Someone correct me if I'm wrong, but as I understand it, the displayed range is based on past performance and is an estimate. If you drove up a HUGE hill at 10°C with the heat on, rear defroster on, and the heated seats on your range will be significantly less than what it may have displayed when you set out with a full charge. The next time you get a full charge the displayed range may be significantly less because past performance indicated that estimate was WAY off. It doesn't mean that the battery is going bad necessarily, just that your usage is burning the energy at a higher rate than anticipated.