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AGM state of charge


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 We purchased a 2005 Dynasty ISL last September, it is equipped with 4 6 volt Full River AGM 250 amp house batteries dated 2014, the coach still has the original Xantrex model 2012 charger/inverter and set at 20 amp charging rate. The coach is equipped with a RC7 GS. The batteries are also equipped with Power Pulse ( https://www.pulsetech.net/pp-12-l-powerpulse-12-volt-battery-maintenance-system.html ) battery maintainer’s and also one solar panel on the roof and the coach is plugged into shore power 24/7. The coach is inside a storage building at a constant 52-54* temp. I’m curious as to the longevity of the batteries, yes I could just replace them but I’m also curious as to how much life I can get out of them. I must say I have not been let dow by them on the few nights we spent boondocking with minimal power usage, one sleep machine on all night and a toe rail light on.

So here’s my question,  if I unplug the shore power and watch the RC7 GS in the percentage SOC mode with the inverter off, I’ve noticed the percentage of SOC drops rather quickly down to the mid to upper 80’s % in approximately 5 minutes, then it seems to stabilize and then the SOC actually starts to increase back to 97%ish and then it holds pretty steady, this could all be very normal, I’ve never really sat and watched batteries SOC so I can’t say what’s normal.  Is this normal or is it telling me something I’m not seeing?

 Thanks in advance!


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The RC seven measures the SOC by voltage which is a very poor way of verifying SOC. The correct way would be to add a shunt like a Victron BMV 712. Measuring SOC with voltage will vary greatly anytime you put on or takeoff a load to the batteries. 


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My RC7 display shows about the same thing and I quit worrying about it.

I don't use the AGS function so I was curious last year and asked the same question.  How does the RC7 determine voltage to be able to use the auto generator start.  Never got an clear answer. 

FWIW, my flooded lead acid batteries were ~11 years old when I replaced them last year.   Prior to a trip out west last Nov I checked them with a hydrometer and all the cells tested good and voltage of each battery was good.  Went on my trip and all was good until I was parked one day and the display started to flash, checked the batteries and one was dead.  Luckily I was near a Sam's club that had 4 of the same batteries and had them replaced.  I actually had been contemplating replacing with AGM prior to the trip but they were hard to come by.  I asked at Sam's and they did not have any and did not know if they could get them so I replaced with the lead acid flooded type.  

Edited by jacwjames
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25 minutes ago, jacwjames said:

I was near a Sam's club that had 4 of the same batteries and had them replaced.  . . . . contemplating AGM . . . . . replaced with the lead acid flooded type.  

"I've heard" that AGM's (high cranking amps) are best as chassis batteries and flooded lead acid (deep cycle) are best as coach batteries, so IMO you did the right thing.  When my turn comes I'm going straight to Sam's.  They (Duracell GC2) may not be the best but IMO best bang (or zap) for the buck. 

There's some good info at a web site called "Battery University" https://batteryuniversity.com/articles  They get pretty deep.

In Jdw12345's case I would run those batteries until they don't do the job anymore . . . . but I don't rely on a CPAP. 

- bob

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53 minutes ago, Gary 05 AMB DST said:

Jim, on your battery disconnect switch, if you have this option, you could put each 12v combo on its own switch, L1, L2, both, and isolate the dead battery. I know it's too late for you but maybe someone else could go this way.

Gary 05 AMB DST

If I was somewhere that I did not have options I probably would have taken the one dead battery out and just kept two of the good batteries to provide power until I could get somewhere to do a more permanent solution.  Since I was near a Sam's club that had batteries I just went ahead and changed them all out.

I then went on with my trip and gave the batteries a good test, I boondocked at the Elk Mountain campground in Wind Cave NP.  This was the first time I had the Samsung refrigerator.   One day it was sunny enough for my 325 watt solar panel to top off the batteries and I didn't have to run the generator.  Went to bed with ~90% battery charge and woke up with ~75% charge.  Nothing was on except the refrigerator.  Over the next ~3 weeks I consistently saw battery consumption overnight about the same.  I had also installed the Bluesea which combined battery banks while driving so at the end of a driving day I didn't usually have to run the generator at all. 

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