How to read a battery case drawing
Every housing we sell comes with a dimensioned drawing, and most buyers read the wrong number off it. This is what each figure means, which one decides whether your pack fits, and what the section view tells you about fitting a BMS.
Short answer: read L2, not L. L is the outside of the case and includes the end caps and the connector block; L2 is the opening, which is the space your cells actually occupy. The gap between them is 25–40 mm on a Hailong-pattern case, and it is the single most common reason a pack that measured correctly still does not fit.
1. The three dimensions, and which one decides the fit
Every drawing we issue carries three length figures: the overall length (L), the opening length (L2), and the body length (L3). The one that decides whether your pack fits is L2 — the opening. L is the outside of the case and includes the end caps and the connector block; L3 is the body between the caps. Buyers routinely order against L and discover the pack misses by 30 mm, because the end caps are not somewhere you can put cells.
2. Why the opening is shorter than the body
The opening is bounded by the internal rib the cell holder seats against at one end and the terminal block at the other. Neither is negotiable: the rib is what stops the pack sliding, and the block is where the discharge connector mounts. On a Hailong-pattern case that costs you roughly 25–40 mm against the body length, and more on models with a lock barrel at the rear.
3. Tolerances, and what ±0.5 mm actually covers
Our drawings state tolerances on the moulded part, not on your pack. ±0.5 mm on an opening means the mould will hold that, which is plenty for a cell holder but not for a pack built to the exact millimetre with no clearance. Build to about 1 mm of clearance against the opening figure and you will not have to shim anything.
4. The section view is where the BMS question gets answered
The section (usually the lower right of the sheet) shows the internal rib positions and the height of the BMS shelf. If the shelf sits above the cells you lose height, not length; if it sits beside them you lose width. That distinction is the difference between a 13S pack fitting and not, and it is why we publish a section rather than just a plan view.
5. Mounting points: measure the centres, not the span
Bottle-boss spacing is quoted as centre-to-centre because that is what you measure on a frame. A drawing that gives you the outer span of the mounting plate is telling you something you cannot compare against your bike without subtracting half a plate at each end.
6. What we do not draw, and why
We do not publish cell positions. The interior is sized for 18650 and 21700 holders, and where the cells go inside that envelope is a decision for whoever builds the pack — it changes with the holder, the nickel layout and whether a BMS sits above or beside. A drawing that specified cell positions would be wrong for most of the builds it was sent to.
Where to go next
If you have the drawing and want to check it against a specific pack, the size finder takes the opening length and lists the housings that provide it. If you would rather start from the pack, the pack calculator works out the cell count and watt-hours for any series and parallel configuration. And if you are not sure what to measure off your own frame in the first place, start with how to measure your frame.
Drawings for any model are issued on request — ask for the one you are considering and we will send the sheet rather than just the headline figures.
Common questions
What does L2 mean on a battery case drawing?
Why is the internal length shorter than the outside of the case?
Does the drawing show where the cells go?
How much clearance should I allow against the opening figure?
Send us your pack layout
If you have the cell block dimensions we will check them against the drawing for whichever mould you are considering, and say plainly if it does not fit.