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Old 06-28-2022, 10:42 AM   #1
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Voltage to set autogen for AGM batteries

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?
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Old 06-28-2022, 11:12 AM   #2
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Depending on the load on the batteries , 11.9 may be a little low. At 11.9v (no load on your batteries) the state of charge is less than 20%. Depending on the load , 11.9 may be just fine. If you want to only discharge your batteries to 50% DOD you should set the autogen to around 12.2v open circuit. A lot depends on how often the batteries are cycled. If they only get used a few times a yr 11.9 should be just fine. If they get cycled on a daily basis for months at a time you should consider a higher setting.
For example , if you have a 100 amp hr battery with a 10 amp load on it , 11.9v will be about 50% DOD which is good.
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Old 06-28-2022, 02:15 PM   #3
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Yes, measuring under load is an inexact science….. if I run the microwave on inverter, the heavy load (167 amps from batteries) will draw the voltage down after awhile low enough to trigger my auto gen start…..it always startles me when that diesel gen starts and I didn’t expect it - lol

Take that huge load off, and voltage bounces back up quickly even without the generator starting.
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Old 06-29-2022, 08:12 AM   #4
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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?
You will never have your batteries at complete rest when using the coach so an at rest setting is of no use. With average loads on the batteries while the coach is being lived in a voltage of 12.0-12.1v is in the ballpark of 50% SOC for an AGS setting with your AGM batteries.
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Old 06-29-2022, 08:17 AM   #5
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If running an inverter off of those batteries and using high draw appliances, like a microwave, you’ll need to keep the setting below 12v unless you want the generator to start every time you use the microwave.
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Old 06-29-2022, 09:12 AM   #6
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If running an inverter off of those batteries and using high draw appliances, like a microwave, you’ll need to keep the setting below 12v unless you want the generator to start every time you use the microwave.
To keep the generator from starting when using a microwave or other high draw appliance on an inverter you would need an AGS setting in the neighborhood of 11.2v. This would be an unhealthy setting for when there was no large inverter draw.

A high draw appliance is probably used at most 5% of the time while using the coach on batteries. Better to set an AGS voltage setting that will protect you and recharge your batteries for the 95% of the time instead of the 5%.

To prevent unnecessary generator starts the AGS can be disabled for the occasional use of high draw appliances.
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Old 06-29-2022, 09:58 AM   #7
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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".

"Charging" 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.

"Discharging" 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.

"Static" 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
100% 12.7
90% 12.5
80% 12.42
70% 12.32
60% 12.2
50% 12.06
40% 11.9
30% 11.75
20% 11.58
10% 11.31
0% 10.5


I wish you good luck and happy trails ahead!
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Old 06-29-2022, 02:52 PM   #8
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To give a simple answer to your original question, yes you can use the same voltage for the AGMs as you used for your previous flooded cell batteries. Whether 11.9v is a wise choice is a matter for debate and you've received plenty of opinion on that!
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Old 06-29-2022, 03:07 PM   #9
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Take that huge load off, and voltage bounces back up quickly even without the generator starting.
Which is the reason I have lowered mine to 11.8 or 11.9v.

My coffee maker was often firing up my gen in the morning when I had it set to 12.2v. Since I have lowered the trigger voltage, I can brew my pot off coffee, and when it's done brewing, the battery voltage will often return to 12.3-12.4v!
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Old 06-29-2022, 05:47 PM   #10
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Yikes, seemed like a simple question but complicated answers.

Keeping that mood going at what voltage should AGS stop? Is their a recommended max run time?
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Old 06-29-2022, 05:53 PM   #11
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Yikes, seemed like a simple question but complicated answers.

Keeping that mood going at what voltage should AGS stop? Is their a recommended max run time?

I set mine to 1 hour run time. That will put a good charge in the batteries if I’m not there, and if I am there, I will monitor the charging and adjust as necessary.

AGS is mostly a safety net, not a totally automated battery maintenance solution.
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Old 06-30-2022, 04:09 AM   #12
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Seems like a time delay on AGS would solve short term high draw inverter issues.
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Old 06-30-2022, 12:28 PM   #13
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If he was happy with the way the 11.9v AGS setting worked before, it will operate the same after changing to AGM batteries. All the rest ifs technobabble.
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Old 06-30-2022, 12:38 PM   #14
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If he was happy with the way the 11.9v AGS setting worked before, it will operate the same after changing to AGM batteries. All the rest ifs technobabble.

YEP^^^

AGM batteries are a sophisticated version of lead-acid batteries
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