Loaded V vs OCV for DOD Lifeline AGMs ?

LY, good info. I am doing a re-test of my big 8D 250AH AGM after a disappointing capacity test the other day and a follow-up Lifeline type 8 hour conditioning session at 16 volts. I am part-way through the re-test but so far it does look better. I will post the results.

Yes, one clue something was wrong was the loaded voltage being unusually low for the load.

The 8D has the same OCV vs SOC graph as the one I linked above with a broad brush range of voltages. It tends to be about 13v when full and 12.4 volts at 50% when the battery is in good shape. IE in the middle of the range of voltages in the graph.

I am using 12.4 bounce back to resting as my marker for 50% for these tests. As long as I use the same marker I can tell if things are better or worse, even though the exact capacity measurement might be off in percentage.

Temperature is an issue. If I pick the 20 hr rate according to ambient, I would have a lower rate--

EG, 250AH rated at 25C, is more like 225AH at 10C, so my chosen rate for the test would be 11.25 amps instead of rated 12.5 amps.

But then what about the battery's actual temperature? Does it heat up discharging? It does charging for sure, so if you want to see the in and out AH come out the same except for heat loss on recharge, what to do? It is messy.

I choose to use the 12.5 amps even if it is cooler out ambient, and let it all happen. I figure the real capacity will be a tad higher than what I get from using the higher rate. Main thing is to do each test the same way so they can be compared.

My opinion is the 50% 'rule of thumb' can be largely ignored with AGM, especially when one can meet 20% charge rate and preferably more, and then hold absorption voltage until amps taper to 0.5% of capacity.

I'll regularly use 65Ah overnight, of my 90Ah battery and have ~1100 deep cycles and thousands of engine starts and many hundreds of shallow cycles in the 85 to 95% range on it in the last 5.5 years.

I judge battery health by loaded voltage, but I will still see 12.2v+ under a 4.5 amp load at 45Ah from full if it is within a few days of a high amp recharge to full. If it has been two weeks since the high amp recharge, this will be 12.1 or perhaps 12.0 at the same 45Ah from full and I view this as an indication it is time to draw it down deeply and parallel my second 25 amp charger to my 40 amp power supply and blast the mofo with ~65 amps until it reaches 14.7v. which took about 24 minutes when the battery was newer and is about 18 minutes now.

I'd also not be trying to compare lifeline loaded voltage to other AGM's. You have the trimetric, reset it after a 20% charge rate to 0.5% at Vabs, then pull 50% the amp hours out of them at the 20 hr rate.

You can then see where 50% loaded voltage resides on your specific batteries at their specific temperature, and then unloaded voltage, and how quickly it rebounds and use this data as referece for future discharges.

Northstar AGm says 50% is 12.11 volts. I dont believe that at all, as if that were true my 90Ah battery should be rated instead at ~125AH. Obviously it is not, so I put no Stock in 50% rested or loaded voltage charts that populate the web.

The green yellow and red chart voltage/SOC chart posted here ad naseum, is likely one of the most ridiculous things I have seen purveyed on any forum, as people take it as gospel.

My AGm rests fully charged at 13.06v, but that chart says 12.73v is 100%, and people will pull out a voltmeter, see 12.68% and declare 98% charged as if in ultimate authority. Freaking ridiculous.

My AGM at 12.73 rested is nowhere near 100% charged. But that chart says so, so it must be true!!!!!

Mex, I just ant to know as in the OP--is there an error in their tables where they have the loaded and unloaded voltages the same at 50%. I could ask them if nobody here knows.

EDIT--I sent them an email. They say to expect answer in a couple days or so.

Is it a toll call from Canada to Concorde? Your question does not generate a fixed answer. You need to interact with a technician to absorb a viable answer for Other Than Lifeline device. Four questions to generate interactive answers.

Some answers are based on OEM amp hours and then further revelations are predicated on AVAILABLE capacity. Kapish?

I have a Trimetric monitor for the AH readings. To measure battery capacity at the 20 hr rate by drawing it down for 10 hours, or when it gets to 50%, you need to know when they are at 50%.

With Flooded batts you can take their SG as your marker. When at 50% by SG, disconnect load and wait till voltage bounces back and settles so you can now use the OCV vs DOD table. It can be your confirmation check with the SG marker. You can then double the AH you got to get what it would be for 100% You also have the time it took. If it took 9 hours to get down to 50%, your capacity is 90% of rated.

With AGMs, you can't get the SG, so you can't really use that method. Lifeline says to use the RC figure and the 25 hr rate for that, and do the full time down to 10.5 volts. No way I am going to do that with my batts! You can't stop at half the time either, since you don't know what the full time would be.

So I am going with the 20 hr rate method and use voltage to mark the 50% level, confirming by waiting till bounce back complete.

It is far from exact, but by doing it the same way each time, you can compare with the previous test to see how much the capacity has changed since then. You can then tell if you need to do something about it or if all is normal.

Could rig up a watt meter and measure how many amps you pull out. Once at 50% check the voltage of the different batteries.

If you read that Lifeline info, you will see that the voltage vs DOD does not change with temperature hardly at all.

I do have a problem with my brands of AGMs (I have two different brands) where they both show a range of voltages per DOD in their graphs for that, so I can't pick an exact voltage. Lifeline does say theirs are not exact but do list just one voltage per DOD. So does Trojan for their AGMs.

It would help to know if Lifeline has made an error in its two tables per the OP question.

Scroll down to the OCV vs Residual Capacity graph for how mine do it.

http://www.wegosolar.com/products.php?product=SKR%252d125AGM-Stark-AGM-12V-Solar-Battery-Sealed-125A

You can only roughly guess at the approximate 50% charge level and with any guessing you will need to have an exact reading and stability for temperature.