[TechniSol]

As Ken said, the lower limit is set either by a hardware circuit preventing discharge below a certain lower limit or through software looking at an A/D converter. The upper limit is key to the issue of capacity as the voltage level at which charging should be stopped varies with aging -it's not simply voltage dependent but sensed by looking for a change in rate of rise(dv/dt) or actual drop, as I recall, depending on the method. It's been a while(years) since I researched data sheets on lithium ion charging/power management chips so cut me some slack if I mangled the exact method.

The things is that the curve chosen to represent the approximate relationship between voltage and remaining current capacity has to be modeled properly and you have to modify it to match the change in upper charge cutoff(terminal charging voltage) as the battery ages. As was said by someone else the proper way to do it would involve monitoring current supplied during a full discharge from 100% to the hardcoded minimum voltage level and then base charge level on the most recent capacity determined through that full discharge cycle, or through it averaged with a previous cycle or two to moderate the change in capacity in case it was skewed through low temperature effects, or the like, temporarily affecting capacity(current).

As Ken said, once every couple of months I run lithium batteries down, especially if they have been charged at 50-75% or higher a lot, just to give the charge metering a chance to recalibrate through experiencing a full cycle.(I mean 50 -75% of capacity as depicted by the battery gauge on the device, not actual capacity as there is always a bottom end that doesn't get discharged to prevent damage to the battery.)

I don't mean to be absolute on this, but I fail to see the benefit of charging to termination, unplugging and repeating, unless the device has a faulty final termination detection algorithm or a thermal cutoff is getting in the way and interrupting the process -been known to happen, but I don't really want that to be the method chosen to prevent my device, possibly my home, going up in smoke...