For the charging of lithium iron phosphate batteries, a special charger is required, which charges the batteries according to a special charging procedure. (CC/CV = constant current, constant voltage.)
Do not charge the batteries with a higher voltage than 3.6 volts. It is advantageous to charge the batteries only up to approx. 3.5 volts, as this significantly increases the service life or number of charge cycles (possibly up to a doubling). Only lithium chargers may be used for charging. Only lithium chargers with a charge up to 3.6V may be used for charging.
It also says this above in german...
And again the manufacturer recommends 3.65 just like headway and other lithium ion phosphate manufacturers. That helps crappy BMS balance and gives faster charge, at the cost of service life.
3.60 good balance, can be used with AGM and an Add-on lithium pack. Voltage matches.
3.55 is what I use mostly,
3.525 is the same as gel batteries CV so good to charge combined lead/lithium add-on setups. And the long termination matches both. And the lithium holds the lead high after the charge so helps de-sulfate/finish the lead charge.
3.50 takes a very long time, needs a very low sub 1000thC termination point, and OK if you have time and a suitable charger with powerful balance capability. This will give best cycle life. If the long wait and less well balanced batteries are not an issue. Still ends up full.
Lower than this voltage will not charge them or balance them properly no matter how long you wait.