How to limit maximum voltage when it's 0.1V too high and the device controller complains?
The extra 0.1 V or so represents minimal energy in the battery.
A relatively low current drain shunt regulator applied WHEN THE BATTERY IS NOT ON CHARGE* can be used to reduce the battery voltage.

Zeners have a too rounded "knee" for this task and will noticeably dissipate battery capacity needlessly. A current limited shunt regulator using a precision shunt regulator IC will meet the need.

A very cheap solution is a TL431 clamp regulator / "adjustable zener"  driving a transistor with a series load resistor to limit current.  

Example only: 

R!, R2 control TL431 turnon voltage.
ZD2 clamps M1 gate to a safe maximum on value.
M1 + Rload acts as a shunt load.   

Rhyst is optional. When equipped it is a small value R such that
V = IR = I_RLoad x Rhyst is say 0.05 to 0.1 V. 
When Vin rises to a level that turns ZD1 on, the voltage across Rhyst steps from 0 to say 0.1V, which adds to the voltage on the R2+R1 divider, increasing the apparent Vin value. Thus Vin has to fall to somewhat below the original Vtrip before M1 is turned off. This has the effect of ensuring M1 is fully on or fully off and not in a partially resistive state. This means that clamp current is set by I = V/R = Vin/Rload and that M1 dissipates minimal power when on.

Dimension Rload wattage to suit. 

*If the load current through Rload is small enough the circuit could be allowed to operate during charging.  







simulate this circuit &ndash; Schematic created using CircuitLab