Tony, ideally, the car should have a battery gauge reading in real time exactly what the charge condition (or capacity) is to always be available to the driver to continuously monitor. This might take the form of an "amp-hour" or a killowatt-hour meter just like we all have connected to our houses from the utility grid. This would be analogous to having a gas tank with a "fuel remaining" gauge connected to a level detector in the tank.
Well, we're not going to get any more gauges at this stage, so we have to put our "faith" in what we already do have in the car, i.e. the so-called GoM. Like all "machines", the more we use them, the better we get to know them and their idiosyncrocies. Eventually, you get conditioned as to what to expect.
In my RAV4 EV, the GoM does seem to read out fairly accurately, although perhaps on the conservative side. I think the same would be true of most gasoline fuel gauges. The point being, if you unexpectedly run out of gas (or in our case, run out of battery charge), the driver will be cursing at his "fuel" gauge unmercilessly! The gauge is doing its basic function, if it warns the driver BEFORE that happens, even if there is a few more (emergency reserve) miles left to go at "EMPTY"!
If you drive in a reoccuring pattern - like commuting to work - and you are the SOLE driver of the vehicle, I contend that the GoM can be used to judge range fairly accurately. The only troubling thing I noticed about the GoM is that it does not seen to be a linear readout. That is, at the top of the gauge, each segment bar, especially for the first bar, varies quite a lot for miles driven before the next bar extinquishes. The miles driven denoted by each segment is non-linear, which is probably unavoidable since miles per kWH can be continuously changing. I think the lower the bar, the less mileage you get for that bar as you drop in range. Of course, this is obviously the case for the first bar, in large part do to the difference in battery capacities from an extended vs. standard charge.
The first bar on the gauge for an extended charge will extinquish after after ~25 or more miles, but after a standard charge, it should decrement after far less mileage. As the gauge decrements, the gauge tends to become more linear (or less non-linear might be a better description), decrementing every 5 to 8 miles driven. Down near the bottom of the gauge it seems to decrement at a faster rate, i.e. less miles driven per segment, perhaps only 4 or 5 miles, instead of 6 or 7 near the top of the gauge.
My point is the GoM still provides useful information and can be somewhat "trusted" (although certainly not 100%), but only if you do not "reset" the driving efficiency variable, thereby intentionally defeating its consistency from drive to drive. A RAV4 EV with a GoM that at first indicates 146+ miles (after an extended charge), but only actually travels 90 miles before the tow truck arrives, is far less accurate and certainly even less useful to the driver.
Just leave the gauge alone, not reset it (including the driving efficiency readout), and driving "normally", the GoM is probably going to maintain reasonably accurate. But, if you reset it, all bets are off!
Any thoughts?
