AGM House Battery Charge/Discharge

[truk4u]

While at Marathon yesterday getting the ole Genmate Auto Gen working, I had a good conversation with the tech who specializes on electronics and we discussed the famous battery percentage of discharge. Marathon for many years has setup a default for inverters/chargers and auto-gens to run the batteries down to 11.4/22.8 and then charge back to 13.3/26.6. He explained that Marathon/Concord/Trace have worked in harmony many years and this method is time tested. So, we setup my auto-start for those numbers. After running the batteries down to 11.4/22.8, the auto-start ran the gen for exactly 5 hours, then shut down with batteries topped at 13.3/26.6. With these settings, the amount of time you can go between charge cycles with the inverters is fantastic.

So, the anal in me (sorry Jon) just couldn't except the fact that Concord insists the magic number is 12.2 and that being 50% discharged. This gives you great battery life, but pretty whimpy usage tiimes with the inverters. I called Concord tech and here is the skinny...

The AGM's are 100% discharged at 10.5/21.0. It is OK to run the batteries down any level above the 100% discharge rate without hurting the batteries. The deciding factor is number of cycles (1 cycle = 1 discharge/charge). What you get is a trade off of battery life vs inverter time. Here's an example based on discharge rates:

10.5/21.0 will give you 450 cycles
11.4/22.8 will give you 650 cycles
12.0/24.0 will give you 1000 cycles

The AGM state of charge follows:

100% 12.80
75% 12.55
50% 12.20
25% 11.75
0% 10.50

So all this time I've been yacking about not going below 12.2 and now I have to change my tune. I'm sticking with the 11.4/22.8 discharge as Marathon suggested and that should equate to 650 cylcles (my batteries are almost new) and if I used 50 cycles a year dry camping, I'm good for about 12 years. Of course we know we're lucky to get 5 years since were always plugging, unplugging, running the gen, etc. and not always taking the batteries down with a full cycle. I think that's what kills the batteries over a shorter time.

One note, Concord told me Marathons logic works, the guy said he's been there 10 years and only remembers 2 battery warranty issues with Marathon.

OK, there's my latest story and I'm sticking with it.....

PS - I ran two A/C's, fridge, tv and a few lights for over 3 hours on the inverters as a test for the auto gen startup. 2, 4000 watt inverters and 6 AGM 4'd's.

[dalej]

Great post Tom, I personally like what you said. I have never had auto start on our 4, 8D's AGM
batteries. I usually run them until I see them hit 11 volts and then charge. Our gauge isn't digital
so its not a exact science. Knock on wood, our batteries were bought in 1999. I was told by the
lifeline salesman that I’m pushing the end.

[jello_jeep]

Nice post Tom! Makes sense.. I will have to reporgram mine when I see it again..

[Just Plain Jeff]

Your overview is interesting.

Could you possibly provide some details?

[jack14r]

Tom,I have been thinking about this low voltage set point and I knew my Marathon was below 12 volts.I was almost at the point of changing the auto gen on to 12,thanks for taking the time to research and get the facts.Marathons electrical system seems to be superior to me to some others because of the 2-4000watt inverters that lets me not run the generator for hours,but still run AC or heat without running the generator.I have recently noticed that other converters have increased the size of their inverters and batteries.I wish Marathon had 8 batteries rather than 6 like Vantari.
Jack

[Jon Wehrenberg]

Tom,

The logic as explained is very clear. Rather than dig through my research previously my reason for not allowing the batteries to discharge as deeply as the autostart allows was predicated on different numbers (such as 5000 cycles at about 12.3 or 12.4 volts versus a quarter of those cycles at the lower voltage) but conceptually your information and mine agrees.

I have a second reason, and it is a seasonal issue. The Webasto likes higher voltages. I would rather manually start my generator prematurely than have to deal with a funky acting Webasto. But further, I am a generator slob. I bought it and by golly I intend to use the damn thing. I want to wear it out. I want to be able sit in my coach with every light on, the AC's running, and the toe space heaters all at once.

I started using our old coach by conserving our battery power. It did not have auto start so I would sit in the damn dark so the batteries would last all night. Not any more. I like the smell of generator diesel exhaust and I intend to get as much as I can.

[truk4u]

Good point about the webasco Jon. On the way down to FL we parked at my sisters house with no power and had to let the webasco run all night. In the morning I still had 12+ volts and it ran fine. In those situations and when needing A/C, I would let the gen run, run, run and not depend on the inverters and auto-start.

Marathon owners with Trace inverters can go back to the factory settings by pushing and holding the reset botton for a few seconds. The trace has a chip in it with the Marathon settings. The only other thing you have to do is manually change the power cord size as needed for 30 or 50 amp.

[Just Plain Jeff]

As I begin to approach middle-age, there seem to be some things I don't recall the way I used to.

Isn't there something about battery 'memory?' That is, that batteries, if left below certain levels won't return to a full charge?

Maybe that's just for certain kinds of batteries?

We were told once that it was a good idea from time to time to fully cycle the batteries from low to full charge. Maybe that was hogwash?

[jack14r]

I believe you are thinking about nicad batteries that do have memory.

[Joe Cannarozzi]

Ya know, we can go to all the technical sites out there and read up on the subject and god knows I've been to plenty myself.

Only here are we able to get "the rest of the story"

Tom GREAT POST!

Imre, great link!

Jon, I once heard the best humor always contains a little bit of truth