There are much development of battery monitors done by digital camera professionals. Sony had done especially many years ago. KDE team could have studied Sony's approach in DSC-p30/p50 cameras. Which is very accurate when battery is freshly recharged. Then monitor failed very badly after the minutes are predicted to half the length of time on infoLithium system. It shows the nonlinear behavior of Lithium batteries when fully charged or partially charged. The use of high capacity(2000ma) NiMH batteries(lower voltage) are worse in the length of time prediction resulting in sudden depletion of battery capacity indicator.

Anyway, digital cameras pioneered zoom technology for vector graphics display, and battery monitors for charged batteries. Once the battery voltage falls below a critical value(circuit demand dependent), the time estimated when fully charged will be totally unusable. Sad but true.

Eventually, one has to have a mathematical formula to predict when the critical battery voltage(charged capacity) will happen? Battery should be recharged before fully depleted then trickle charged to maintain battery condition of internal memory(chemical uniformity or homogeneity or damaging oxidation)?