(LIFEPO4) Battery is at 13.3V when I put a 15watt DC load Voltage drops to 12.6. Cant figure it out

[SolarHawaii]

I have a 12V 115AH LIFEPO4 Battery from Simpliphi. I am not sure if something is wired wrong or battery is bad. All terminals are very tight. Battery is charged and reads 13.3V and when I plug in a 15 watt load it drops to 12.6. Help would be much appreciated I have been working to try to get this system going for way to long now haha