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The Energizer L91 Lithium AA cell is a fantastic power source. For high-drain applications, it offers great advantages over ordinary alkaline cell and rechargeable NiMH cell. However, it is also very expensive as a disposable battery. So it is important to recognize the strength of L91, and use it only in applications that give you the best results. Otherwise you'll be wasting your money.
According to data sheet published by Energizer, the capacity of the Lithium AA cell (L91) is 3000mAh. This number remains nearly constant with load current. In comparison, the ordinary Alkaline battery (such as the Energizer E91 AA) is rated as 2800mAh, but only at a very light load of 25mA. At a heavy load of 1000mA (typical for a digital camera or flash unit), its capacity drops to below 1000mAh. This number gets even worse at low temperature. At zero degree C and 1000mA load, the L91 can provide about seven times more energy compared to the E91. This is where the marketing claim of "Last up to 7 times longer" comes from.
When used in a digital camera at room temperature of 25 degree C, the L91 is more likely to last 3-4 times longer than alkaline. This is enough to justify its price, which is also around 3-4 times higher than alkaline. However, in low-drain applications (such as smoke detector, wireless mouse or remote control) alkaline cells will last just as long at a fraction of the cost.
The L91 also contains much higher energy when compared to an ordinary rechargeable NiMH cell such as the Energizer NH15-2500. This is partially due to its higher terminal voltage of 1.5V, which is 20% higher than that for NiMH.
Energy stored in NH15-2500 : 2.5Ah * 1.25V = 3.125Wh,
Energy stored in L91 : 3Ah * 1.5V = 4.5Wh (44% higher).
Higher terminal voltage for L91 is especially advantageous in digital camera applications, because your camera flash will recharge up to 20% faster. An added benefit is that the L91 weighs only 14.5 grams (0.5 oz) each, compared to 30 grams (1 oz) for NiMH. So your camera becomes much lighter.
The NH15-2500, like all ordinary NiMH cells, suffers from rapid self-discharge problem. If you charge up a set of those cells and leave them in your camera for a few weeks, you're likely to be greeted by 'Low Battery' sign when you try to take a photo. The new generation of 'ready-to-use' NiMH cells such as SANYO eneloop and Rayovac Hybrid solved this problem by having very low self discharge rate. But they have lower capacity of only 2000-2100mAh.
Bottom line: If you need AA batteries for a long hiking trip, then L91 is obviously your best choice. For most other situations, I recommend using low self-discharge NiMH cells for lower operating cost.
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