[Original Article: Mighty Mouse battery life indicator is very conservative • comment by: iKarith]
More than you probably ever wanted to know about batteries…

Part of the reason why people see that Apple’s battery charge indicator is … well, let’s just say MORE than conservative … is that Zinc-Carbon, Alkaline, and NiMH batteries all function differently. Let’s first dispense with carbon-zinc because even if you could find them anymore, they totally suck for Bluetooth devices which are meant to be low-power. They’re older tech than the typical alkaline batteries sold by Energizer/EverReady (same company!) or Duracell. If you stumble across them, they’ll say “Heavy Duty” instead of “Alkaline” and the only reason they still make them is because alkaline cells die faster in high-current applications. You might find them in like D cells, but but probably not AA/AAA.

Alkaline are “nominally” 1.5 volts. Actually, fresh out of the package and not connected to anything, they may read as high as 1.65v, but they’ll drop just as soon as you start using them. And they’ll keep dropping, linearly over time at a constant power usage level, until they reach 0.9v (which is where they’re basically dead. Dead alkaline batteries still having power drawn from them tend to eventually leak, and most devices won’t work on less than 0.9 volts anyway, so … replace ’em at this point OK?)

NiMH batteries have a lower nominal voltage of 1.2v, as others have noted. This is why a freshly charged set of rechargeable batteries will tend to report back like 85% capacity in your Bluetooth menu. Yeah, they start higher than that and taper off, but they tend to spend most of their usable life between between 1.2 and 1.0 volts. Their discharge voltage isn’t linear given a constant power draw like alkaline cells, and so the voltage warning thresholds for “buy new batteries if you need to”, “replace them soon”, and “could die any time now” are lower than alkaline, even if the “it’s dead, Jim” voltage of 0.9v is the same either way. NiCd batteries work the same way, though … why are you still using NiCd? They’re like carbon-zinc batteries—yes, there are still reasons to use them, but you don’t need or want them.

The problem with NiMH/NiCd is that they drain (self-discharge) whether you use them or not. Within a month, they’re flat. Enter the Low-Self-Discharge (LSD) NiMH cell! These are your Sanyo Eneloops, your Apple NiMH batteries (which are probably Sanyo), and some others made by TEnergy and a few other companies you maybe haven’t heard of. They will last a YEAR or more in storage. Gen 2 Eneloops last longer than that. But they will eventually die, and if allowed to stay dead they tend not to come back to life when you put them in a charger. LSD NiMH cells are good. I wouldn’t use anything else anymore.

Note NiMH cells are sometimes physically bigger around than alkaline cells. They’re within tolerances, but sometimes the things you put batteries into … aren’t. I’m thinking of a couple of headlamps I’ve owned for example… And for best results, don’t use “quick” chargers on the batteries if you can help it. They tend to cut the lifespan of the batteries by as much as half! Apple’s is basic but gets the job done. Sanyo’s own work well enough. Maha and LaCrosse make the super awesome tweak knobs and charge odd numbers of batteries chargers. I’m gonna assume you’re using the Sanyo or Apple chargers with your Apple or Sanyo batteries and call it good enough.

New kid on the block: Lithium AA/AAA made by Energizer. These aren’t better for bluetooth devices than alkaline cells, and they’re not rechargeable. These are gonna replace the carbon-zinc cells eventually and are meant for power-sucking applications. They’re more expensive, though, for a disposable battery cell. The only reason I buy them is because if you can keep them dry, they have lower self-discharge than alkaline cells do, so they’re good to have in earthquake-preparedness kits and the like, which is TOTALLY off topic.

FWIW, if you came to this hint looking for a means to tell the bluetooth menu that you’re using NiMH batteries and to stop harassing you about replacing batteries that are nowhere near dead yet, sorry I haven’t found it yet. …

Source: Mac OSX Hints