Lithium batteries
Lithium cells are available as both primary (non-rechargeable) and rechargeable types. They offer higher energy density, lighter weight, and better performance at temperature extremes than alkaline equivalents.
Covered in this section
18650
18mm × 65mm Li-ion rechargeable. The most common format for flashlights, laptops, vaping devices, and EV battery packs. 3.7 V nominal, 1800–3500 mAh.
21700
21mm × 70mm Li-ion rechargeable. Newer, larger format with higher capacity (3000–5000 mAh). Increasingly used in high-performance flashlights and EV packs.
CR123A
3 V lithium primary (non-rechargeable). Used in cameras, tactical flashlights, smoke detectors, security equipment. Also as 2×CR123A = 1×18650 in many flashlights.
Comparisons
Not AA batteries. 18650 and 21700 cells are 3.6–3.7 V — more than double the voltage of an AA (1.5 V). They are physically similar in shape but are not interchangeable with AA. Using them in place of AA will damage your device.