Originally Posted by ascdds
I have recently converted from lead acid house batteries to AGM batteries. I had my autogen set to come on at 11.9 volts with the lead acid. Should the setting be the same with the AGM’s?
I see a lot of good advice above. The optimum gen start voltage depends on many factors.
To keep it simple; if 11.9 volts was working for your flooded cell batteries, then 11.9 is still good for AGM batteries. They will be be very close to the same as the old flooded cell batteries.
11.9 volts is 40% state of charge (SoC). However, that only applies when the batteries have been at rest for a couple of hours. As soon as current starts to flow from the battery, voltage drops. The higher the current flow the greater the voltage drop.
Just because voltage drops when the inverter is working hard, does not mean SoC is low, so starting the generator may not be necessary.
A freshly charged AGM battery may start with a static 13.0 volts. Even if the inverter pulls the voltage down to 12.0 volts, the SoC is still nearly 100% SoC.
If the SoC starts at 12.2 volts (60% of charge), and the voltage drops to 11.2 volts, the SoC is still 60%. The voltage will slowly rise back to near 12.2 when the microwave shuts "off". Technically the generator still does not need to run.
However, the inverter will stop working if voltage drops below its cut off voltage. You want the generator to start before battery voltage drops below the inverter cut off voltage. See inverter manual for cutoff voltage.
Still more complicated. The generator starting motor will probably cause the voltage to drop enough to cause the inverter to stop working. Start the generator before inverter cuts off.
Complicate that with the fact that voltage drops less with a larger battery bank and drops more with a smaller battery bank.
So, what to do? If 11.9 works for you and your RV configuration and use, then continue with the same voltage.
If you get nuisance generator starts, lower the start voltage. If you find the batteries deeply discharged for long periods, raise the auto start voltage.
AGM batteries tolerate deep discharge better than flooded cell batteries. However, they do not tolerate being discharged for long periods, like a week or more.
Lead acid batteries have three basic voltage profiles
. They are "charging", "discharging", and "static".
voltages depend mostly on the charger. Voltage starts at the static voltage and raises slowly to the preset charging voltage. Typical charging voltages are 13.2, 13.6, or 14.4 volts.
voltage will always be lower than "static" voltage. The faster the discharge, the lower the voltage. Voltage will slowly raise to "static" level when discharge is stopped.
voltage can be used to monitor state of charge (SoC). It is when no charging or discharging has taken place for a while. Voltage will slowly stabilize while resting. Accurate static values can usually be made after 3 hours of rest.
"Static" State of Charge (SoC) voltage
After 3 hours rest.
Lead Acid Batteries
I wish you good luck and happy trails ahead!