What precaution should be taken when connecting a solar panel to a lithium iron phosphate battery?
Why Solar panels put out an unregulated voltage that rises well above the battery's safe charging range in full sun (a nominal 12 V panel can reach 20 V or more open circuit), and lithium iron phosphate cells are damaged or made hazardous by overcharging. A charge controller regulates the current and holds the battery to the correct charge profile, typically constant current then a constant voltage of about 14.6 V for a 12 V LiFePO4 pack, then stops. Lithium chemistries are less tolerant of overcharge than lead-acid, so the controller must also be set for the lithium profile, not the lead-acid one.
Watch out A series resistor only limits current, it cannot stop the panel voltage from climbing above the battery's safe limit, so it is no substitute for regulation. Grounding the panel frame is good general safety practice but does nothing to protect the battery.
Panel to battery always goes through a charge controller, and set it to the lithium profile.