IBM/Lenovo ThinkPad line is basically the only manufacturer/brand of mobility devices that includes circuitry and software that allows one to vastly prolong battery life of laptops used as stationary workstations by making sure to never charge the battery prior to it being discharged to below 96%, and having options to start charging only when the battery goes, say, below 46%, and stop charging at, say, 50% or 80%, vastly improving long-term battery health.
Why is this apparently useful feature very uncommon? - Is the circuitry behind such feature somehow intrinsically expensive and not worth it? (Or is it just the case that stationary usage is not accounted for in the design of such other devices?)
There is no technical problem with doing this, and there are advantages in allowing partial state of charge, although not necessarily always as great as you may imagine. There are a number of factors that affect battery cycle life and manufacturers could choose other ones than you suggest.
$V_{min}$ is already controlled by the control circuitry to a level that manufacturers consider safe and this probably varies somewhat between manufacturers. About 3V/cell is usual.
I do not know if patents cover any related aspects but this seems unlikely to have a blanket effect as partial charging in general has general prior art and LiIon technology is now old enough that early patents will have expired.
A major gain can be obtained by simply limiting $V_{max}$ from the usual 4.2V to 4.1V or 4.0V, with a significant gain in cycle life and loss in capacity per cycle.
A significant gain in cycle life can be had by increasing the current at which charging terminates. Usually LiIon / LiPo charge at 1C (mA = mAh) until Vmax is reached then hold voltage at $V_{max}$ and allow $I_{charge}$ to decrease under control of the battery chemistry. If $I_{charge}$ is allowed to decrease to about C/10 = 10% of max rate then the battery attains full capacity but the cell is pushed very near to its upper limit (close to the point where metallic Lithium may "plate out") and the cycle life is decreased. By stopping charge at C/4 or even C/2 capacity is somewhat reduced but cycle life is increased.
Users can achieve their own improvements in cycle life in everyday use at the expense of available capacity. When charging from "empty" at C/1 a LiIon battery will charge at CC (constant current) until $V_{max}$ is reached - usually at around 70% + of full capacity. If charging is terminated at this point (nop CV "tail) then cycle life is increased significantly - maybe 2x to 3x. If charging from empty at C/1 this point is reached at about 0.7 hours = 42 minutes. So a say 40- minute charge from empty about reaches this point.
Generally reducing battery capacity per charge gives diminishing returns. eg if you use half capacity per charge cycle life would need to at least double before you got the same whole-of-lifetime capacity. ie N x 100% = 2N x 50%. In practice you may get say 3N cycles for C/2 capacity so a 50% plus gain in gross capacity over lifetime.
Reducing charge to 1/3 capacity may give 6 x as many cycles.
But reducing charge to say 10% of capacity may give not too much more than 30 x as many cycles so still a not much more than 3x gain, if that.
Those figures are 'out of my head' but I believe they are in of the generally correct order and will be enough affected by other factors that they can only be a guide.
Lithium Ion batteries that must have extremely long cycle lives may be charged to about 2.8V/cell. This greatly reduces cell capacity (maybe to 30% of normal max) but allows maybe 8000 cycles. The Mars Opportunity Rover battery is run at this sort of level plus "all the magic they can work" to achieve this sort of result.
References:
Look at www.batteryuniversity.com for more related information.
These pages are relevant:
BU-409: Charging Lithium-ion
"Find out how to prolong battery life by using correct charge methods."
BU-808: How to Prolong Lithium-based Batteries
"Discover what causes Li-ion to age and what the battery user can do to prolong its life"
