Here are details on how the Classic's (NHW11's) battery icon works. I've simplified a few things (e.g. there is hysteresis in the transitions). The source of my data is Graham's website (
http://home.earthlink.net/~graham1/ and specifically
http://home.earthlink.net/~graham1/Priu ... yItems.htm) in addition to my own observations.
1. There are only four bars. The posts never get filled.
2. The battery is only displayed as "empty" when its state of charge (SOC) is below about 20%. This can't happen unless you run out of gas or have some major system failure.
3. The first bar is displayed when the SOC is between 20% and 43%.
4. The second bar is displayed when the SOC is between 43% and 56%.
5. The third bar is displayed when the SOC is between 56% and 73%.
6. The final bar is displayed when the SOC is between 73% and 100%. This only happens at the bottom of long hills.
7. Under normal conditions, the battery will be maintained between 45% and 80% SOC. However, the battery icon shows the full range of possible SOCs. This means the "empty", "1/4", and "full" states will rarely be displayed. For typical driving, the SOC is maintained between 55% and 65%, so most drivers see only the "3/4" state.
I've heard that the model year 04 Prius (NHW20) has some changes to make the battery icon more meaningful: "empty" is 40%, "full" is 80%, and there are more bars, which are evenly spaced across this range. Note, I've heard the NHW20 allows the battery SOC to fall to 40% (instead of 45% in the NHW11) before it forces recharging. However, the battery icon is still just for fun, because the Prius handles all the battery maintenance, so you don't have to worry about it.
Douglas (2002 Silver, Wisconsin)