21700 and 26650 Li-Ions
Over the past decade the 18650 format (18mm diameter, 65mm long) has dominated the lithium-ion marketplace and it's still the sweet-spot in terms of compatibility, availability and value. However the 18650 cell seems to have peaked at a capacity of 3,500mAh, which has resulted in greater demand for Li-Ions such as the 26650 and 21700 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 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's designed for a high-capacity cell. High capacity batteries are also more expensive – depending on your needs there may be a case for buying two sets pf 18650s rather than one set of the bigger batteries, so you can charge one set while using the other and/or carry the spares for those priceless moments when you realise you forgot to charge your battery after your last outing!
Click on the pictures below to see details, photos and prices and drop me a line to discuss discounts.