Ignoring the data sheet of a batterie in some way. Is it bad?

[Volta]

If I have a Lifepo4 cell and its data sheet say "chargeing current: 500mAh, 3.6V"...is it bad for the batterie to charge it with less current and only up to 3.5V?

 

[hamhanded]

Perfectly safe. Those particular specifications are maximums and LiFePo4 doesn't generally experience memory effect.

 

[KITROBASKIN]

If you also avoid numerous severe discharges (and let them sit there), your battery will likely serve longer.

Some people charge no higher than 3.4V, claiming that it is over 90%(?) charged at that level.

diysolarforum is a good source for LFP battery health.