Reconditioning battery question.

[Terrahawk]

I thought I'd do a deep cycle of my 11 month old battery today because I ran it down the other day in normal use and the reading from Battery Maximiser suddenly dropped from about 12% to 5% in one fell swoop and a beep. I thought it would be nice to have the percentage shown be closer to the actual charge in the battery.

So, I charged it fully and then discharged it with a light load, playing MP3s with all shutdown and warning timers disabled. I watched the load and it was pretty much a constant 13.8W give or take 0.3W. The initial battery capacity was 55.35 Wh (from a design capacity of 57.60 Wh) and I let it run down. It predicted a run time of just over four hours, which happened.

The low battery beep happened and the predicted remaining time then was 5 minutes. It ran for a further fifteen minutes before the "siren" came on and the full charge capacity was updated, which went down to 54.50 Wh at that point. The computer then continued happily for another twenty minutes before shutting down.

I did a quick calculation and the 4.5 hour run time at an average power usage of 13.8W is over 62 Wh. That is greater than the design capacity of the battery, which is quite odd to me. What bothers me though is that the recorded capacity is only 54.50 Wh.

Why is it that I appear to have a highly conservative capacity rating stored in the battery? The computer can run for more than 15 minutes with the Battery Maximiser software showing 0%.

I'm not overly disappointed in that after a year, Li-Ion batteries typically go down to about 70% capacity but I'd like a more accurate reading - when it says 0%, I expect the thing to shut down in about a minute or less, and have no jump from 8% to 5% with no corresponding 7 and 6% reading.

Oh yeah, this is for my R40 by the way.
Does anyone have any ideas on this?

[GomJabbar]

Lithium Ion batteries have a somewhat flat voltage curve while discharging making it difficult to tell how much capacity is actually left in the battery.

This article explains it pretty well...
Battery Fuel Gauges: Accurately Measuring Charge Level