BMV-712 battery monitor incorrectly displaying 100% SOC

[steve&renee]

I've got the Victron BMV-712 monitor installed to monitor a 420 AH lithium battery bank. I've had the RV parked at home for about a week now -- not plugged in, solar disconnected, but inverter and its sub-panel were on the whole week. The parasitic loads, my running electric loads intermittently, and constant loads like the entertainment center and theater seating ran the battery down to 11+ volts. I found out when I tried to extend the front slide and it wouldn't operate.

I tried to activate the phone app for the BMV-712, but the battery voltage was so low that the monitor couldn't even be discovered. So I activated the solar, and with plenty of mid-day sun, the batteries started charging back up. Pretty soon, the BMV-712 was discoverable via the phone app, and that's when I saw the battery voltage. 12+ and inching up.

But the BMV-712 was also showing 100% state of charge. ??? I monitored the SOC for a good 10 minutes and it never varied from 100%.

So I did some internet searching and found out that 100% SOC is reported based on three threshold parameters: Charged voltage, Tail current, and Charge detection time. To declare 100%, the charged voltage must be exceeded and charge current must be less than tail current (simultaneously) for the entire charge detection time. My charge voltage is set at 13.9V, the tail current is 16.8 Amps, and the charge detection time is 3 minutes.

Well, without meeting the threshold, the BMV is still showing 100% SOC. Clearly something is wrong.

Any ideas what to do to get an accurate SOC?

-Steve

[howson]

I've got the Victron BMV-712 monitor installed to monitor a 420 AH lithium battery bank. I've had the RV parked at home for about a week now -- not plugged in, solar disconnected, but inverter and its sub-panel were on the whole week. The parasitic loads, my running electric loads intermittently, and constant loads like the entertainment center and theater seating ran the battery down to 11+ volts. I found out when I tried to extend the front slide and it wouldn't operate.

I tried to activate the phone app for the BMV-712, but the battery voltage was so low that the monitor couldn't even be discovered. So I activated the solar, and with plenty of mid-day sun, the batteries started charging back up. Pretty soon, the BMV-712 was discoverable via the phone app, and that's when I saw the battery voltage. 12+ and inching up.

But the BMV-712 was also showing 100% state of charge. ??? I monitored the SOC for a good 10 minutes and it never varied from 100%.

So I did some internet searching and found out that 100% SOC is reported based on three threshold parameters: Charged voltage, Tail current, and Charge detection time. To declare 100%, the charged voltage must be exceeded and charge current must be less than tail current (simultaneously) for the entire charge detection time. My charge voltage is set at 13.9V, the tail current is 16.8 Amps, and the charge detection time is 3 minutes.

Well, without meeting the threshold, the BMV is still showing 100% SOC. Clearly something is wrong.

Any ideas what to do to get an accurate SOC?

-Steve

Look under the BATTERY SETTINGS and check if BATTERY STARTS SYNCHRONIZED is on or off. (I have mine off, so when there's a reset there's no charge indication.)

[steve&renee]

Look under the BATTERY SETTINGS and check if BATTERY STARTS SYNCHRONIZED is on or off. (I have mine off, so when there's a reset there's no charge indication.)

I saw that setting, but it confused me. I don't know what a "reset" is. Can you explain?

-Steve

[howson]

I saw that setting, but it confused me. I don't know what a "reset" is. Can you explain?

-Steve

If the BMV power cycles (loses power) it will reset the SOC either to an unknown condition ("---") or 100% depending on the setting of the control I highlighted when power is restored. Since I have the control highlighted set to OFF, the BMV resets the SOC to --- (unknown) when my BMV power cycles.

I typically have some idea of the SOC, so I'll select SYNCHRONIZE SOC TO 100% then select STATE-OF-CHARGE (Manually set the current state of charge) and manipulate the setting to the approximate value. (Usually the only time my BMV loses power is when I intentionally turn off all 12v power to my trailer.)

If you don't know the state of charge when the BMV is showing --- on the display, charge the batteries to a known state and then set the BMV manually to 100%. With the Battle Born batteries I have it is easy since the internal BMS controls the charging. Once the Victron goes into Float mode I know the batteries are at 100%, so I manually set the BMV accordingly.

