[TechniSol]

The fast dropoff from 25-30% to the recharge point is pretty common, not just for Kobo devices. The issue is that the battery voltage falls off at a different rate as the battery is discharged. When you get down near the end of the curve the battery voltage falls off VERY rapidly compared to a nearly straight line slope for most of the charge. The algorithm determining remaining charge is probably based on a linear or straight line approximation rather than modeling the curve.

If Kobo employed either a lookup table or derived the equation for the battery curve, they could give a lot more accurate picture of remaining power... As it is, I assume 25-30% is the end of the line and figure 65% is really about 50%, etc. I usually charge before I get anywhere near the low end because it tends to be easier on the battery and you can probably charge twice as many times if you're only subjecting the battery to mild to moderate discharges. I think deep cycles are A LOT harder on lithium batteries based on my day to day experience with the batteries in my cell phones over the years. I recall that most of the battery literature points this out as well.