Battery size names are a mix of old British codes, international codes and plain descriptions. The good news: most of them describe a physical shape only. The chemistry inside can differ completely.
The everyday sizes
These are the formats you will find in drawers, toys and remotes across the UK.
| Common name | Also marked | Size | Nominal voltage |
|---|---|---|---|
| AA | LR6, MN1500, AM3 | 14.5 x 50.5 mm | 1.5 V alkaline, 1.2 V NiMH |
| AAA | LR03, MN2400, AM4 | 10.5 x 44.5 mm | 1.5 V alkaline, 1.2 V NiMH |
| AAAA | LR8D425 | 8.3 x 42.5 mm | 1.5 V alkaline |
| C | LR14, MN1400 | 26 x 50 mm | 1.5 V alkaline |
| D | LR20, MN1300 | 34.2 x 61.5 mm | 1.5 V alkaline |
| PP3 / 9V | 6LR61, 6F22, MN1604 | 26.5 x 17.5 x 48.5 mm | 9 V |
| CR123A | CR17345, DL123 | 17 x 34.5 mm | 3 V lithium |
Size is a shape, not a chemistry
An AA is a size. What is inside could be alkaline at 1.5V, NiMH at 1.2V, lithium iron disulfide at 1.5V, or even a lithium-ion cell at 3.6V in a similar case.
This matters because devices are designed around both the shape and the voltage. Buying the right size with the wrong voltage is the most common battery mistake we see.
A 14500 lithium-ion cell is the same diameter and a similar length to an AA, but it is 3.7V instead of 1.5V. Putting one in an AA device can destroy it instantly or overheat the cell. Only use it where the device explicitly states 14500 support.
Rechargeable versions of the same size
AA and AAA rechargeable NiMH cells are the sensible default for anything used often: controllers, torches, toothbrushes, toys. A decent low-self-discharge NiMH holds most of its charge for a year on the shelf.
NiMH cells are labelled 1.2V, which worries people. In practice most devices designed for 1.5V alkalines run fine, because alkaline voltage sags under load anyway. The exceptions are a few devices with strict voltage cut-offs, such as some cameras and smart locks.
PP3 and the 9V stack
The 9V PP3 is a rectangular case containing six tiny 1.5V cells in series, or a folded stack in rechargeable versions. That is why a PP3 has far less capacity than it looks: often only 500 to 600mAh.
Rechargeable PP3 batteries are usually 8.4V to 8.7V rather than 9V. Smoke alarms are the one place to be careful: always fit the type the alarm's manufacturer specifies, and never fit a rechargeable unless the alarm says it is suitable, because the low-battery warning depends on voltage.
Lithium-ion numbers: 18650, 21700, 26650
These codes are dimensions in millimetres. An 18650 is about 18mm across and 65mm long. A 21700 is 21mm by 70mm. Both are nominally 3.6V or 3.7V.
They are not consumer batteries to swap casually. Bare cells need a protection circuit, correct charging and a device designed for them, and counterfeits are common. Buy from a known UK supplier and treat any damaged wrap as a reason to stop using the cell.
Loose lithium cells in a pocket or drawer with keys or coins can short and catch fire. Store them in a plastic case with the terminals covered.
Which battery for which device
Low-drain devices such as clocks, remotes and scales are happy with alkaline or NiMH. High-drain devices such as camera flashes, torches and motorised toys benefit from NiMH or single-use lithium, which hold voltage better under load.
If a device sits unused for months, alkaline cells can leak and corrode the contacts. Either use low-self-discharge NiMH or remove the cells.
The short version
- Size codes describe physical dimensions, not chemistry.
- AA and AAA rechargeables are usually NiMH at 1.2V and work in most devices.
- Never fit a 14500 lithium cell into an AA device.
- 18650 and 21700 codes are millimetre dimensions, and are not casual swaps.