21700 and 26650 Li-Ions
Over the past decade the 18650 format has dominated lithium-ion battery chemistry, and it's still the sweet-spot in terms of compatibility, availability and value. However the demand for greater energy reserves in a single cell has led to the development of new formats.
While the 18650 cell seems to have peaked at about 3,500mAh, new-generation Li-Ions such as the 26650 and 21700 are appearing with capacities of 5,000mAh and more. More mAh does not make a torch any brighter, but does make it run longer on a charge.
However these cells are physically bigger (the first two digits give the diameter and the third and fourth digits give the length in millimetres before a protection circuit and outer wrapper are added) so they don't fit into torches or chargers that aren't designed for them – although the opposite is NOT true, 18650 batteries can usually fit into a torch or charger that is designed for a new-gen battery.
There's still a good case to be made for buying two 18650s rather than one of the bigger batteries – having two cells means you can charge one while using the other and/or carry the spare for those priceless moments when you realise you forgot to charge your battery after your last outing!
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