eta, ok finally found what i *think* is the answer.

kind of long-winded but apparently there are some 'technical difficulties' that make LiON ill-suited as a generic AA replacement. In some cases LiON rechargeable actually don't last as long as a NiMH rechargeable.

Any rechargeable LiON you see on ebay is suspect and probably not what you're thinking it will be.

quote:
A system designed to run from a 1.5 Volt source (SMqV) will handle input voltages from about 2 Volts down to 0.8 Volts This is in order to suck the last dregs of power from a dying alkaline battery. A rechargeable Li-Ion will come off the charger at about 4.2 Volts. This says the switching power supply now has to handle a 5.4 ratio of input voltages (4.2 divided by 0.8). It can be done, but efficiency of the switcher suffers. A common voltage for DSPs and other digital stuff is 3.3 Volts. With a nominal 1.5 Volt battery, you are always running as a step up switcher (boost switcher). If you have a higher input voltage than 3.3 Volts, such as the 12 Volt external input on some gear, you need a step down switcher (buck switcher). This means the designer only has to worry about one type of supply. The topology of the step up and step down devices is totally different. The problem with a 4.2 Volt battery is that now you need a switcher that steps the voltage down to 3.3 Volts for most of the Li-Ion battery life but with an alkaline or other 1.5 Volt battery, you need the same switcher to step up to 3.3 Volts.

There are setups that will handle both but they add complexity and worse, are not as efficient. Taking half an hour to an hour out an SMv's battery life is not a way to be popular.

Finally, since the power/volume density of a NiMh is very close to that of Li-Ion battery, there really is no good reason to design for a Li-Ion battery that is hard to find in the middle of the Amazon, let alone Walmart. (Alkaline and NiMh AA's are ubiquitous.) For the power/volume numbers, the power of a NiMh is 2.5Ah times 1.2 Volts or 3 Watt-Hr. The 14500 Li_Ion rechargeable is 3.6 Volts times 0.8Ah or 2.9 Watt-hr, pretty much equal numbers. Where the Li-Ion has a big advantage, is in weight. The power/weight density is much better at about half the weight of NiMh battery for equivalent energy storage. This is very important in cars where Li-Ion battery weight can be 25% of the mass of the car (Tesla).

So, hard to find in an emergency, same power level as NiMh, complicates power supply design ($$) and reduces run time in general. Keep in mind this discussion is for AA size batteries. Obviously Li-Ion is very successful in many devices, most of which are permanently installed (cell phones) and sized mechanically to fit a particular, one off package. Being able to remove the protective can from a battery as in a flat pack battery for phones is a huge advantage for Li-Ion, one that the spiral structure of a NiMh does not enjoy.