AA-batteries can be purchased anywhere and are relatively small. Also, penlites can be found in both rechargeable (NiMH) and not rechargeable (Lithium and Alkaline) versions. These versions each have their specific properties, which have their advantages and disadvantages for each application.
AAA-batteries can also be purchased anywhere and are even smaller than the AA-version. This is the reason why AAA-batteries are often used in keychain-sized torches. Also this battery can be found in both reachargable and non-rechargeable versions.
18650-batteries offer a very high capacity and can take a very heavy load. They are very suitable for intense use. With sporadic use they must periodically be charged, because they can discharge slightly. Torches that work on 18650-batteries, can often also be used with two CR123a-batteries. Therefore, in case of need, you could even dodge to this different battery type.
The CR123A-batteries are very small and therefore often used in very compact torches. These batteries keep their charge for 10 years and are therefore suitable for sporadic use. A disadvantage is the lower capacity and the high prices at which these batteries are offered in most shops. However, at Knivesandtools.co.uk we purchase these batteries in big quantities and can therefore offer them with a competitive price.