RVGreen
Houston, TX
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Joined: 10/18/2002
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Regardless of the brand, type, size, age, or cost of your battery or battery bank, do you know its true capacity?
Let say I have two 125AH marine deep cycle battery. The total capacity is 250AH, according to the manufacturer, at 20hrs discharge rate.
How do I know if that number is correct? Do I just trust the "published" number? The only way to verify it, in my mind, is to put on a 12.5A (C/20) current on the fully charged battery and keep track of the hours until the load no longer run. But doing so, I have shorten the battery life.
What's your REAL LIFE, ACTUAL method, if any?
Sam (Houston, TX)
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BFL13
Victoria, BC
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Joined: 02/15/2006
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I have run several trials on my batteries to see what they can do and I do not have an ammeter, or monitor. What I can do is run them down to an estimated 50% based on a resting voltage of 12.2 which I have found is close. 12.06v works for about 40% with mine.
To get the capacity you need to know your temperature and have that chart of reduction of rated ah vs temperature. You need a charger with an amps read-out and a clock.
From 50% you charge them up and note the amps at times. You can then calculate the amp hrs you have put in over the time by adding the incremental amounts between times (the more increments the better because it is a curve, not linear.) So lets say your charger did 100ah. Deduct 10% for heat loss during recharge and declare you have put 90ah back in the batteries and they are "full" (Full means maybe 97% really)
Ok, so now you know that 50-97=47% is 90, so 100 would be 191 at that temperature. Say the temp chart says they are down 20% from what they are rated for at 80F. So 191 is 80% of 239.
So you say your pair of batteries rated at 232ah are as good as new where 239 is close enough considering your margin of error.
BTW you have to be a total nut to do all this 
2003 Chev 2500HD ExtCab, LB, 2WD, 6.0/4.10 Gas
2003 Komfort 26FS 5er (28.5 Ft-RK-10,000lb GVWR)
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RVGreen
Houston, TX
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Your method sounds logical to me, except for one major problem: How do you know that 12.2 is your true 50% point? What if 12.2 is 40%? What if 12.2 is 60%?
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BFL13
Victoria, BC
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Joined: 02/15/2006
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If the batteries only took 70 and should have taken 90 then you can either think your 50% mark was wrong and it was a higher SOC start or your batteries are in bad shape. If they took more than they should you can figure your 50% was really say 40% and adjust next time.
It is all very rough, but I have found a high degree of predictabilty with my batteries over the years. The biggest factor is temperature. In cool weather you have far fewer amp hrs and will have to recharge more often just for that. This is made worse by the fact then the daylight hours are shorter so lights on longer and the furnace is on. A double whammy.
You might have to recharge every day in cool weather instead of every second day in summer.
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RJsfishin
Winston Or.
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Joined: 10/16/2007
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BLF13 is right,...you're nuts !
Who cares what the exact AHs are ?
What we care about, is if our battery bank is capable of doing what we want it to do. If its not, you add another battery. And either way, what are you going to do, sue the MFG if the AHs don't come out to your non scientific calculations ?? Good luck !
Rich
'98 Flair, 454, Onan Microlite 4k, Intel PD 9155 w/ wizard, Sta-power 1500 watt Inv, 2 6v batts, ammeters, KingDome/sat, Oly Catalytic Heat, hauling 2 Bent Bikes and sometimes towing a Tracker F&S boat.
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skipnchar
Topeka Kansas USA
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Joined: 12/17/2003
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I've never tried to use my batteries mathematically. I just MONITOR them and don't let them discharge below 50% if I can help it. I could care less what a pencil and paper tells me about how long they'll last, but i KNOW how long they'll last while camping under different circumstances.
2004 F-250 SCREW Long Bed (new)
OR 2004 F-150 HD (85,000 towing miles)
Rockwood 8314SS 34' travel trailer
We have enough YOUTH...how about a fountain of SMART
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Bubby's RV
CA
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Joined: 07/22/2003
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Good Sam RV Club
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I compare what the 50% voltage should be compared to the 50% SOC reading on my Link 10. If they agree, I figure I'm close enough.
I'm not going to run my batteries down to 0% SOC for a test because, as you say, it will shorten their life.
BTW, battery capacity will change over the battery's lifetime. It is generally accepted that the peak capacity isn't reached until the battery has about 30 cycles on it. (US Battery says 50 to 125 cycles.) I used to know the location of a graph, but I've forgotten where.
John, Winnebago Minnie 24V
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wa8yxm
Wherever I happen to park
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Joined: 07/04/2006
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If you want to know the TRUE CAPACITY you need to use a good brattery monitor system (The Link 10 and 20 systems come to mind) that can measure voltage and current and do a bunch of math for you.
Then you test
Nothin adds excitment like something that is none of your business
Kenwood TS-2000 housed in a 2005 Damon Intruder 377
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professor95
Mechanicsville, VA
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Joined: 10/03/2004
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RVGreen wrote:
What's your REAL LIFE, ACTUAL method, if any?
This is not intended to be a smart or sarcastic answer - but it will probably sound like one
You measure the time it takes using a "normal" battery load until the propane gas alarm connected to your 12 VDC battery pack starts beeping.
I am an EE and have read and taught most everything imaginable related to batteries. This is the best answer I have for RV use!
BTW - I have two 127 Ah AGMs on the house side and four of the same on the auxilary/inverter side. (606 lbs of added weight). The AGMs have given me the best overall performance of any battery I have ever used.
Professor Randy T. Agee & Nancy Agee. Also Oscar, the spoiled rotten Dachshund, Mechanicsville, VA
2009 Cedar Creek 34SATS 5th Wheel - GMC 3500 dually
Even a bad day camping in our RV is always better than a good day at work!
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Mousefart
New Jersey
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Joined: 08/15/2004
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professor95 wrote:RVGreen wrote:
What's your REAL LIFE, ACTUAL method, if any?
This is not intended to be a smart or sarcastic answer - but it will probably sound like one
You measure the time it takes using a "normal" battery load until the propane gas alarm connected to your 12 VDC battery pack starts beeping.
I am an EE and have read and taught most everything imaginable related to batteries. This is the best answer I have for RV use!
BTW - I have two 127 Ah AGMs on the house side and four of the same on the auxilary/inverter side. (606 lbs of added weight). The AGMs have given me the best overall performance of any battery I have ever used.
Also not a sarcastic answer...
At what voltage does the propane alarm start beeping? If it starts at a 20% SOC, your test just significantly reduced you battery life, especially if it is a "marine/RV" glorified starting battery. 
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