Thanks guys! That curve looks great, but I was beginning to wonder about temperature sensitivity when I saw all those diodes.
The other thing I'm curious about is that Alec's latest schematic, while it produces the collector currents I've determined are necessary to drive the gauge in a linear fashion, doesn't actually include the 200 ohm path to ground that I mentioned in Post #36. It's not intuitively apparent to me whether, if the plot were of Ic(Q1), would that value change at all with the addition of a 200 ohm resistor between the collector and ground?
If the temperature sensitivity could be compensated for with a thermistor, that would be pretty slick. (But as with this transistor solution in general, I wouldn't know where to start in selecting the specific component to order...) Failing that, I wonder what's worse, a state-of-charge gauge that exhibits wild temperature sensitivity (keep in mind that this car is powered by lead-acid batteries, so leaving it parked outside all day in -10 C really will change how much energy you have left to get home from work--maybe underreporting state of charge in that situation isn't such a bad thing, since the battery monitor won't actually know the batteries are compromised from cold soaking until you get underway), or just reverting to a linear solution that doesn't account for the nonlinearity of the gauge (i.e. I'd have to explain to my brother that the gauge will read 50% when he's actually at 25% and then move rather quickly over the bottom half of the scale)?