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Old 10-10-2021, 10:11 AM   #1
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Time for a new house battery?

Ann and I were on a weekend camping trip and while relaxing at Chickahominy Reservoir, photo below, I noticed that the house battery voltage was getting low after a night of power usage.

Evening no load voltage when arriving and engine off 13.25
Morning no load voltage after fridge and lights running 12.5
Morning voltage with fridge running 12.16

Am assuming it is time for new house batteries.

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Old 10-10-2021, 11:25 AM   #2
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I assume you have a traditional flooded lead acid or AGM battery.

It's difficult to say whether your experience is problematical or not without knowing the amp hours you consumed overnight along with the amp hour (AH) capacity of your battery. Note that, in general, your useful AHs will be something around 50% to 60% of your battery's AH capacity. This is based on two factors, the more you discharge over 50% the lower your battery life and secondarily, the further you discharge, the lower the available voltage. Most battery manufacturers recommend that you never discharge more than 80% state of charge (SOC). This is not the case with Lithium (LiFePO4) batteries.

Your 13.25V is misleading since it is high due to what's known as surface charge:

https://www.batteryequaliser.com/bat...tenance-facts/

There's also some good information here (even though it's written for boats, it's still applicable to your situation):

https://marinehowto.com/under-load-b...oltage-vs-soc/

I'd also suggest getting a hydrometer to test your specific gravity as described in the first link.
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Old 10-10-2021, 12:20 PM   #3
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larrie, how old are the batteries?

Seems this is not normal behavior so what would the normal voltage be in the morning?

Are you using some sort of SOC monitor? Watching outgoing amps?
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Old 10-10-2021, 08:38 PM   #4
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Have a Victron meter that monitors the two 75 amp hour (150 amps total) flooded batteries that are about four years old. The fridge draws about 3.5 amps and runs about 50% of the time. So with two batteries I should have 75 amps capacity. If the fridge runs at 3.5 amps for 17 hours at 50% it would use about 30 amps. Throw in a few LED lights and some use on the water pump and call it 40 amps used out of 75 amps available. It sure seems like I should have a higher ending voltage if the batteries are still healthy.

I would use a hydrometer but the batteries are a pain in the but to get at. They are stuffed between the frame rail and barn door step.

Thanks for the links.
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Old 10-11-2021, 05:30 AM   #5
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I have a pair of 1-year old Interstate 6v GC2 batteries (around 210 AH @ 12V total). With small 2.5cf fridge overnight, voltage is 12.8v in the morning. I run a small 700W electric tea kettle for coffee in the morning. Only then does the voltage drop to 12.0v when kettle is on, but comes back-up a few minutes after I make coffee. Batteries lose approx 0.1v per pot of coffee I make.

Sounds like your batteries are on their last legs....

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Old 10-11-2021, 06:14 AM   #6
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Agree. Sounds like either you need to top off the batteries or replace.
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Old 10-11-2021, 10:09 AM   #7
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Flooded batteries with caps require you to check the water levels from time to time. Even maintenance free flooded batteries can loose water. Once the plates are exposed, your batteries can be damaged. It could be just one cell causing the problem but as mentioned a hydrometer is key to testing. That's why AGM batteries excel because they are sealed & not prone to water level issues.
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Old 10-12-2021, 06:31 AM   #8
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So remember, as the battery ages it is the capacity that degrades. So the 150 amp hour capacity is not that anymore. The 50% target is great, but the fact is you are basing 50% on a new battery. This actually helps speed the process in degrading the battery.

You can do a "Pseudo" load test, hook up a 20 hour load, in this case 7.5 amps. I suspect you won't get close to twenty hours. A proper load test needs to done in a controlled environment, but you could get close.

-greg
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Old 10-12-2021, 09:35 PM   #9
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Well sounds like, as I thought, I need new batteries. I really like the lithium system I built for my Jeep so will probably go that way. Just need to find the wiring diagram I made for the van and see how I need to modify it for lithium.
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Old 10-17-2021, 10:22 AM   #10
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Is it worthwhile to run the van engine while using a 5K inverter having 2, 12 vdc 75 amp ea. amp gel batteries when heating water via 35 amp. 120 vac.? It takes about 20 min. or so to heat the small tank. Usually it's night and no solar charge available.
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