House Batteries Draining with good batteries

[DanR]

Thank you in advance for any assistance. We have been through the forums and blogs and haven't found a similar occurrence. I must be missing something obvious.
2013 Journey 42E
6x VMaxTanks 135AH batteries (installed May '22 and load tested today as good)

PROBLEM: at full charge the batteries last less than two hours under nominal inverter load (~16A, i.e. the fridge and some DC lights). With 810AH, I expect 20+ hours.

DETAILS:
- full timing - mostly with full hookups
- Magnum charger/inverter
- inverter stays on all the time
- charger stays on all the time
- new VMAX batteries installed May '22 (6x 135AH AGM)
- charger setting set to VMAX parameters:
-- absorb 14.7V, 2.5 hours
-- float 13.7V
-- low battery cut out set at 12.1V


TROUBLESHOOTING so far:
- voltages and currents tested at the batteries/cables to ensure what is displayed is accurate
-- charge voltages and current test (at the batteries) consistent with what the Magnum controller is saying
-- discharge currents are also consistent (allowing for the DC loads)
-- normal discharge (with fridge and DC loads) is about 16A (ranging from 3A to 21A, based upon items cycling)
- load tested batteries - test good
-- disconnected all batteries and tested each voltage after rest
-- load tester showed 775+/- MCA @ 12.91V
- checked BIM - appears to be isolated properly

At full charge, the batteries display 13.7V when disconnected from the charger and drop to 13 after an hour (no load). At load, the battery bank drops to 12.5V after 10 minutes and will hold there for an hour or so. Then, the voltage drops quickly below 12V.
The items running are the inverter (powering the fridge and a few ancillary items) and a few DC (LED) lights. The electric water heater is off. I've checked (several times) the current at the batteries and am not seeing any additional draws.

I think I am missing something obvious. Any thoughts?
We full time, so dropping off at a dealer is not a great option.

[eyecando]

If we can assume the batteries are OK, there are only two possibilities. Either the batteries are not getting fully charged or the discharge current is higher than you think. If you read the current at the battery terminals, it should approach zero after a full charge.

I would take a clamp on ammeter and record the discharge current at the battery terminal after disconnecting the 120VAC. If more than one wire is connected to each terminal, make sure the clamp goes around all of them. Connect a voltmeter to the battery and record the voltage at the same time.

Leave the ammeter connected and connect the 120VAC. watch for the current to reverse when charging starts. The actual current from the charger/converter would be the sum of the two readings, but the charging current is the actual reading on your meter. Record the voltage. I would expect it to go to around 14.7 volts at least initially.

I would continue to monitor the voltage until it drops down to the float level (13.7 volts). When the voltage drops to 13.7 volts, I would expect the current to be low, probably no more than 3 or 4 amps and approach zero after a long time. Once that occurs the battery is fully charged.

One problem with all the batteries connected in parallel is its very difficult to determine how each battery is performing. If the batteries are actually fully charged at the start and you are only getting 2.5 hours out of them, that would correspond to at most one good battery.

I suspect the actual problem may be the batteries are not getting charged properly but taking the voltage and current readings I suggested should prove or disprove this.

[Morich]

I would agree with the idea that they may not be getting fully charged but there are times when simple stuff can be a problem.
Maybe take a good look at how the batteries look. Are they in a space that gets a lot of dust, etc. that create a path from positive to ground?
A bit too much damp dust and dirt or something like a battery clamp a bit too close to one of the positive posts, so that a continuous short to ground can be working as an additional drain?

[DanR]

Eyecando,
Thank you for the thoughts. I agree with you and Morich that the likely culprit is undercharging.
I have watched the current and voltage (at the battery and monitor) and am not seeing the current taper down significantly before the charger enters float mode. The current will stay at 12 +/- amps at 14.7V until the set absorb time is reached (2.5 hours) and then enter float mode. I would expect that the current should taper down to just a few amps prior to entering float mode.
I just got off the phone with tech support at Magnum and they suggested using the charging voltage to 14.9V and to try increasing the absorb time incrementally to see if that helps capacity.
I have reset the absorb time to 4 hours for now to see if that helps. I will know more this afternoon after they charge.
I have disconnected the entire bank, ensured the connections and leads are clean, and measured each battery individually. Everything seems in order for now.
Initially, I suspected a single bad battery in the bank, but that doesn't appear to be the case.

