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I have a BMV-712 and it seems the SOC percentage vs the Consumed AH is off. The SOC was showing 75% when the consumed AH showed 248. That’s more like
My total battery bank AH is 645. So 322AH used should be right at 50%.
Does it take time for the BMK to configure itself?
Thank you!
There is most probably no issue.
The battery AH rating entered applies ONLY to a constant 20 hour discharge rate / current.
But when you use your battery the discharge current will vary and can be anything
So when you have a small load you will get MORE capacity out of the same battery.
Conversly, if you are running a huge load that would flatten the battery in a few hours then you will get far LESS than the 20h discharge capacity.
The BMV constantly monitors current in & out of the battery and compensates for this behavior by using the Peukert Exponent specified in the BMV setup parameters.
You can read up more on this if you are interested. You can also take a look at the data sheet for your battery, it should have the battery capacity at many different discharge rates.
No problem.
Based on the info you provided it looks OK to me.
The current draw shown in your screen image is very low, so if this is typical of normal usage it would be expected for SOC% to be higher than a basic calculation between 'Consumed Ah' & 'battery capacity'.
The only thing that I would probably change is the charged voltage. If you are using solar then set it to be just below the absorption voltage by ~0.2 to 0.4v. At the moment you might be getting a premature sync, before the battery is really at 100% SOC.
Open circuit voltage for a typical 12v battery at ~75% SOC should be ~12.5v, but as your drawing a small load that would bring it down a touch. At 12.3v you are not far off.
You really just need to watch the behaviour over some time and slowly fine tune as required.
Hey Mark. Thanks for taking the time to reply to my issue. I appreciate it.
I have a large pusher RV with no solar. We like to dry camp and It pulls a decent amount of power when we do so. For this test I was hooked up to 50amp shore and turned off the charger and let the batteries power the DC things on board for a couple of days. (Like the lights). When we dry camp I usually see around 25-35 amps on average drawn from the batts.
Well 645Ah / 20h = 32.25A, so IF you were to draw that exact load constantly from 100% SOC then the BMV SOC should match a basic calculation of Ah drawn vs total capacity quite well.
However, if the current draw is lower (on average) then you will get MORE than 645Ah capacity from the same batteries. Accordingly the SOC % will be correctly HIGHER than what you would estimate from a simple calculation of Ah drawn vs total capacity.
In summary I cant see anything obviously wrong with the data its showing you. Just try to make sure that the batteries are FULLY charged at least once a week & that the BMV performs a SOC sync to 100%. As over times any small errors can compile and cause the BMV SOC to drift from reality.
Do you feel I should go by amp hours consumed to figure out when to start the generator and ignore the SOC percentage?
No I expect SOC% to be more reliable because with Ah consumed it does NOT consider the discharge rate when that capacity / energy was consumed.
My point is that only SOC% represents the 645Ah capacity (@20h discharge rate) that you have entered.
To see how well the BMV is tracking you could disconnect ALL loads overnight & then check the battery bank resting voltage in the morning & compare to the data sheet for your particular batteries (there should normally be a SOC% vs voltage table or graph). But also note that this can be influenced by battery temperate & age.
When charging the batteries, try to monitor the BMV towards the end of the absorption phase. The charge current needs to drop to about 1 to 2% of your battery capacity (so ~6 to 12A) for the batteries to be considered fully charged. Around the same time the BMV should sync to 100% SOC. This needs to occur regularly for the BMV to accurately track the true SOC%.
For your reference;
Below is a data sheet extract from a 250Ah battery (@ 20hr discharge rate). Notice how the capacity for the same battery varies significantly depending on the actual/different discharge rate;
Below is a data sheet extract showing open circuit voltage vs SOC % (note that this must be measured after everything has been totally disconnected for ~6+ hours, to allow the battery to stabilize & find its own natural state);
Both these tables should only to be used as an example, you should refer to the specific data sheets for your particular batteries.
Hey Mark, thanks for sharing this great info. I learned a great deal.
:)
Very helpful! I had the exact same question. thx!
