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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

  • they have a significantly lower voltage than AA's
  • they lose their charge over time

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

AA Li-cells Dummy AA

Rather amusingly named, given that primary Lithium cells can burn, and these will get hot and smoulder if short-circuited.

I try to remember to date my batteries, but often forget..

Two 'Dummy AAs' . The leftmost is 1/2" wooden dowel, wrapped in aluminium foil with a small domed screw for the 'pip' and a drawing pin for the 'flat' end, all wrapped in parcel tape.

Dummy AA

Chemistries

The second (and best) Dummy AA uses aluminium bar 10 or 12mm diameter cut to 54mm. I formed the 'pip' with an angle grinder (mounted in a cut-off stand) and turned the bar in a drill.

1 or 2 layers of heatshrink tube or some duct tape makes an OK sleeve. For devices that use 4 cells then a double-length Dummy AA works.

Comparision of different cell discharge curves. Lithium is plotted on the RIGHT column adjusted to compare with TWO cells on the left.

The original of this diagram with a lot more info. is at http://www.mpoweruk.com/performance.htm to whom I give full attribution for their work.

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.

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