|Creative Commons: Share & Enjoy|
Rechargeable AA Batteries
We use loads of AA batteries in all our everyday stuff and it makes sense to use rechargeable (secondary) cells.
Unfortunately the only commonly available secondary AA battery is NiMh (or NiCd) but these have 2 major problems
Alkaline batteries deliver 1.5V for most of their life, but NiMh batteries are 1.4V briefly after charging and then 1.2V until they are empty. Most devices are designed for a nomical cell voltage of 1.5V (typically 3V for 2-cell or 6V for 4-cell) and so they often won't work for very long, even with 1400mAh NiMh cells which are often only half-depleted when the device has stopped working.
NiMh cells have a self-discharge rate of about 1% per day at room temperature - more if the cells are poor quality or if they are hot. So if you put 2 or 4 of them in your camera expecting it to work for a month or so, it will only have 70% of its charge after 4 weeks, and it may only take half as many shots if the camera is supposed to be used with 1.5V cells.
Lithium to the rescue!
AA-sized single-cell Lithium batteries are available with a nominal voltage of 3.7V.
DON'T USE THESE AS DIRECT REPLACEMENTS FOR AA'S
You must use the correct Lithium battery charger for these cells, NOT a NiMh or NiCd charger
Each Lithium cell has a nominal 1440mAh capacity: I use 1 x Li battery and 1 x dummy in to replace pairs of AA's. This actually gives a higher voltage than intended, so cave emptor - I'm not responsible if the device fails! I have used these in all of my 2-cell or 4-cell devices with no problem - my electric tootbrush is very vigorous!
However, there is no protection against excessive discharge: ideally a Lithium Ion battery should not be taken below 3 Volts or it's lifespan is reduced. I haven't noticed this yet, but then I haven't had enough use from my cells that have probably only been run down 10-20 times.
I have conceived of a protecton device - you can get low voltage alarms for Radio Control models - and mine would fit inside the Dummy AA with a wire to connect to the 'hot' end of the battery. I'm not sure I'll find the time, though - I guess I'll accept a shorter life battery.