Mastervolt AGM Open Circuit Voltage State of Charge SOC

[phorvati]

Per mastervolt datasheet i did the Open Circuit (OC) voltage SOC test. I disconnected each battery and left to stabilize around 13H. I am getting 13.07-13.08V across the three AGM160Ah batteries i have. The way i tested is I have 200A post fuse in each + terminal. That is where i disconnected each battery. I left the negative terminal connected. Then i used DMM, the same one i always used, in DC Volts setting, across plus and minus terminal of the battery with plus still disconnected.

These batteries were purchased by PO in 2014. i bought the boat in 2016. I did the same test in 2016, and 2018. In 2018 result was in the range of 12.57V to 12.60V across the three batteries. That made more sense.

We live on the dock and AGM shore charger is constantly trickle feeding the batteries. Monitor typically shows 12.9 to 13.0V and some mA give or take depending on if i load up the DC side of the panel. I.e Use raymarine stuff or other DC loads while plugged into shore charger.
If we go away for the weekend or longer and i use the batteries they seem ok. I did notice that my 12V PC would reset if i sail overnight. That was a bit strange as system voltage would be around 12.3V. (electronics and autopilot typically draw 8-10A). But i have since traced the problem to a 20A circuitbreaker that was dropping 0.5-0.75V.

I was expecting the OC SOC result to fall somewhere below the 2018 measurement.
In 2018 the boat was stored away (drydock) and charger was not plugged in.
I wonder if 12H per datasheet is not enough or are my batteries damaged due to keeping them at 12.90-13V for a long time? Is this the memory that sometimes batteries are refereed to develop?
I am trying to figure out if i need new batteries before my trip south in the fall. Perhaps a load test might be in order.

[rslifkin]

Some AGMs (including a set of Mastervolts I used to have) seem to hold higher open circuit voltage than many other lead-acid batteries. I often saw around 13 volts with mine. My current Fullriver AGMs in my house bank settle to about 12.75.


For what it's worth, when my Mastervolt AGMs hit end of life, it was just a fairly sudden, large capacity loss (seemed to be about half of their capacity). They worked fine and handled heavy loads fine, but as you drew them down, you'd hit a point and the voltage would start to drop off quite significantly (much earlier than expected). They were only about 3 years old at that point too.

[Dogscout]

Originally Posted by rslifkin

Some AGMs (including a set of Mastervolts I used to have) seem to hold higher open circuit voltage than many other lead-acid batteries. I often saw around 13 volts with mine. My current Fullriver AGMs in my house bank settle to about 12.75.


For what it's worth, when my Mastervolt AGMs hit end of life, it was just a fairly sudden, large capacity loss (seemed to be about half of their capacity). They worked fine and handled heavy loads fine, but as you drew them down, you'd hit a point and the voltage would start to drop off quite significantly (much earlier than expected). They were only about 3 years old at that point too.

I have a set of MAstervolt AGM, group 31, 12v, 90A that are doing the exact same thing for the past month. New Odyessey's on the way.

[donradcliffe]

Over 13v on an AGM does not make any sense after a 12 hr open circuit period. I would suspect some solar or other charger has not been disconnected.

Make SURE you have unplugged the shore power and disabled all charging sources, then put a constant 10 amp load on the bank and measure voltage vs time under load for 24 hrs. The voltage should rapidly drop to 12.6-12.7v, and end up around 12.1-12.0

[rslifkin]

Originally Posted by donradcliffe

Over 13v on an AGM does not make any sense after a 12 hr open circuit period. I would suspect some solar or other charger has not been disconnected.

Make SURE you have unplugged the shore power and disabled all charging sources, then put a constant 10 amp load on the bank and measure voltage vs time under load for 24 hrs. The voltage should rapidly drop to 12.6-12.7v, and end up around 12.1-12.0

13+ volts OCV is technically wrong for a 12v lead-acid battery (including AGM). But I've seen some AGMs (from multiple manufacturers) that truly do hold an OCV around 13 volts or just over even after sitting disconnected for a few days.

