battery voltage and state of charge percentage

[bcarli]

I am having a hard time getting my battery voltage and my Victron state of charge percentage to make sense.
I have 900 amp hours of new T-105 batteries. I just did a test where for 20 hours (I had the charger off) and a constant house load of 12 amps my
voltage was 12.0 but the percentage was 82% left. I started with test with batteries charged to 100% by shore power. In my mind 12.0 is real close to 50% of charge but when I multiply 20 hours times 12 amps it should be 240 amps used which is about 1/4 off full capacity or about 1/2 of usable power if I stop at 50%. I talked to a service tech who said I should trust the Victron % not Voltage but it seems to me Voltage doesn't lie.
All Negative ground wires lead first to the shunt.
Not sure what to do

 

[Comodave]

I am not familiar with that monitor but the one I put in my last boat said it took multiple discharge cycles to learn the batteries before it would be accurate. Does yours say that?

 

[DavidM]

Well, in addition to Comodave's advice, don't believe battery voltage as an indicator of SOC while drawing 12 amps. That will cause the voltage to drop a couple of tenths from its resting voltage.

So, 12*20/900 = 27% used or 73% left. That is a bit less than the 82% that the monitor is indicating, but first try a few discharge/charge cycles and check it again.

David

 

[H2O_Doc]

I think using voltage to determine state of charge is only a rough guess.

Victron sells a smart shunt, and that measures actual amps out and amps in, and I find that to be accurate and reliable.

https://www.victronenergy.com/battery-monitors/smart-battery-shunt

 

[DDW]

You don't say what Victron, but most of them integrate the amps in and out and are pretty accurate if installed and set up correctly. Voltage as a state of charge indicator is only accurate (and marginally so anyway) when they are disconnected from any load and left to rest for an hour or two.

To have the Victron work well, you must set the Peurkert constant, charge efficiency, and make sure the indicator being used for full charge (there are a couple of choices) is properly set. It is all in the instructions.

 

[Cigatoo]

Starting with a fully charged battery I tend to look at amp hours used on our Victron as opposed to state of charge. With four 6 volt Trojans I have a theoretical 450 amp hours available. Half of those are usable….again theoretically. I start the generator when I have used 100 amp hours or so. Seems to work well.

 

[BruceK]

If you want to use voltage as an approximate soc indicator do it with no load applied.
12.6 is full. 12.00 is seriously discharged. I suspect your reading reflects the load. When my portable fridge compressor runs the voltage drops significantly but returns to >12.6 after the compressor cycles off.

 

[Art]

Multimeter - Wire it direct to batt bank. Have the meter in easy access location.

https://www.amazon.com/Multimeter-M...t=&hvlocphy=1014242&hvtargid=pla-897758219168

https://www.harborfreight.com/7-fun...MIkf_lrL-s-wIVSRXUAR3TrwzyEAQYAiABEgKvY_D_BwE

 

[C lectric]

Already said but an accurate state of charge voltage can be had ONLY when there is no load and no charge going on. The battery needs a rest of approx. 24 hrs.

In use voltages can be a guide to what is going on but only in general terms.

 

[Mischief Managed]

I had a similar problem this Summer and discovered a bad main DC distribution switch was the cause. It allowed plenty of current but created a little voltage drop that showed up on my SOC meter as unexpectedly low voltage compared to the SOC percentage. Check the battery voltage with another meter and you may find that it's fine.

 

[bcarli]

thank you everybody

 

[DDW]

If you are really interested in the state of your batteries and DC systems, you should periodically check both current and voltage. The two together tell you everything there is to know. Looked at separately, you are missing some information.

 

[Patrick4002]

depending on the type of battery the internal resitance is diffent and it changed bij the load of the battery, when u hav no load on the battery you can measuere the voltage, with a load on even a small load of 10amps it can give not so accurated soc

 

[cafesport]

So I think your intuition is right, but perhaps your victron settings are off. It’s crucial that you get the settings correct especially when changing out batteries.

Before you clear the history and do a meter reset, make a record of all settings and note the amps in vs amps out. You’ll need this to make an estimate of charge efficiency. The other crucial settings are peukart factor (1.25) and true bank capacity. Unfortunately only way to be sure about the later number is to do a full capacity test. I’ve seen new batteries start off under spec and without intervention fail prematurely because they never ever get to full charge. With lead acid batteries an amp out seldom equates to an amp in hence the importance of the correction factors. These values also change over time adding further inaccuracy to the readings.

Those are good batteries, my brother ran them for a long time in his boat up your way. Check the specific gravity periodically against voltage and the victron to verify everything is as it should be and do a capacity test annually to properly set the victron. “Trust but verify.”