In reply to Dan Bailey - UKHillwalking.com:
If you happen to wander past an IKEA then their batteries have always served me well, but I haven't tried any of the current batch, mine are the old 2000mAh ones that got a decent write-up here:
http://www.candlepowerforums.com/vb/showthread.php?387711-Ikea-Ladda-(AA)-A... .
The current ones are 2450mAh
http://www.ikea.com/gb/en/products/lighting/batteries-chargers/ladda-rechar...
Don't necessarily be fooled into thinking that a higher mAh rating (milliamp hour - basically capacity) means a better battery. Straight out of the charger a new battery should give its full advertised capacity, but some batteries lose their charge faster than others whilst just sitting around doing nothing, so if you come to use your batteries several days or even weeks after charging, then a large capacity cell that loses its charge quite quickly could last less time than a lower capacity cell that keeps its charge better. Hybrid cells (low self-discharge cells) are still NiMh cells (nickel–metal hydride) the chemistry is fundamentally the same, it's more a marketing name than anything else. The lower self discharge is achieved by better quality electrodes and by having a better separator between the positive and negative electrodes inside the battery, partly by using a higher quality separator and partly by making the separator thicker, which is why they have a lower capacity than the highest capacity regular NiMH cells, there's less room inside for the electrodes.
Of course as batteries go through charge and discharge cycles, over time their overall capacity is reduced. People upgrading to new batteries after a few years may well extol the virtues of their new cells in comparison to the old ones, not remembering that the old ones were just as good when new.
Rechargeable batteries have a fairly flat voltage curve compared to regular disposable batteries, alkaline batteries have a nominal voltage of 1.5V but that drops steadily over time, NiMh batteries have a nominal voltage of 1.2V but it stays close to 1.2V for most of the discharge cycle and suddenly drops off as the cell gets close to being empty. If your head-torch has circuitry in it to maintain a constant voltage to the LED as the battery is used then you wouldn't see a difference in brightness. With cheaper head-torches that don't have that circuitry, fresh disposable batteries will start out brighter than rechargeables but get progressively dimmer over time. Rechargeables will provide a relatively constant light output and then suddenly dim very rapidly as they get close to exhaustion.
It isn't a good idea to completely discharge NiMH batteries, this is because they don't all become exhausted at the same time, once one cell becomes discharged the remaining cells push current through it, which causes loss of capacity, which in turn means that it goes flat sooner the next time and gets damaged some more. Very fast charging also reduces the capacity more than slow charging, so if you have the time (and your charger has more than one setting) it is better to charge your batteries at 200mA for fifteen hours than at 2000mA for one and a quarter.