Reention, Polly, Dolphin and Shark case sizes — what the names actually mean

The short version: the letters are the mount, the number is usually the cell count, and the dimensions vary by factory. So “Reention DP-9” tells you the fitting and roughly the envelope — it does not tell you the interior, and the interior is the number you actually need.

The names you will see, and where they come from

Reention is a Taiwanese mould maker whose shapes became the de facto standard for down-tube cases. When a listing says “Reention compatible” it means the case uses that mounting pattern — the same dock, the same rail — not that Reention made it. Most of what is sold worldwide now is moulded by other factories to the same pattern.

Polly is one of those Reention patterns, the one where the case slides down a rail and the body carries a USB port. Hailong is a different pattern and the most widely copied of all — it is the format most assemblers re-order because the docks are everywhere. Dolphin describes the slim, long down-tube profile. Shark is the larger Hailong-pattern body.

None of these are brand names, which is the practical point: two factories selling a “Dolphin” will ship you two different interiors.

Size and cell count reference

Figures are the ranges we see across the moulds in circulation, not a single factory's datasheet. Treat the exterior as a guide and ask for the interior before you commit to a pack.

Why the listing quotes the 18650 number

Because it is the larger and more impressive one. A 21700 is wider than an 18650, so the same mould holds fewer of them — a case listed at 91 cells typically takes around 70 on 21700. If you are building on 21700, the headline figure on the listing is not about your build.

What to ask for, whichever name you start from

The shape name gets you to the right family. Four numbers get you to the right case:

• Interior length, width and height — not the exterior
• Usable length after the end caps, which is typically 20–40 mm shorter
• BMS compartment width and position — above the cells or beside them changes the usable length more than anything else, and it is the usual reason a 13S pack “does not fit”
• Mounting centre spacing, so the case matches your frame rather than the other way round

There is a longer guide on those four numbers in what internal dimensions to ask a supplier for.

Which of these we mould

Our range covers all of the families in the table above — Hailong, Polly-style, in-frame, down-tube, rear rack and bottle-mount — across 110 moulds. Rather than match names, the reliable way to find your case is to work from the cell count you need:

• Case selector — enter the cells or the pack configuration and it lists the models that hold it, with the opening length for each
• Format comparison — the six families side by side, including which one is wrong for a given build
• 13S4P vs 13S5P vs 13S6P — what each configuration is in cells