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Old 05-11-2021, 09:02 PM   #1
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2020 28' Flying Cloud
North Oaks , Minnesota
Join Date: Apr 2019
Posts: 13
AGM battery performance


Thanks in advance. I have a 2020 FC 28 with AGM batteries and 150 watts of solar. I have some questions about the battery's performance. I am surprised how quickly the battery discharges to 50% per the Sun explorer II read out. For example, boondocking with temps below 32 degrees will cause the furnace to cycle around 10 times per night and will result in the battery showing 50% remaining power. Even with using the battery disconnect switch activated and without known parasitic load, the battery discharges at a rate faster than expected (in 2 weeks it will discharge from 100% to 65%). Is the Sun explorer's readout accurate and in real time? I am impressed how quickly it re-charges with the solar package, but overall I disappointed that I have to ration use of the battery to avoid discharging the battery beyond 50% when boondocking. If anyone else has this set up I would appreciate hearing about your experience.
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Old 05-11-2021, 09:31 PM   #2
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Old 05-12-2021, 12:25 AM   #3
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2021 16' Bambi
Nashville , Tennessee
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Might be time to buy a clamp meter and start taking some current measurements to see what's drawing power.

The furnace in my '21 16RB is 12000BTU and the fan draws 3.5A when running. The furnace in yours is 25000BTU. I'm not sure if the current draw scales linearly or not, but assuming it does that's 7A. If it fires 10x per night @ 15 min per cycle, that's 150 minutes @ 7A, or 17.5 Ah of draw on 80Ah of battery (50% of 160Ah), or roughly 22%. Combined with all of your lights, water pump, phones charging, etc. it can add up pretty quick.

In the interest of dispelling some exhausting RV myths that just won't die, and do a great disservice to confusing people: no you will NOT kill your deep cycle AGM batteries by discharging below 50% capacity, or 12.2 volts, or whatever. There is this misconception I've seen repeated all over that if a battery dips below a 50% Depth of Discharge (DoD), it does permeant irreversible damage to the battery or something.

For the TL;DR: By routinely discharging down to 11.8v/80% DoD instead of 12.2v/50% DoD, your cut your battery lifespan by a whole 12 percent. Whoopty doo. Discharging to 12.0v/70% DoD cuts the lifespan by, wait for it......2%.


A Lifeline GPL-24T is rated for 1000 discharge cycles to 50% DoD, 700 cycles at 70% DoD, and 550 cycles at 80% DoD.

These values map to the following voltages displayed on your Airstream control panel, when the battery is *at rest* for at least 4 hours (i.e. no load and not being charged).

100% Charged - 12.8V
50% Charge (50% DoD) - 12.2V
30% Charge (70% DoD) - 12.0V
20% Charge (80% DoD) - 11.8V

I hear you saying "But NeonFlamingo, 550 cycles is almost half as many as the 1000 cycles the battery is rated for!" And that's true. But it is also only half the equation. The other half is the fact that each of those 80% DoD discharges gets you more usable amperage than a 50% DoD. So you must multiply the DoD capacity by the number of rated cycles to get the estimated total lifetime charge capacity and then compare between values.

Example: The 160Ah battery array (the standard 2x GPL-24T's on 2021's with factory-equipped solar) would have 80Ah of usable power factoring for a 50% DoD. 80Ah * 1000 cycles is 80,000Ah over the life of the battery.

That same battery array with an 80% DoD provides 128Ah. 128Ah * 550 cycles is 70,400Ah over the life of the battery, nearly 90% the lifetime charge capacity at a 50% DoD. Suddenly that "half as many charge cycles" thing is put into perspective.
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Old 05-12-2021, 06:34 AM   #4
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Actually there is a parasitic load with the battery switch in ‘store’ position: your LP gas detector. If you disconnect all the negative cables in the battery box, you’ll find the AGM’s will hold their charge for a long time in storage.
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Old 05-12-2021, 07:12 AM   #5
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Are those discharge longevity specific to Lifeline or are they pretty much general for AGM batteries? Are they for the 12 volt batteries or for the 6 volt that many people use? For my use 550 cycles is a awful lot of use in itself.
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Old 05-12-2021, 07:29 AM   #6
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2021 16' Bambi
Nashville , Tennessee
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Originally Posted by Bill M. View Post
Are those discharge longevity specific to Lifeline or are they pretty much general for AGM batteries? Are they for the 12 volt batteries or for the 6 volt that many people use? For my use 550 cycles is a awful lot of use in itself.
These figures are specific to Lifeline, though other high quality manufacturers should be comparable. It doesn't matter the voltage of the battery, because voltage is just a function of the number of cells within it (three for a 6v battery, six for a 12v battery). The cell technology and performance would be the same.

Page 40 (last page) of this PDF shows you cycle life vs depth of discharge for any Lifeline AGM battery:
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Old 05-13-2021, 05:11 AM   #7
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To complicate the equation, the battery capacity declines in the cold. Approximately 50% at freezing with some variation based on load and time. If batteries are stored outside the climate controlled interior, the need to run the furnace in the cold night will further challenge your battery capacity.

We were able to boondock without concern until the 24 F overnight in the mountains. Woke up to 12.1v, we have 300Ah Lifeline stored in drawer under the chassis (motorhome)
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Old 05-14-2021, 10:35 AM   #8
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This Spring has been very cold for May.

Camped at the 7800 elevation mesas of New Mexico where the mornings were in the mid-20's and at sunrise goes into the upper 60's / lower 70's for Solar Heat, but the air is still nippy. That was the first two weeks of May this year. Snow in Arizona mountains to the West.

After Sunrise... mid 70's, low humidity and pleasant clear skies.

Used five minutes of furnace the first day AM to get prepared for the 2nd Day.

Dog's water bowl outside froze to the bottom (about 1-2 inches of ice). Inside 10 to 15 degrees warmer. Say 28F outside... about 38F inside. The furnace was to just take the 'nip' out of the morning. After that first day, the following ten the furnace was not needed... at all. AGM Interstates did not go below 12.8v.

Soon after Sunrise, open windows and free heating all day... until Sunset. Repeat entire previous routines. Repeat, the next. Etc.

We use multiple covers and wear sweats. We have 160watts on the roof and 100 watt Costco portable Solar Panel directed to the Sunrise that recharges our batteries to 100%.

We have done this since 2006. The Furnace is great when hooked up to power at a RV Park. Out Boondocking... you have to be on top of performance and your personal comforts. We are prepared with bed coverings and covers for the two Blue Heelers. It is learned... and you will develop a system... or not.

The Airstream is not a 12 month home on wheels for comfort. There are plenty of posts on the Boondocking Threads how to develop a SYSTEM. Comfort at 25F outside and 38F inside is what to expect. Kill your batteries and run out of Propane it will be 25F out and inside...

Add a Little Buddy with the small expensive propane cylinders for taking the 'nip' off the cold. We survive. We get tough. Our Blue Heelers have no complaints. Our water system does not freeze up. These techniques are on the Boondocking threads.

We have survived 18F in Wyoming in July. Water lines froze in some areas and this was one of our first trips with the 2006 Safari. We learned how to prepare. You will as well. One step at a time. Eventually you will be on top of it and how to park the trailer for heat or for cold when Boondocking.

The option is Tent Camping. If they can survive without incident... you have a Trailer with many options. They do not. Tent Campers to an Airstream... problems have already been solved. That was our evolution. The learning curve was not as steep for us.

Good luck. You will get it figured out sooner than later. We survived. You will as well.
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