Community Forums

TT_TR250RDS · 01-07-2025, 11:41 AM

Hi all,

I recently upgraded my 2003 Bigfoot 15C 9.5FS with two Interstate Marine 31M-AGM batteries, replacing the original lead-acid ones.

Before installation, I noticed one battery was at 10V and the other at 12V. After charging both for 24 hours, they held over 12V each.

I tested the setup by running only my Atwood 8012-II heater on the lowest setting, which raises the camper temperature about 20°F above ambient. It was around 30°F outside, and the camper reached 50°F.

However, after just 3 hours, my battery meter showed 50% capacity (down from 100%). Based on the heater specs—12V, 1.8A (21.6W)—it should draw ~4AH per hour, or 32AH over 8 hours. With two 100AH batteries (200AH total), even at 50% depth of discharge, I’d expect at least 24 hours of runtime for just the heater.

Does this seem right? I feel like I should be getting more runtime, or could I have missed a necessary adjustment when switching from lead-acid to AGM batteries?

Thanks!

d23haynes57 · 01-07-2025, 12:12 PM

Is your battery meter a shunt type monitoring current in and out or just a voltage meter?
New batteries need to be cycled a few times for full performance. The plates, grids, whatever you want to call are still forming.
Under load battery voltage will drop and a voltage only meter will give an inaccurate reading during charge or discharge.
What are you charging them with? Does the charger have an AGM profile? Enough capacity for fully charge them. Again, smart battery monitoring is very helpful.
For AGM you need a charger that can bring them at least 14.6 volts and hold there for 3 hours for absorption, and then drop to float, 13.2 to 13.6 volts.

It probably would have been better to go Lithium.
These Group 31 AGM tend to disappoint. Interstate is highly marketed but in no way a high-end brand.

BTW, the battery at 10 volts is either overly discharged from sitting for way too long or it may have a bad cell. As a battery guy I would individually charge and then capacity test each.

Dave Pelletier · 01-07-2025, 01:38 PM

Well something's not right.....I'd echo the comments that group 31 12v's wouldn't be my first choice (I'd have gone lithium or GC2 6 volts) but with 100 AH's each they should be able to run your furnace for a couple days not 3 hours. Ditto on the comment for testing the batteries and a real smart shunt (I have a Victron) is needed to tell you how much energy you are actually using.

Cheers,
Dave

SafariBen · 01-07-2025, 02:35 PM

Charge them up, disconnect the cables from both negative terminals, let them sit 24 hours and check the voltage, I bet the one has a bad cell and will be less than 12V, get that one replaced, and hopefully the new one is close enough to the other one to work reasonably.

TT_TR250RDS · 01-07-2025, 02:47 PM

Quote:

Originally Posted by d23haynes57

Is your battery meter a shunt type monitoring current in and out or just a voltage meter?
New batteries need to be cycled a few times for full performance. The plates, grids, whatever you want to call are still forming.
Under load battery voltage will drop and a voltage only meter will give an inaccurate reading during charge or discharge.
What are you charging them with? Does the charger have an AGM profile? Enough capacity for fully charge them. Again, smart battery monitoring is very helpful.
For AGM you need a charger that can bring them at least 14.6 volts and hold there for 3 hours for absorption, and then drop to float, 13.2 to 13.6 volts.

It probably would have been better to go Lithium.
These Group 31 AGM tend to disappoint. Interstate is highly marketed but in no way a high-end brand.

BTW, the battery at 10 volts is either overly discharged from sitting for way too long or it may have a bad cell. As a battery guy I would individually charge and then capacity test each.

It is just a basic voltage meter. It will drop when the heater runs as there is draw on the system then bounce back up in % when the heater kicks off. The charger does have an AGM profile, but will need to double check if it has enough capacity to charge to full.

I would have went lithium, but I wasn't prepared to revamp the entire RV electrical. Additionally, I would not have much luck with any solar set up.

I will double check my charger, and then do an individual charge. When I had them connected to the charger they were reading above 13.2 so I assume it has enough capacity.

Arch Hoagland · 01-07-2025, 02:48 PM

I'm curious where you bought the batteries and what the date code was on them.

Pretty unusual for a battery to be down at 10 volts when brand new.

d23haynes57 · 01-07-2025, 02:50 PM

I have pretty much given up on "off the shelf" AGM batteries.