The BMV will also set itself automatically, though I'm not completely familiar with when and how. The manual probably has that info....but I haven't read it through completely (gasp).

[Guest]

If the BMV power cycles (loses power) it will reset the SOC either to an unknown condition ("---") or 100% depending on the setting of the control I highlighted when power is restored. Since I have the control highlighted set to OFF, the BMV resets the SOC to --- (unknown) when my BMV power cycles.

I typically have some idea of the SOC, so I'll select SYNCHRONIZE SOC TO 100% then select STATE-OF-CHARGE (Manually set the current state of charge) and manipulate the setting to the approximate value. (Usually the only time my BMV loses power is when I intentionally turn off all 12v power to my trailer.)

If you don't know the state of charge when the BMV is showing --- on the display, charge the batteries to a known state and then set the BMV manually to 100%. With the Battle Born batteries I have it is easy since the internal BMS controls the charging. Once the Victron goes into Float mode I know the batteries are at 100%, so I manually set the BMV accordingly.

The BMV will also set itself automatically, though I'm not completely familiar with when and how. The manual probably has that info....but I haven't read it through completely (gasp).

howson Howard,

I figured that setting would either assume the same SOC as before the power was shut down or 100% SOC . My shunt is on the negative battery cable with no disconnect on the negative side. When I disconnect the battery on the positive side and then reconnect, my BMV712 will show the same battery SOC. My "Synchronize to 100%" is turned off. Not sure why the BMV would show "unknown condition", but I am using the temperature probe with a positive and negative input to the shunt.

Reading further back I see its Steve that is having the issue where the synchronize to 100% should be turned off. A reset is a battery reset which means the positive input to the shunt was also disconnected. The BMV712 has a very small current draw and needs to be kept on at all times.

Jim

[howson]

A reset is a battery reset which means the positive input to the shunt was also disconnected. The BMV712 has a very small current draw and needs to be kept on at all times.

Ideally, sure, but there are times when power will be removed. This puts the BMV in an unknown state, aka "reset". What the BMV will display when powered back on is determined by the setting.

[Guest]

Only way to mess up the BMV if setup correctly is to disconnect the battery such as for winter storage. Once the battery is re-installed, and after a full charge, the Sync to 100% button is pushed and then with all power removed except the BMV, the Zero Current Calibration button is pushed.
Looks like Howard's system is setup the same. Except my BMV is powered directly from the battery via the temperature probe.

I was also pleasantly surprised to see that my BMV-712 app can be viewed from inside the truck while traveling. This is a great battery monitor.

Jim

[steve&renee]

howson Howard,

Reading further back I see its Steve that is having the issue where the synchronize to 100% should be turned off. A reset is a battery reset which means the positive input to the shunt was also disconnected. The BMV712 has a very small current draw and needs to be kept on at all times.

Jim

Interesting discussion -- but not my problem. My problem is the BMV is showing 100% SOC before the unit reaches the Charged voltage. What I've been doing to try to understand the problem is calculating a percentage based on the BMV's displayed voltage and the Charged voltage, then manually setting SOC to that percentage. This calculated percentage is typically 95 or 96%. The BMV shows this SOC immediately, and then it begins to creep up to 100% in a few minutes.

The BMV displayed voltage at the battery during this time holds steady at around 13.4 volts. The charging amps are above 40. I think what's happening is the Lion internal BMS is holding the charge voltage low. What I can't understand is how/why the BMV ignores the Charged voltage/Tail current parameters and declares 100% SOC before the parameter settings are met.

-Steve

[howson]

Interesting discussion -- but not my problem. My problem is the BMV is showing 100% SOC before the unit reaches the Charged voltage. What I've been doing to try to understand the problem is calculating a percentage based on the BMV's displayed voltage and the Charged voltage, then manually setting SOC to that percentage. This calculated percentage is typically 95 or 96%. The BMV shows this SOC immediately, and then it begins to creep up to 100% in a few minutes.

The BMV displayed voltage at the battery during this time holds steady at around 13.4 volts. The charging amps are above 40. I think what's happening is the Lion internal BMS is holding the charge voltage low. What I can't understand is how/why the BMV ignores the Charged voltage/Tail current parameters and declares 100% SOC before the parameter settings are met.

