The designation is a code, and it is readable
The IEC codes on cells are structured rather than arbitrary, and once you can read one you can identify a cell you have never seen. Take LR44. The leading letter gives the chemistry: L for alkaline, S for silver oxide, C for lithium manganese dioxide, B for lithium carbon monofluoride, P for zinc-air, H for nickel metal hydride, and no letter at all for the old zinc-carbon type. The R means round. The digits identify the size within the round family.
Cylindrical lithium-ion cells use a different and even more literal scheme: 18650 is 18 mm across and 65.0 mm long, 21700 is 21 mm by 70.0 mm, 14500 is 14 mm by 50.0 mm — which is why it fits a AA compartment. Coin cells split the difference: CR2032 is lithium (C), round (R), 20 mm across and 3.2 mm thick. Once you notice that, a shelf of coin cells stops being a wall of numbers.
A leading digit means cells in series inside one package. 6LR61 is six alkaline cells, which is where the 9 volts of a rectangular battery comes from. 4LR44 is four of the LR44 button cells stacked in a can, giving 6 volts. 8LR932 is eight tiny cells making the 12 volt A23. That is also why those packages have no single-cell equivalent: they are packs, not cells.
Nominal voltage is a chemistry property, not a size property
Every alkaline cell — AAA, AA, C, D — is 1.5 volts, because the chemistry sets the cell potential and the size only sets how much charge it holds. A D cell is not more powerful than a AA in voltage terms; it lasts longer at the same voltage, and can supply more current without sagging. Rechargeable NiMH in the same shells is 1.2 volts for the same reason, and the 20% deficit is real. Most devices tolerate it. Some do not, and the ones that do not are usually those with a low-battery cutoff tuned to the alkaline discharge curve, which a NiMH cell crosses immediately and then sits below for its whole useful life.
Discharge curve shape matters as much as the starting voltage. Alkaline sags steadily from 1.5 volts downward through its life. Silver oxide holds close to 1.55 volts until it is nearly exhausted and then falls off a cliff. Lithium primary cells behave similarly flat. That is precisely why a film camera light meter or a mechanical watch specifies silver oxide: the reading depends on the supply voltage being what it was calibrated against, and an alkaline substitute will read correctly on the first day and progressively wrong afterwards.
The substitutions that cause damage
Fitting a lower-voltage cell where a higher one belongs is a disappointment: the device runs briefly, or refuses to start, or reports a flat battery. Fitting a higher-voltage cell where a lower one belongs is a different category of event. The pairs that share a physical size and do not share a voltage are listed in the tool output, and the sharpest of them is CR123A against 16340. A device designed for a 3 volt primary cell, given a lithium-ion cell straight off a charger at over 4.2 volts, is being run at forty percent over its design voltage. Some devices tolerate this by design and say so. Assuming yours does because the cell fits is not evidence.
The mirror image of that mistake is stacking. Two CR2016 coins are the same total height as one CR2032, so they fill the compartment and make contact, and they deliver 6 volts instead of 3. A few devices genuinely specify that stack. Most do not, and the fact that it physically works is not a signal either way.
Handling, and what this page will not advise on
Primary cells are not rechargeable and this page gives no guidance on attempting it. Beyond that, a few things are worth saying plainly and none of them are controversial: do not mix chemistries, brands or ages within one device, because cells in series discharge unequally and the weakest one can be driven below zero volts by the others. Remove cells from anything being stored for months, since a leaking alkaline cell destroys the contacts around it and the damage is usually worse than the device is worth. Coin cells are a serious ingestion hazard for small children and for pets, and compartments with a screw retainer exist for that reason. Lithium-ion cells are a different class of item from everything else here, with charging, storage and transport considerations that belong with the equipment manufacturer rather than with a size chart.
For working out how long a given cell will run something, or how a pack should be sized, the battery runtime calculator and the battery bank sizing calculator take capacity and load as inputs. This page only tells you what the thing in your hand is.
Questions people ask
Can I use a rechargeable AA where the device asks for alkaline?
Usually, with one caveat worth understanding. NiMH is 1.2 volts nominal against 1.5 for alkaline, so a four-cell device sees 4.8 volts instead of 6. Most electronics run happily on that. What suffers is anything with a low-battery indicator calibrated against the alkaline curve, which may report empty from the moment the cells go in, and anything that needs the higher voltage to start a motor under load. Neither is a damage risk; both are an annoyance.
Is CR2032 the same as CR2025?
Same 20 mm diameter, same 3 volts, different thickness — 3.2 mm against 2.5 mm — and therefore different capacity, with the thicker cell holding meaningfully more. A CR2025 in a CR2032 holder is 0.7 mm short, which often means intermittent contact rather than clean failure, and intermittent contact in a device that keeps a clock or a configuration is a frustrating fault to chase. The reverse does not fit at all.
What is the difference between LR44 and SR44?
Chemistry, and it changes behaviour more than the numbers suggest. LR44 is alkaline at 1.5 volts and its voltage falls steadily throughout its life. SR44 is silver oxide at 1.55 volts and holds almost flat until it is nearly done. They are physically identical, so both will fit anything. Where a device specifies silver oxide it is because it depends on a stable supply voltage, and an alkaline cell will work at first and drift.
Is an 18650 just a long AA?
No, and this is worth being emphatic about. An 18650 is a lithium-ion cell at 3.6 volts nominal, more than twice the 1.5 volts of a AA, and it is 18 mm across against 14.5, so it does not fit a AA compartment anyway. The cell that does fit a AA compartment at lithium-ion voltage is the 14500, and that is exactly the trap. Cylinder shape carries no information about voltage.
Why does my 9V rechargeable read 8.4 volts?
Because it is seven NiMH cells in series rather than six alkaline ones, and seven times 1.2 is 8.4. Some are built with eight cells and read 9.6 volts instead. Neither reading is a fault. It matters mainly for smoke alarms and measuring instruments, which are specified against the alkaline discharge curve and may signal low battery early on a rechargeable pack.