What is the maximum static voltage of LiFePo4 batteries?
I may have found a possible answer: https://endless-sphere.com/forums/viewtopic.php?t=51586

The charging voltage is 3.6-3.65v for LiFePo4 (4v for the chemistry with the addition of yttrium) and a voltage of 3.6v is 100% charged (safely. This chemistry used to be charged to 4.2v max but it was discovered the this decomposed/oxidized the electrolyte). Voltage does decrease normally after charging has terminated, and this decrease is determined by how long you charged at 3.65v and how low the current was allowed to fall. The voltage decreases because the chemistry is still absorbing the electricity/energy from the charge. The voltage of the terminals at the end of the charge cycle may be higher than the voltage of the interior of the cell. As this equalizes within the cell, the voltage drops to match.

A fully charged LiFePo4 battery will stay at 3.6v (or close to it) even after resting for 24hrs. Cells that have gone through many cycles may fall to 3.33-3.4v due to cell degradation. 3.37v is considered 80% charged, 3.4 is 90% and 3.6 is 100%. There is very little useable energy between 3.4v and 3.6v with the LiFePo4 chemistry and the voltage drop will reflect this when the battery is in use. This cell under load will reach 3.3v quickly and then remain between.3.2 and 3.3v for quite a while (flat discharge curve coming into play).