When the battery was at 30 miles and -3 surplus I had approximently 40 more miles to go. This is the only time I ended up in the very low battery range (10 miles or less). I ended up at my destination with 7 miles on the battery and it showed +5 surplus. (Remember the battery shows how many estimated miles you have left and includes the surplus.) In that 40 miles of driving I ended up with a total of 13 surplus miles. I want to let you know I do not change my driving habits very much. Typically it is 60 mph, cruise control, AC on, radio and sync active.
One other thing to keep in mind is the difference between "Battery Gauge" miles and "actual" miles driven. The gauge sets the budget, at the beginning of the trip, lets say 77 miles. When you finish your trip and look at the battery gauge and subtract your original budget amount from the remaining battery balance, 77 miles - 7 miles = 70 miles, it rarely equals the number of miles driven according to the trip odometer. In this case I drove 83 miles and had a remaining balance of 7 so I should have been able to drive a total of 90 miles.
This whole system is a "guesstimation". I understand why it is done and it actually makes sense, however, when you test the logarithm, you can get some different results. This is no different than the warning light on your gas gauge. There is always a couple more gallons buried in there somewhere. You just have to figure out how many.....
The variable we have in the electric car is "degradation" over time. That is one of the things I am trying to get a handle on. Right now I feel very safe driving 85 miles on my battery. But at what point will degradation, and it will occur, shrink that to 83 or 81 or 77 miles?
2012 Ford Focus Electric. Delivered 5/29/2012