[coopec43]

After reading the above posts I am more than happy because I get a reading of 25.7V (AGM 24V system). I was starting to get worried that my batteries were knackered.

[smac999]

Do a c20 load test. A volt meter is never going to tell you the condition of battery capicity. Only if it is charged or not.

[sailinglegend]

Originally Posted by smac999

Do a c20 load test. A volt meter is never going to tell you the condition of battery capicity. Only if it is charged or not.

Absolutely correct!!!!!

The Merlin Smartguage can tell the capacity of a battery but measuring the voltage, but it measures it over 1000 times per second - and uses very complex computer algorithms to achieve a very accurate result. This technology is now used in US tanks and other military vehicles.

A C20 test is the only way to test the capacity - but it must be over 20 hours under the correct load - which has to be adjusted as the voltage falls. End the test when the voltage falls dramatically to 10.5 volts. If this takes 15 hours not 20 then your batteries have lost around 25% of their original capacity.

Lead acid batteries should be replaced at this point but AGMs might last longer. Lifeline suggested that my AGMs were replaced when they reached 50% - which makes them very good value for money - they lasted 14 years as a liveaboard.

One technical error by the OP - always remove the negative cable - not the positive as this could flap around and touch something that is earthed, like a through hull fitting which would short out the battery. Yes that has happened to me!!!!

[phorvati]

Originally Posted by smac999

Do a c20 load test. A volt meter is never going to tell you the condition of battery capicity. Only if it is charged or not.

Then why does manufacturer put out a graph showing remaining capacity vs. OC voltage? Thats a spec that manufacturer puts out and stands by. I agree that C20 test is the best, as thats nominal AH rating, but not most feasible to implement. I'm going to reach out to them.

[phorvati]

Originally Posted by sailinglegend

One technical error by the OP - always remove the negative cable - not the positive as this could flap around and touch something that is earthed, like a through hull fitting which would short out the battery. Yes that has happened to me!!!!

I have to disagree with you on which cable to remove. Electrons cannot flow from battery plus to earth. They have to return to battery minus. So technically it doesn't matter which cable you remove.
In my case, Once positive was removed, no source of power. So flaping red cable means not much.. same can be said for removing negative cable.

[donradcliffe]

Originally Posted by phorvati

Then why does manufacturer put out a graph showing remaining capacity vs. OC voltage? Thats a spec that manufacturer puts out and stands by. I agree that C20 test is the best, as thats nominal AH rating, but not most feasible to implement. I'm going to reach out to them.

Just remember that you are doing the test to see if your battery bank has the capacity to run your house loads for at least 24 hours. The best way to do that is to unplug the shore power and leave things on. It doesn't matter much if your load is C20 or C10 or C30, if you have dead batteries in 6 hours you have a problem

AGM voltages open circuit voltages above 13 volts mean that either (1)you have a charger on or (2) you have just disconnect a charger and have a surface charge or (3) you have performed a miracle.

[rslifkin]

Originally Posted by donradcliffe

Just remember that you are doing the test to see if your battery bank has the capacity to run your house loads for at least 24 hours. The best way to do that is to unplug the shore power and leave things on. It doesn't matter much if your load is C20 or C10 or C30, if you have dead batteries in 6 hours you have a problem

That's not the purpose of a C20 test at all. The purpose is to find out how much capacity the batteries actually have (compared to their spec) to determine their health. It's not to determine if you have enough battery capacity installed.

[smac999]

Originally Posted by phorvati

Then why does manufacturer put out a graph showing remaining capacity vs. OC voltage? Thats a spec that manufacturer puts out and stands by. I agree that C20 test is the best, as thats nominal AH rating, but not most feasible to implement. I'm going to reach out to them.

That chart is Soc. A 15 year old battery is still going to be full , or 50%. Or 0% at those voltages. Sure it shows capicty left. You have 50% left. But 50% of what? what is the current total capacity? Only a c20 test will tell you that.

you might only have a 50ah battery instead of a 100ah battery.