-Steve

Please post how your BMV is configured to calculate the Lion Safari's battery's SOC (Battery Settings).

[steve&renee]

Please post how your BMV is configured to calculate the Lion Safari's battery's SOC (Battery Settings).

https://gdrvowners.com/forum/operati...rge-controller

[steve&renee]

...

The BMV displayed voltage at the battery during this time holds steady at around 13.4 volts. The charging amps are above 40. I think what's happening is the Lion internal BMS is holding the charge voltage low.

...

I was reading the Lion battery manual, and there's a test button on each battery that shows state of charge for each battery. I disconnected the batteries and charger and tested each battery. Two of the four were fully charged. The other two were 1 LED light away from full charge.

So I did another experiment: Began charging with the AIMS 50 amp charger, the WFCO OEM supplied charger, and my Victron 100/50 Charge Controller -- all at once. This gave me a 70+ amp charge rate. Checked back after charging this way for about 2 hours and discovered the BMV monitor showed 14.2 volts at the battery. Disconnected the batteries and checked the on-battery SOC meters. All four batteries showed 100% SOC. And the AIMS charger had transitioned from Bulk (fast, constant current) charging to Absorb (fast, constant voltage).

So. . . I'm finally getting my batteries charged all the way up. I went into the BMV settings and selected the SYNCHRONIZE tab just for fun (because it was already and had been constantly showing 100%).

Anyway, this post is just more info. I still don't know why the BMV showed me 100% SOC when I had 11.9V in the bank.

One last tidbit: When we're out camping and not hooked up and Renee is running her hair dryer and curling iron and such, I'm constantly monitoring SOC. Mostly because I'm amazed that we can run all that stuff for 3 days with just solar recharging the bank. Anyway, the SOC goes down predictably. Or at least it has all spring and summer long.

-Steve

[gbkims]

Discussion got me reading some more on the battery monitor.

Battery Monitor - BMV-700 Series https://www.victronenergy.com/upload...S-SE-PT-IT.pdf

2.2 Synchronising the BMV
For a reliable readout, the state of charge as displayed by the battery monitor has to be synchronised regularly with the true state of charge of
the battery. This is accomplished by fully charging the battery.
In case of a 12V battery, the BMV resets to ‘fully charged’ when the following ‘charged parameters’ are met: the voltage exceeds 13.2V and
simultaneously the (tail-) charge current is less than 4.0% of the total battery capacity (e.g. 8A for a 200Ah battery) during 3 minutes.
--
2.3 Common problems
The BMV synchronises too early
In solar systems or other applications with fluctuating charge currents, the following measures can be taken to reduce the probability for the BMV to
reset prematurely to 100% state of charge:

a) Increase the “charged” voltage to only slightly below the absorption charge voltage (for example: 14.2V in case of 14.4V absorption voltage).
b) Increase the “charged” detection time and/or decrease the tail current to prevent an early reset due to to passing clouds.
See section 4.2.1 for set up instructions


Long read:
Making Your Battery Monitor More Accurate https://marinehowto.com/programming-a-battery-monitor/
...“RC, Isn’t my battery full when the ampere hours on the screen are returned to zero?”
"Contrary to popular misconception Ah counting battery monitors do not always reset based on Ah’s returned to the battery. Instead they use a number of factors such as; voltage, time, current and time at both current and voltage to try and determine when the battery is actually full. Unfortunately, with multiple charge sources, and house loads, they can get confused and tricked into prematurely re-setting to 100% SOC.."

[steve&renee]

Discussion got me reading some more on the battery monitor.

Long read:
Making Your Battery Monitor More Accurate https://marinehowto.com/programming-a-battery-monitor/

Excellent stuff. Especially:

"The “Gotcha” Conundrum:
Now let’s look at this in a real world scenario. Your boat has a solar array and it’s early to mid morning. The batteries are still in bulk charge mode, and the solar system is just barely able to get them to your Charged Voltage setting of 13.2V.

Because of the low current supplied by the solar array, due to it being early in the day, all the PV system can muster is 13.2V, and at a current below your Tail Current. If you’ve dipped below 90% SOC over night & the system proceeds like this for longer than the Charged Detection Time, regardless of where you are in the SOC range, the monitor can actually reset itself to 100% SOC.

