The most confusing family on the shelf, and all of it fits the same hole. LR = alkaline (1.5 V, sloping); SR = silver oxide (1.55 V, flat). On silver oxide the suffix means something — W = high drain, SW = low drain. AG10 and CX189 are trade codes for the cheap alkaline cell, so “LR1130W” and “389A” printed on an alkaline card are marketing: a W suffix has no meaning on alkaline. Rule of thumb — if the device shows a number (caliper, thermometer, glucose meter), buy 389; if it is a toy or a light, AG10 is fine.

The workhorse button cell — same 11.6 mm diameter as the AG10 family, but 5.4 mm tall. Alkaline LR44 is fine for a toy; a caliper, a glucose meter or a camera light meter wants 357 silver oxide.

Small and very common. Watch out: the identically sized PR41 is a 1.4 V zinc-air hearing-aid cell, not a substitute.

The single most common wristwatch cell. When a watch shop changes your battery, it is usually a 377.

The thin watch cell. Same diameter as the AG4 family but half a millimetre shorter — they are not interchangeable.

Between the AG10 and AG13 families in height. A 4.2 mm holder will loosely accept a 3.05 mm cell and then lose contact, which is the usual explanation for a “dead” new battery.

Same diameter as the AG3 family, much taller. The identically sized PR48 is a 1.4 V zinc-air hearing-aid cell.

Common in slim watches and calculators.

The thinnest of the 11.6 mm cells.

Same 9.5 mm diameter as the AG7 family, a millimetre taller.

The slim 7.9 mm cell, mostly watches.

Between the AG11 and AG3 families in height.

The thin 9.5 mm cell.

Zinc-air breathes. Peeling the tab lets air in and starts the reaction, and the cell runs down in 3–4 weeks whether the aid is worn or not. In exchange it packs five to six times the capacity of the same-size alkaline at a flat 1.4 V. PR41 shares a shell with LR41 and PR44 with LR44 — but at 1.4 V and with air holes in the case, so do not cross them.

Same diameter, different thickness, so capacity scales with height and nothing else. A thinner cell drops into a thicker holder and works until the contact springs back — the usual cause of an intermittent key fob. A thicker cell will not fit, and forcing it bends the holder. Two CR2016 stacked make 6 V, which is why some fobs take a pair.

The big coins, used where a CR2032 would not last: alarm sensors, tyre-pressure monitors, medical wearables, Bluetooth trackers.

Same rule as the 20 mm family: thickness is capacity. Common in car keys, where the case dictates the size.

The small coins, mostly watches and small medical devices.

Three things shaped exactly like a CR2032 that are not one. BR is a different lithium primary — less capacity, far wider temperature range. LiR is 3.6 V rechargeable and over-volts a 3 V circuit. ML is 3 V rechargeable and is the correct part for a device that charges its own coin cell, such as some laptop clocks.

One physical size, five chemistries and a 5× spread in what you actually get. Rule of thumb: zinc-carbon for a clock, alkaline for a remote, NiMH for anything you use weekly, lithium FR6 for cold or high drain. Never a 14500 — same shell, 3.7 V.

The same chemistry choices as AA at roughly a third of the capacity. The 10440 Li-ion is the same shell at 3.7 V — never a substitute.

The big cylinders, kept alive by torches, radios and lanterns. Watch for spacer-shell “D” cells — an AA rattling inside a D-sized plastic tube. Legal, and a tenth of the capacity.

Six 1.5 V cells in a box, which is why the capacity is so small. Smoke alarms, multimeters, guitar pedals. NiMH versions are 8.4 V from seven cells, and a meter reads them as nearly flat when they are full.

Eight tiny alkaline cells stacked in a tube. Small capacity, high internal resistance, and a habit of going open-circuit — a gate remote that only works when you squeeze it is almost always this. Not rechargeable, and no relation to AA/AAA despite the “A”.

3 V lithium cylinders that outlived the film cameras they were made for, because torches and alarm sensors adopted them. Each has a 3.7 V rechargeable twin (16340, 15270) that is a whole volt higher.

Sizes that turn up in exactly one device you own and nowhere else.

The most-made lithium cell in history. Two things to check before buying. A protected cell carries a small PCB in the base and is 2–5 mm longer, which sometimes will not fit. And a button top will not make contact in a device wanting a flat top, or the reverse. Any listing claiming 5000, 6800 or 9900 mAh in an 18650 is fake — the physical ceiling is around 3600 mAh.

Where the industry moved after the 18650. More capacity per cell means fewer cells, fewer welds and fewer failure points in a pack.

These are the dangerous ones. A 14500 is exactly AA-shaped and 3.7 V — two and a half times the voltage. Torches that accept both say so explicitly on the label; nothing else does.