[eyecando]

DanR

I think the advice your received from Magnum was good advice, especially about increasing the absorb time. You will know when the batteries are fully charged when you get into float mode and sometime later the current into the battery approaches zero.

Mike

[eyecando]

DanR

No problem. Glad to help.

What is your maximum charge rate from your converter/charger? I suspect you may need to increase the absorb time depending on the charge rate. With 810AH of batteries, you probably should have a charger rated for at least 100 amps. I believe the charge rate could be up to around 400 amps but that should be verified with Magnium.

Mike

[creativepart]

What Magnum inverter/charger do you have? And, what remote are you running?

If it's the 3-button ME-MR remote you might consider upgrading to the ME-ARC50 remote. The ARC family (ARC=Advanced Remote Control) lets you set many more of the charge properties. Though, the way you talked about leaving the charger on I assume you must have an ARC remote because you can't turn off the charger with the 3-button ME-MR remote.

With the ARC when setting Absorb Done you have a choice of time, SOC or AMPs. If you set it for zero amps or .5 amps then when your charger nears full charge and reduces the amperage being delivered to almost zero it will turn off the charger when the batteries are full. And consider setting Max Absorb Time to 4 hrs or 6 hrs.

[powercat_ras]

You can estimate the state of charge of your battery bank when not on generator or shore power charging from DC voltage.

Better yet, you can install a battery monitor that will show you current, voltage, and battery state of charge to the nearest 1%. They involve installing a shunt between the batteries and ground so the thing can track exactly how much energy moves in or out, so it can then track the exact state of charge.

Here is an inexpensive one I have installed on a RV for a friend in 2021 that is still working with no issues after 2 years in service. There are also slightly more expensive alternatives that don't involve installing a display in the RV and having to run the associated signal cable that just display the information on a smart cell phone app.

Here is the link to the cheap hardwired battery monitor.

Here is the image of the battery state of charge vs. voltage for lead acid batteries that are not being charged that have no load or a small load, compared to the battery bank capacity. This should give you a reasonable estimate when the only think loading the inverter is the refrigerator, or less load than that.

[MadMaxWinny]

All great advice. Be careful with voltage as a measurement of capacity. It's handy to know when your charger is running, but almost worthless for anything else.

x2 on the monitor upgrade. Accurately tracking amp-hrs and real time amp loads will give you a ton of useful info.

That fridge (through the inefficacies of the inverter) is quite a continuous load. I also agree that 40 amps of charger is nowhere near what you need on a 800+ amp-hr bank.

Finally: "load testing" a deep cycle battery is not useful. The industry standard is the "20 hour test," but it is tedious and only easily accomplished on a test bench.

[Four Seasons]

I have a friend with a 2014 Winnebago Adventurer 38Q who is experiencing the same symptoms. He replaced his batteries and still has the same problem. In his case it appears to be a voltage leak somewhere. After weeks of troubleshooting he has left his rig with a Winnebago service center to let them try to find the culprit.

[JimStreit]

I could be wrong, but typically the most practical depth of discharge (DOD) for an AGM battery is 50%, which means you really should cut the AH in half when factoring how much usable power you have. With six 12v batteries you get a total of 810 AH but I think you should base your usage and time calculations on 405 AH.

[bigb]

Hold the phone here a minute, you stated you had 6 batteries at 135ah each but are they 6 volts or 12? I somehow saw the 6x Vmax and took the 6V for 6 volts but they are probably 12?

edit: Just looked them up.....carry on

[Ray,IN]

Dan, you said you have " new VMAX batteries installed May '22 (6x 135AH AGM)".
For clarification, are these 12V batteries or 6V batteries?
When you used a DC current meter, what was the reading for battery draw?
What happens to current draw when you turn off the inverter?