Thank you all for the input! I do intend to keep it simple.
I do have to say living with the unitless "Gids" for nearly two years I'm actually rather fond of them. As a more or less constant unit of remaining energy with a range of roughly 0-300 it's rather handy. Given the imprecise nature of the Leaf's current sensor my ideal measure (Ah remaining) may be unrealizable. Would a few more bits of precision be nice? Yeah.. would it be nice to see the credit you're getting for long regen sessions without the obnoxious hysteresis? Yeah.. but I guess I shouldn't complain. The precise SOC number in Group 1 is fun to watch but it's not a constant unit and will vary from vehicle to vehicle.
There's been a lot of talk of kWh and that's where the discussion gets interesting. Obviously we can measure kWh into the vehicle (or battery) and it'd be nice to have a constant measure of "fuel" but unfortunately batteries are odd beasts. Fortunately lithium batteries are a tad more efficient and consistent than many others. What we really would like to know is how many Amp-hours are left because Ah are much less effected by how you drive. If you start with 66Ah and drive 80 miles and take out 60Ah, when you charge chances are you'll put 60Ah back in to reach 66Ah again. kWh don't work that way. Due to the internal impedance of t he cells the voltage of the cell is always higher on charge than discharge which results in the round trip efficiency of the battery. You may have 66Ah (24kWh) at your disposal with a fully charged new pack, but if you chose to discharge that at low current (driving really slow) vs high current (driving really aggressively) You will net more kWh on the slower drive, but your Ah consumption will be the same in both cases.
Having a "fuel" that varies depending on how you use it and a "tank" that decreases with age makes EVs more uhm.. interesting.
If the precise SOC was based on Ah remaining then SOC * Ah CAP would give an ultra precise measure of remaining Ah.. If however like Gids its a fudged guess at remaining kWh (not knowing how you will be using it) then that equation won't work.
My inclination at the moment is as earlier noted.. to provide Gids for those who like the semi-constant remaining energy display and the precise SOC for folks who really like watching the numbers move (and getting that oh so rewarding credit for regen!)
'17 blue Volt Premier w/ACC
'12 SL black Leaf
'11 SL blue Leaf
Gen1 & Gen2 EV1 1997-2003
PV 2.4Kw, 10kWh lithium battery SCE TOU-DA