Downtube battery case opening lengths — the model number is not the size

Short answer: a downtube housing does not have one length. Our 15 downtube moulds are cut at 20 different opening lengths between 278 and 531 mm, and 12 of the 15 are offered at more than one — ALX030B ranges 176 mm within a single model. So the length you measured off your frame selects a step, not a model. 9 of the 15 can be ordered at 355 mm or less; 13 of them start below 392 mm.

Every listing in this trade names the mould. Almost none names the step, because the step is decided at the order and it changes the part. That is why "will this downtube housing fit my frame" has no answer until the length is fixed, and why two buyers with the same model number can end up with housings 176 mm apart.

Every downtube mould at every opening length we cut

Sorted by the shortest opening in the family, so you can read down the second column until you pass your own measurement. The last column is the cell ceiling menu for each format — a separate list, not one value per length.

A dash in the 21700 column means the 21700 opening is not published on our sheet — not that the mould is 18650-only. 6 moulds show a dash, and 2 of those (ALX119A and ALX127) still carry a 21700 cell ceiling, so the 21700 version exists and its length has to be asked for.

What a measured clear length actually buys

The useful question is not "which model" but "how much clear space do I need before this family starts". Read the table down from your measurement:

The shape of that table is the point. 278 to 355 mm is where the family fills up — one mould at 278 (ALX143), one at 340, then 9 of the 15 are available at 355 mm. Above 392 mm you are adding the last 2 moulds, and the longest single opening we cut is 531 mm. If your frame measures under 340 mm, the downtube family is effectively one mould.

The steps are not evenly spaced

Across the 22 gaps between consecutive openings in the same mould, the jump runs from 36 mm (ALX127) to 130 mm (ALX086). There is no standard notch, which means "go up one size" is not a quantity. On ALX127 the last step adds 36 mm of housing; on ALX086 the single step adds 130 mm. Two buyers both asking for "the next size up" are asking for different amounts of housing.

The cell menu in the table above is not a ladder you can climb by length either. On 7 of the 12 multi-step moulds the number of cell ceilings does not match the number of openings — they are two menus quoted per mould, not a pair per step. Capacity follows the interior, and the interior is not a straight function of the opening.

Choosing 21700 shortens the menu

9 moulds publish a 21700 opening list, and on 7 of them it is shorter than the 18650 list. ALX030 is the clearest case: 3 openings on 18650 (355 / 415 / 465 mm) and 1 on 21700 (435 mm). Only ALX100 and ALX100A keep the same number of steps in both formats. If the build is on 21700, the length you can order is a narrower set than the catalogue suggests — check the 21700 column before you design the frame around an 18650 figure.

What the opening length does not tell you

Opening length is the measured internal span the pack sits in — L2 on our drawings. It is not the space available to cells: end caps, the connector block and the cable exit all take length out of it, and the BMS sits inside the same envelope. A housing ordered at the right step can still refuse to close once a 13S board is in it, which is a separate problem worked through in why your 13S5P pack will not fit.

So an order needs three numbers, not one: mould, cell format, opening step. A supplier who quotes a price from the model number alone is quoting a housing you may not be able to use.

The measured size table carries the opening and section for all 90 models if you want to cross-check a quote against a published figure, and the downtube series page lists all 15 moulds with their cell ceilings. If you have not measured the frame yet, the order of operations is in how to measure your bike frame.