18650 and 21700 Pack Formats

Notes from a pack bench. Not a manual, not a datasheet, just the things that keep getting written in the margin of a drawing that is already crowded.

The holder on the shelf does not fit the cell in the hand. That is where most format trouble starts, and it starts quietly. The 18650 family names NCR18650GA, NCR18650BD, and UR1865ZM2 in Panasonic catalog data, at a family nominal voltage of 3.6 V, and the 21700 and 22700 family names NCR2170M, NCR2170N, and NCR2270A, also at a family nominal voltage of 3.6 V. Same family nominal voltage. Different formats. The holder was drawn for one format, the enclosure was drawn for one format, and the wiring assumed one format. Then the other format arrived, and nothing fit the way the drawing promised. The note to write first, before anything else: which format does the design specify, and does the design tolerate a format change at all? Ask early, ask in writing, and ask with the drawing beside the question.

A second note, taped to the same drawing: the family nominal voltage is the only figure this article states, and it states it for the family, not for a model. Everything else below is a question. That limit is deliberate, and it is worth understanding why. Per-model capacities are held back for the 18650 family on this site, because the archived capacity columns do not align cleanly across models. No capacity figure is published for the 21700 family either. A pack sized on a capacity read off a family summary will not hold what the design assumed. The question that replaces the missing figure is this: what is the datasheet capacity of each named model, on the datasheet's own basis, and can the models be compared on a shared basis? A capacity without its basis is a headline, not an input.

Dimensions. Same discipline, different note. Dimensions are published for the 21700 and 22700 models on their own page. Dimensions for the 18650 models are not republished in this article, because the archived 18650 columns do not align cleanly. A holder cut to an unconfirmed dimension is scrap with a delivery note attached. The inquiry question is the same shape as the capacity question: what are the datasheet dimensions of each named model, and do they confirm against the pack design? A dimension read off a family summary is not a dimension confirmed for a named model.

Then the pack itself. Arrangement, connections, protection. The bench note here is short, because the article publishes no configuration string and no pack figure. A configuration string is a listing label, not a specification, and no pack figure should ever be read off one. Ask how the configuration is specified, what arrangement the pack maker proposes, and what must be confirmed for the named models in that arrangement. Ask what protection the pack maker proposes for the named models, and how monitoring is arranged. Protection described at family level is protection described for no particular pack at all, and treating it as an answer builds packs on assumptions.

Charge behavior gets its own note, because it is where family logic fails most politely. A pack charged on an arrangement chosen for a family can fail at the named model without ever looking wrong on paper. Ask what charge voltage the pack design specifies for the named models, and how the pack maker states the charging arrangement. This article states only the family nominal voltage, and it does not publish charge-voltage variants or charging figures. Put the named models and the charging requirement in the inquiry together, in the same message, so the answer has to cover both.

Operating range. A note about real life rather than catalog life. Ask what operating range the pack design needs, and whether each named model covers it. The catalog pages note temperature ranges for listed ranges, but the exact range for a named model lives in its datasheet. The range question belongs in the inquiry, not in an assumption made after delivery, and not in a note attached to a pack that came back.

Service life. Often answered with a number, rarely answered well. No cycle-life figure is published for either family on this site, and this article adds none. A pack sold on a service life quoted without a basis was sold on the basis, not on the figure, and the basis turned out to be the whole argument. Ask how the pack maker states service life for the named models, and on what basis. Ask for the basis alongside the figure, and compare models only where the bases match. Where the bases differ, the comparison cannot be made yet. Admitting that is better than building on a comparison that was never valid.

A note on substitutes, because the bench sees them often. Do not swap a format on the strength of a family summary. Do not accept a substitute model on the ground that it shares a format name. A substitute that shares a format name may still differ in every figure that matters to the pack, and none of those figures are published here. Name the format. Name the models. Ask for the datasheets. Then decide whether a change is safe, with the drawing open and the answers in writing.

One more note, about the drawing itself. A drawing that names a format but not the models is half a drawing. A drawing that names models but not the arrangement is half a drawing. The inquiry should carry the same completeness the drawing should: format, models, arrangement, protection, charge requirement, operating range, and the basis behind any service-life statement. If any of those is missing from the inquiry, the supplier cannot answer completely, and the gap will come back as a delay or a return.

The last notes are commercial. Ask what documentation accompanies the named models, and what documentation is available on request. Ask what the ordering steps are from inquiry to delivery, and what must be stated in the inquiry so the supplier can answer completely. This site publishes no prices, quantities, or timeframes, and this article asserts none. A commercial question left out of the inquiry comes back as a delay. Put it in writing at the start, while it is cheap.

So the bench notes add up to one habit: a family answer is not a model answer. Name the format. Name the models. Ask for the datasheets. Check the arrangement, the protection, the charge requirement, the range, and the basis behind any service-life statement, each for the named models rather than for the family. Do it in writing, before ordering, and treat any figure that cannot be confirmed for a named model as unconfirmed rather than as a property of the family.

For cylindrical-format inquiry about any model on this site, use the inquiry form on the contact page. State the format, the model names where they exist, the pack design requirement, and each open question from this article. Confirm the figures, the configuration, the documentation, and the ordering steps for the named models before ordering. Confirm capabilities before ordering. Certificates available upon request.

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