Even from the big boys like Varta and Tadiran who make Lithium Thionyl Chloride coin cells, they are primary batteries. When used for memory backup on a computer motherboard, for example, a small leakage current is allowed back into the battery, which makes the battery last a bit longer than it would without the current. However, it doesn't 'recharge' it in the conventional sense. Have you found a manufacturer that claims to have one that is rechargeable? Are you sure it is Li-SOCl2 chemistry? You can see from my part-7 post in the series, these are in the energy-density range to be primary batteries.
My good buddy in high school showed be why the car gas gauge is so non-linear -- when you look at the shape of the tank, it is rectangular (vertical walls) at the top, and then slopes at the bottom. A gauge that floats on top, is going to move faster at the bottom...simple as that. The simple fix for that is just to make the 'tick marks' for the remaining fuel be non-linear on the meter as well. Voilla.
David: yep, pics of batteries would be nice. I tend to shy away from showing one, for fear that someone take issue that the one I picked was a lousy non-representative sample, or some such. For the truely bizarre technologies, I will provide a pic or two, however.
Duane: the calibration is for the measuring system, yes. It has to compensate for the fact that our world changes, and not for the better (be thinking -- entropy and such other random thoughts). Just a very very little piece of humor, there. But really, if everything always behaved in a well known/predictable manor, re-calibrating would not be needed.
Duane -- I really don't find either of the display systems more common than the other. The gas gauge is a coulomb integrator. Good ones also monitor the terminal voltage, and in this way can also tell when a battery is unserviceable. The game can be played that at the current rate the battery has this much left. Personally, I prefer the % capacity meter instead of time -- but this is because I have to deal with the non-linear battery (think of adding water to the flodded lead-acid cells during charging, to throw a kink in the total charge calibration, for example). We humans like to think of the world as balanced: if we take out half of the amount of charge from the battery when we start fresh, then put back the half we took out, then we want to be back at full charge again. But this doesn't give an accurate picture. Therefore, I will cover gas guages in a future post. That's cooking with gas!