Is the battery meter in the PRS505 linear, or is it like the gas gauge in my car - it takes a long time to reach the 1/4 mark, and but then moves toward "E" so fast you can almost see it move... In other words, is there the same amount of reading time between the battery meter indicating full (4 bars) and 3 bars and between 3 bars and 2 bars, etc.?

I've never let my 505 battery meter drop more than 1 bar but it seems like it takes a long time just to do that

I'd say it's linear, certainly more so than those hinky gas gauges tend to be, but I don't really have anything to support that beyond "it seems that way" to me. :shrug:

When you are charging, if you disconnect it before the little red light has gone out then the meter is apt to be unreliable.
Also, in the beginning, it takes a few charging cycles before it settles down and becomes accurate.