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Sign that AGM batteries need replaced?

FriendsofMarioFriendsofMario Member Posts: 14
in Battery/Electrical & Solar
2019 400 BDL. One sunny day last week in Tucson (on which we received 280W of solar charge) I decided to try out our inverter for the first time. I turned off the main breaker to simulate no 'shore' power, logged into the Victron BMV via the Bluetooth app so I could watch things in realtime, and made a pot of coffee using the little ol' 600W coffee maker. 

The moment that I turned the maker on, the battery monitor popped up a warning that the battery voltage was low, 11.8V. The app also showed that the amp draw was 65.5A, which I really can't understand (can't make the math work on that). During the approximately 5 minutes it took to brew the coffee, the SOC dropped from 100% to 98%. 

In his "Unofficial nuCamp t@b 400 Resource Guide," the estimable David Weinstein states that "our batteries are fully charged at 12.65 volts and are fully discharged at 11.8 volts. If allowed to reach full discharge, it reduces the lifespan of the battery each time that happens and one day, they read 0-volts and must be recycled," and he AGM batteries have a "5-year typical lifespan" (p. 54).

This being the case, I'm wondering whether our now 7-year-old batteries (trailer was built in April of 18) are telling me, through the 11.8V reading when the inverter was in use, that they need replaced. We're the third owners of this 400, and while the Victron history shows me the lowest voltage, deepest discharge, etc., it doesn't tell me how many times in the past the batteries were taken very low. 

I should add that I am very new to the solar game, our only previous experience with solar being a Renogy 100W portable panel that we use to trickle-charge the flooded battery of our other camper when it's not in use.

Any thoughts are appreciated. 




2019 T@B 400 Boondock
2002 Sunline Solaris Lite 2363
2024 F-150 XLT SuperCrew 4x4
Mario, the porch cat
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Comments

  • qhumberdqhumberd Administrator, Moderator Posts: 695
    edited October 25
    We have the same trailer and year and noted the same issues with loading the battery. Our battery seemed to lose capacity much more quickly than when it was new to us.

    If you look at prices of AGM replacements with the same amp hour rating versus a Lithium, they are not that different these days. We went with a LiTime 230 amp hour battery with low temp protection and I found that the replacement was very easy. Didn't have to buy any new cables or anything. And the WFCO converter does not have to be upgraded since you have solar already to bring a lithium battery to 100% allowing it to balance the cells.

    We have used our new battery on two trips now and my battery anxiety is a thing of the past.

    https://tab-rv.vanillacommunity.com/discussion/19831/2020-tab-400-lithium-battery-upgrade#latest

    2019  T@B400 Boondock Lite "Todd"

    2016 Toyota Tundra 5.7 Crew Cab
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  • MountaintrailsMountaintrails Member Posts: 8
    edited October 24
    600 watts divided by 120 volts is 5 amps, but you are now pulling 600 watts through an inverter off a dc 12 volt source. That’s a lot more amp draw at 12 volt Plus the inverter that takes energy to operate. You are drawing to much . A watt is a watt no matter what!
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  • HoriganHorigan Member Posts: 803
    The voltage verses state of charge values referenced in the Unofficial guide are rested voltages, after the battery sits for a period with no load.  Having it drop to 11.8V while drawing 65 amps is normal.  You pulled 5aH from your battery for the coffee, which is relatively close to the 2% drop in SOC.

    What did the battery voltage recover to after the inverter was turned off for 30 minutes?  If it was close to 95%, your battery is fine.
    Rich
    2019 T@b 400
    2013 Toyota Highlander 3.5L V6
    2025 Toyota Highlander 2.4L Turbo
    Bellingham WA
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  • rh5555rh5555 Member Posts: 526
    I think I can explain the math:  Your little ol' coffee coffee pot is 600W nominal so that number may well be higher.  Also heaters draw more current when cold (did you notice if the current consumption went down a bit as the water temperature rose?)  So, in reality you may have had a 700W draw.  Next up is inverter efficiency, which might well be around 90%, so load on your batteries is ~770W.  For the draw to be 65.5 Amps, this means the voltage at the input to the inverter is 770/65.5 = 11.75V.  The last 0.05V variance from your measured 11.8V is due to voltage drop in the wiring.  These numbers are approximate.  Maybe the coffee pot uses less power, maybe the voltage drop in your wiring is greater, but you get the idea.
    The take home message is that it looks like your AGM batteries are doing really well.  The batteries "are fully discharged at 11.8 volts" only applies under no load.  However you are nearing the time when one or both of these batteries may fail (mine did last year) and then they won't take or hold a charge.  But for now, you're good to go.
    Roger and Sue Hill | 2020 T@B400 Boondock (Cryst@bel) | 2022 Land Rover Defender 110 - P400 | San Juan Island, WA
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  • Grumpy_GGrumpy_G Member Posts: 648
    Voltage and voltage drop are not the best indicators for the health of a lead-acid battery. What matters is how long can the battery supply a decent amount of current. This is the amount of charge or capacity it can hold. I do have some old lead-acid batteries that have less than 5 Ah of capacity left but still will deliver 60-70 amps for a couple of minutes and recover to 12V+ after. 
    If you start with fully charged batteries and can draw 100 Ah (about half of the nominal capacity) before the voltage gets too low the batteries are still useable. 
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