I am using an AGM deep cycle battery 130Ah 12V connected to 200W solar panels and a 500W inverter to power electrical devices such as fans, laptops and lamps. The inverter has an automatic cutoff feature at a nominated voltage. I understand that deep cycle batteries should not be discharged below 12V (approximately 50% SOC). I have noticed that when the inverter is in a state of providing charge that the measured battery voltage is considerably lower than that measured if I switch the inverter off. Sometimes the measured battery voltage can jump as high as 0.5 to 1V when the inverter is powered off while it is powering devices. On this basis, I have set the inverter's auto cut off setting to 11.5V. Sometimes the voltage drops to 11.5V or less and I lose power even though the battery's actual voltage is much higher.
If the battery should not be discharged to less than 12V, does this refer to the battery's resting voltage or the lower voltage that I am seeing while it is powering devices?
What is a safe level to set for the inverter's auto cutoff voltage so that I don't destroy the battery and also to prevent devices from cutting out while the battery still has a sufficient SOC?
I don't have the battery's datasheet handy. This information is available on the battery itself:
Valve Regulated AGM Rechargeable