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?
UPDATE:
I don't have the battery's datasheet handy. This information is available on the battery itself:
Valve Regulated AGM Rechargeable
BATT130LED
Charging Details:
Float 13.6-13.8V
Equalisation 14.4-14.7V
Best Answer
Battery aging is inevitable. It occurs during charge transfers and life cycles reduce 50% for every 10'C rise above room temp and to some extent by the minimum % DOD or cutoff voltage you choose.
You must define a spec to choose desired %DoD and thus capacity and thus lower cutoff voltage and thus lower cumulative Ah capacity* charge cycles.
As always, refer to your AGM datasheet.
This datasheet from Victron shows the the number of Charge cycle always increases by using a lower %Depth of Discharge (DoD). This implies a higher "resting" cutoff voltage gives longer life. But how much? When I multiply the # of cycles from the barchart by the amount of charge used (DoD) the total life cycle charge goes up from 300 effective total charges to 435.
( measurements taken from bargraph by counting pixels with I_view)
Conclusion: Reducing your cutoff will extend inverter runtime but also reduce battery number of days of charge life, so you lose time later.
Aging is directly related to total charge transferred during life. For longest lifespan, keep at room temp! (20~23'C) and choose Mfg's advice wisely.
If you need more capacity, use more batteries rather than lower the cutoff, but if desperate, you can reduce cutoff to 10.8V for the best short term capacity.