TT_TR250RDS · 01-07-2025, 02:52 PM

Quote:

Originally Posted by Dave Pelletier

Well something's not right.....I'd echo the comments that group 31 12v's wouldn't be my first choice (I'd have gone lithium or GC2 6 volts) but with 100 AH's each they should be able to run your furnace for a couple days not 3 hours. Ditto on the comment for testing the batteries and a real smart shunt (I have a Victron) is needed to tell you how much energy you are actually using.

Cheers,
Dave

Yeah, I should have probably done the GC2 6 volts. I agree, I figured I would easily get 24hrs with some lights on. There is no microwave, tv, etc in the camper.

Quote:

Originally Posted by SafariBen

Charge them up, disconnect the cables from both negative terminals, let them sit 24 hours and check the voltage, I bet the one has a bad cell and will be less than 12V, get that one replaced, and hopefully the new one is close enough to the other one to work reasonably.

I will do that, thanks for the advice!

TT_TR250RDS · 01-07-2025, 02:54 PM

Quote:

Originally Posted by Arch Hoagland

I'm curious where you bought the batteries and what the date code was on them.

Pretty unusual for a battery to be down at 10 volts when brand new.

Directly from an interstate dealer. I felt the same, I should have tested them at the store before leaving. It is an hour away.

TT_TR250RDS · 01-07-2025, 02:54 PM

Quote:

Originally Posted by d23haynes57

I have pretty much given up on "off the shelf" AGM batteries.

What other AGM would you go for?

d23haynes57 · 01-07-2025, 02:59 PM

Quote:

Originally Posted by TT_TR250RDS

It is just a basic voltage meter. It will drop when the heater runs as there is draw on the system then bounce back up in % when the heater kicks off. The charger does have an AGM profile, but will need to double check if it has enough capacity to charge to full.

I would have went lithium, but I wasn't prepared to revamp the entire RV electrical. Additionally, I would not have much luck with any solar set up.

I will double check my charger, and then do an individual charge. When I had them connected to the charger they were reading above 13.2 so I assume it has enough capacity.

The 13.2 may have been the float voltage of the charger. If one is bad it will be a load to the other. That is why for testing they need to be separated. As these are smaller batteries an auto starter battery tester will give a quick test looking for failure.
This is a reasonably priced one that drops a load for 15 seconds. You set for cranking capacity. 600 amps for those.
https://www.northerntool.com/product...125-amps-37800

Not a true capacity test but will pick up a bad cell really quick.
You should also get a multimeter, one with a clamp on ammeter and learn to use it.
Read from this web site. https://batteryuniversity.com/articles
Learn Ohm's and Kirchoff's laws and you will be a DC circuit expert.

Dennis

TT_TR250RDS · 01-07-2025, 04:09 PM

Quote:

Originally Posted by d23haynes57

The 13.2 may have been the float voltage of the charger. If one is bad it will be a load to the other. That is why for testing they need to be separated. As these are smaller batteries an auto starter battery tester will give a quick test looking for failure.
This is a reasonably priced one that drops a load for 15 seconds. You set for cranking capacity. 600 amps for those.
https://www.northerntool.com/product...125-amps-37800

Not a true capacity test but will pick up a bad cell really quick.
You should also get a multimeter, one with a clamp on ammeter and learn to use it.
Read from this web site. https://batteryuniversity.com/articles
Learn Ohm's and Kirchoff's laws and you will be a DC circuit expert.

Dennis

Ok, well looks like I have some work to do. I do have a multimeter with a clamp on ammeter. Will review the resources. Thanks!

Bobby F · 01-07-2025, 04:11 PM

If your new AGM tested at 10v when you got it home, I think I'd just automatically return/exchange it.

TT_TR250RDS · 01-07-2025, 04:15 PM

Quote:

Originally Posted by d23haynes57

The 13.2 may have been the float voltage of the charger. If one is bad it will be a load to the other. That is why for testing they need to be separated. As these are smaller batteries an auto starter battery tester will give a quick test looking for failure.
This is a reasonably priced one that drops a load for 15 seconds. You set for cranking capacity. 600 amps for those.
https://www.northerntool.com/product...125-amps-37800

Not a true capacity test but will pick up a bad cell really quick.
You should also get a multimeter, one with a clamp on ammeter and learn to use it.
Read from this web site. https://batteryuniversity.com/articles
Learn Ohm's and Kirchoff's laws and you will be a DC circuit expert.

Dennis

Do you have any suggestions on a decent charger that can bring the batter up to 14.6 v? I was looking at this one...
https://www.amazon.com/Battery-Charg...B3CV4S2Y&psc=1