I have been sitting next to batteries 30-40Ah’s known discharged when a battery monitor has reset to 100% SOC. I glance over one minute and it is at 85% SOC and the next it has reset to 100% SOC. This is simply poor programming and leads to vicious counting errors.

System loads on your boat, such as water makers, refrigeration etc. can also pull charging voltage and net charge current below the charged detection parameters, if your charge source is not able to muster it all. These house loads can also indirectly cause the monitor to reset on voltage, tail current and charged detection time.

There are many scenarios we can paint that can cause some of these monitors to reset falsely & prematurely thus creating more counting errors.

For the average boater, simply turn off or program out auto-sync…

For the die hard electrically minded individuals out there it is possible to program auto-sync to still work, but this is a recipe you’ll need to figure out for your own system and your system only. There are far too many variables to give a cookie-cutter recipe for auto-sync especially when we start mixing in solar or wind etc..

I set foot on far too many boats with battery monitors reading 100% SOC where I fire up the motor and the alternator is pumping 20-40A plus into the batteries. These are not 100% SOC batteries… A full & healthy battery will accept less than 1% of its Ah rating at absorption voltage. A Manual Sync current of 1.5% – 2%, of Ah capacity, is a safe number for a cruising reset target."

I think that's getting at my problem of reporting 100% SOC when it wasn't anywhere near 100%.

What I'm starting to think now is you have to start out with a good, long, shore-power based charge with no battery loads active, until you see 14+ volts at the battery and 0 amps flowing into the battery (that's the BMS doing its thing). When you get to that state, you're at 100% SOC. Now you can manually SYNCHRONIZE and the BMV will take over monitoring the discharge. Go camping and enjoy. When you get back home, sometime before your next trip, repeat the shore-power based charge, SYNCHRONIZE, and rinse. This seems to be the recipe suggested by howson in post #4.

Anybody agree or disagree?

-Steve

[ncitro]

I would agree with this assessment. I had a few times when we were boondocking where when I woke up the system was at 100%. I determined it was because when the sun came up and was making a very little amount of solar, it was possible to meet the "fully charged" criteria. I think I decided to turn off auto sync and I manually sync it myself every few weeks when we have full hookups and I can look at the current and know they are full. IIRC the older BMV (700) used a fair bit more power itself, and that was not accounted for in the BMV. Over time this could add up and throw off the readings. Thus needing the occasional sync. I think I read the 712 uses less power, and this would be less of an issue.

[howson]

gbkims (aka "Gandalf") strikes again! Really good stuff, Gene. I'd never read the impact that solar can have on the BMV. From page 3 of Victron's manual:

Remarks: a) In case of solar applications or Li-ion batteries several settings may have to be changed.

From a quick glance through the settings it looks like just adjusting the Charge Detection Time may be enough? I've changed mine from 3m to 30m just to see what will happen. (The BMV is a passive device--not going to hurt anything.)

steve&renee -- I think your post is stating exactly what the manual is recommending (page 10):

2.2 Synchronizing the BMV
For a reliable readout, the state of charge as displayed by the battery monitor has to be synchronized regularly with the true state of charge of the battery. This is accomplished by fully charging the battery.In case of a 12V battery, the BMV resets to ‘fully charged’ when the following ‘charged parameters’ are met: the voltage exceeds 13.2V and simultaneously the (tail-) charge current is less than 4.0% of the total battery capacity (e.g. 8A for a 200Ah battery) during 3 minutes.

The BMV can also be synchronized (i.e. set to ‘battery fully charged’) manually if required. This can be achieved in normal operating mode by holding the + and – buttons simultaneously for 3 seconds, or in setup mode by using the SYNC option (see section 4.2.1, setting number 10).

By default, the BMV is configured to start-up in a synchronized state and will indicate a state of charge of 100%. This behavior can be changed: see section 4.2.1, setting number 70.

If the BMV does not synchronize automatically, the charged voltage, tail current, and/or charged time may need adjustment. When the voltage supply to the BMV has been interrupted, the battery monitor must be synchronized before it can operate correctly.

After having synchronized for the first time (automatically or manually), the BMV keeps track of the number of automatic synchronizations: see section 4.3, history item SYNCHRONIZATIONS.