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.