What internal dimensions should you ask a battery case supplier for?
Short answer: ask for four numbers — interior length and width, the usable length after the end caps, and the mounting centre spacing. Any supplier who answers those within a day is one worth using.
Short answer: ask for four numbers — interior length and width, the usable length after the end caps, and the mounting centre spacing. Any supplier who answers those within a day is one worth using.
This is the single most common complaint from people sourcing empty housings: the outside dimensions are on the listing, the inside ones are not, and the inside ones are the only ones that matter.
Why factories do not publish them. The interior is not fixed. The same mould takes different cell holders for 18650 and 21700, the BMS lives in whatever space is left over, and the cable exit can move. So one exterior dimension maps to several interiors, and a single number on a listing would be wrong for most buyers.
The four numbers to ask for.
| Ask for | Why it decides the build |
|---|---|
| Interior length, width, height | The starting point — but not enough on its own |
| Usable length after the end caps | Where the cell block actually has to sit; often 20–40 mm shorter than the interior |
| BMS compartment width and position | Above the cells or beside them changes the usable length more than anything else |
| Mounting centre spacing | So the case matches your frame rather than the other way round |
Two more things worth settling at the same time. Whether the cable exit can be specified, because "almost right but unwireable" is a real failure mode. And whether the cell holder is supplied with the shell, because a case that fits your cells but ships without the holder will not hold them.
How to read a supplier's answer. A reply that gives you four numbers and a drawing is a manufacturer. A reply that says "please tell us your requirement" without offering the interior is usually a trading office passing your question upstream — which is fine, but it tells you what to expect on lead time and on changes.
Every model page on this site carries the opening length, and the format comparison lists the interior envelope for each range, so you can check a supplier's numbers against a known one.
Common questions
Why do battery case listings not show internal dimensions?
What dimensions should I ask a supplier for?
What is the difference between interior length and usable length?
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.