A few weeks ago I reviewed the small handheld Energizer “Hard Case” 2 cell AA swivel head LED torch. Given I also own a Cateye EL-530 LED cycle headlight, it was only ever going to be a matter of time before I decided to compare them, although in fact this is the first time I have had the opportunity. As I noted the PROSW2A claims a 75 lumen light output and 5.5 hours runtime (presumably on the supplied 2 Energizer alkaline cells). Cateye chooses to measure the cycle lamp’s output as 7000 candlepower with the centre of the beam at 10 metres being 1000 candlepower. This means a direct comparison of the outputs is rather difficult to compute. Cateye also says the 4 AA batteries will last up to 90 hours. Since no definition is given by either manufacturer of what level of useful output is obtained at the different times, these figures are somewhat useless. Therefore I can only use an unscientific direct comparison of the usefulness of the two lights when placed side by side and observed illuminating the same target at various distances. All of these comparisons showed the Energizer was much brighter than the Cateye.
Since there is an inverse square law at work, the actual light output of the Energizer could well be more than double the Cateye, perhaps 3 or more times the brightness which could account for the substantial difference in battery life although the Cateye has twice the power available from its battery. And the Cateye has a much bigger reflector which, however, in my opinion is wasted by the small emitter which is basically the same size in the Energizer yet the reflector is much better tailored to its characteristics in the latter. I am guessing overall that a combination of factors in the Energizer including much more power output and substantially higher (by percentage) amp-hour current draw off the smaller battery accounts for the significant battery life difference. As we all know, the higher the discharge current, the lower the capacity. An average battery with a capacity of say 2500 mAh is usually rated to produce that at a current of say 250 mA, if the discharge current is 1 A or 2 A or 5 A the capacity is much less and drops roughly inversely proportional to the current. This effect is caused by the internal resistance of the battery, at the higher currents more power is being wasted across this resistance before it gets anywhere near a load and this also accounts for the battery heating up at high discharge currents. At a very rough guess the 3 watt emitter in the Energizer has a 5 watt input power, this would require a current of nearly 2 amps which is pretty high even for alkalines. There is a possibility of course that 5.5 hours is not continuous. Likewise the 90 hours of the Cateye is not continuous perhaps. At 6 volts for say a 1 watt emitter 2 watts input the current is something like 350 mA. But that doesn’t really account for the 90 hours, surely this must be a mistaken number. The mystery continues…