AGM House Battery Charge/Discharge
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.
AGM battery charge/discharge debate
Here is some more food for thought relative to the Marathon discussion at the top of the thread...
Comments from our VP of Operations...
"This flies in the face of both the common wisdom in the marine industry as well as my research and anecdotal experience. I've been selling, installing, servicing (and replacing) AGMs for nearly the entire time I've worked here at ZMI. We wrote an entire article about AGM batteries and why they fail (posted on the website). I would most definitely take with a large grain of salt anything battery manufacturers have to say on the subject of life cycles, they all exaggerate, it’s an accepted fact. I had a number of conversations with two representatives from Concorde/Lifeline and got conflicting info, the engineer was more frank than the front office guy (big surprise). In the marine industry, we use 50% discharge cycles and experience and testing shows this gives a good balance between cycles and longevity and, this is important, the key metric being amp hours delivered over the life of the battery. I wouldn't discharge an AGM below 12.2 (technically, this voltage has to be measured at rest, that is with no load on the battery for 24 hours, so some interpolation is often necessary). If the batteries are at 11.8 or 12 under a load, they may come up to 12.2 or 12.4 when the load has been removed for some time. Additionally, running batteries this low produces unwanted side effects for any motor loads. As the voltage falls, amperage rises on these inductive loads, which causes the motors to run hotter and die sooner, not to mention tripping circuit breakers. Simply put, working a battery, and the motors attached to it, this hard, regardless of battery chemistry, will, in my opinion, dramatically shorten its life as well as the life of motors, fans, pumps etc. 50% discharge has been accepted for quite some time as a standard in the marine business. 13.3, by the way, is the float voltage, the voltage at which a battery is maintained. The charging voltage is typically much higher, on the order of 14.7, depending on temperature, if the charge source is temperature compensated, AND IT SHOULD BE, particularly for AGMs. The leading cause of their failure is positive grid corrosion, which results from floating at too high a voltage."
Hope this adds to the discussion,
When do I start the generator...
I have 6 - 8D AGM batteries connected in parallel in my coach. When I am dry camping and the Link System inside is set on amp hours when should I start the generator to recharge the batteries? The only accessories that run off of my inverter are refrigerator/freezer, TV, Aqua Hot, & lights. My basement ACs do not run off of the inverter.
Someone at POG II (I think that it was Peter vS) told me that each battery had 300 amp hours. Therefore, I assume that 1800 amp hours would be completely discharged. So would I then start the generator @ 600 amp hours of discharge if I wanted to not allow the batteries to go below 50%?
Also, what is the difference between a 4D and an 8D battery?:eek: