answer: a longer housing does not always give you more capacity — it depends on the cell format. On one of our shells the 18650 version comes in 4 steps from 40 to 78 cells, while the 21700 version has only 2 steps, 50 and 52, 2 cells apart. Lengthening that housing buys you almost nothing on 21700.
Range questions usually arrive as "how big a battery do I need". The honest answer has two parts: how much energy a cell holds, and how many cells the housing will actually accept. The second part is where most builders lose, because it is not linear.
| Housing | 18650 — steps we offer | 21700 — steps we offer |
|---|---|---|
| ALX099 | 40 / 52 / 65 / 78 cells (38 apart) | 50 / 52 cells (2 apart) |
| ALX061 | 91 / 130 cells (39 apart) | 78 / 91 cells (13 apart) |
Why the 21700 version saturates sooner. A 21700 is 21 mm across against 18.5 mm for an 18650. Across the interior width of a housing, that difference can cost a whole column. Once the column count is fixed, the only direction left to grow is length — and cells pack in whole numbers per row, so the rounding loss eats most of the gain. On a narrow interior the 21700 arrangement runs out of ways to be laid out long before the 18650 one does. That is what the table above is measuring: same shell, same mould, two formats, very different headroom.
What it means if you are chasing range. Do not assume the bigger cell format gives you a bigger pack — in several of our housings it gives you a shorter menu instead. Three practical consequences:
- Check the ceiling for your format, not just the housing size. A housing that goes to 78 cells on 18650 may stop at 52 on 21700. If your target pack is above that, the format — not the housing — is the problem.
- Building on 18650 is not automatically the low-capacity choice. Where a housing has more 18650 steps than 21700 steps, the 18650 build can be the larger pack in that specific shell.
- Changing the housing beats lengthening it when you are already near the top of the range. A different interior gets you more column width, which is the dimension that actually unlocks cells.
How to check before you buy. Every housing we mould is published with its cell ceiling for both formats — that is on each series page and on the full measured size table, which lists all 110 housings with openings and cell counts. If you already know your target configuration, the case selector narrows it to the models that accept it.
Common questions
Does a bigger battery case always mean more range?
Is 21700 better than 18650 for a long range pack?
How do I work out what pack a housing will take?
What actually limits range on an e-bike?
Send us the pack and we will confirm the fit
Cell type, series and parallel count is usually enough. If the answer is that none of our moulds fit, we will say so rather than sell you the nearest thing.