Victron BMV-712 Battery Monitor

[BrentB]

Originally Posted by srellis

My BVM-710 has been giving me odd readings for some time, telling me I was at 100% charge when the voltage numbers indicate that I actually had a quite dead battery. Some research leads me to think that this was because I had the 'Charged Voltage" number set far too high. A Victron video leads me to believe that it should be 0.3 volts below the "float charge" voltage supplied by the charger. However, that float charge number is different depending upon several things. The solar controller floats at 13.4. The shore power charger bulk charges at 14.4 to 90%, then enters "normal mode" at 13.6 volts to full charge (the implication is that this mode may run many hours), then floats at 13.2.

I suspect my description has made your heads spin. Mine certainly is. Clearly the 14 volts I have been using as a "Charged Voltage" parameter was far too high. But which of those many numbers should I use as "float voltage"?

--Scott

What is your battery capacity in amp hours? (Ah) the Victron's default setting is 200ah. If your battery's capacity is less than that, it would throw off your readings.

[babock]

Originally Posted by srellis

I'm going to repeat this, more simply, because it's still going on and it's making me crazy.

My Victron 700 still, after a definite full charge two weeks ago, says 100% capacity . . . while simultaneously displaying a voltage that's dropped to 12.42 volts. Obviously, one of these things is not true. What's going on?

Can you post a picture of all your settings or type them out here if yours doesn't have Bluetooth.

[srellis]

BrentB, that's a good question. I initially set that capacity number to the actual "20 hr. capacity" rating of my batteries. I have two sixes in series, each 190 Ah, and since it's voltage and not capacity that adds in series, 190 Ah for the two. Good so far? Then I got to thinking about it, and decided that that was the TOTAL capacity, and I never wanted to use more than 50% of that, so I set the capacity number at 95. That DID seem to make other readings make more sense, but had no effect on this sort of slow-drain thing I'm seeing now.

Twinboat, I understand that voltage drops instantly under load. But what I'm seeing is a very, very slow drop, like from 12.6 v to 12.4 over a week or more. I like your idea of testing with a known load, though, and I will do that. If nothing else, that will clear up my question above, about where my useable Ah number should be set.

Babock, the "current state" numbers have all changed because I plugged it in last night, but here are my settings:

Battery capacity 95 Ah
Charged voltage 13.1 V
Tail current 4%
Charged detection time 3 m
Peukert Exponent 1.25
Charge efficiency factor 95%
Current threshold 0.10 A

There is other info available, of course; please let me know if I missed something that might be helpful.

And one more thought: clearly, something is slowly draining the battery even with the disconnect switched off. (CO detector, whatever.) I set the "Zero current calibration" with that thing running. Is it possible that the system is seeing a small draw as zero current, and therefore not "counting" it toward a percentage discharged? Should I zero with the battery physically disconnected at the terminals?

Thanks for any and all help, folks.

I often say, "Campers are like boats: if you don't like messing with them, you shouldn't own one." And I believe that . . . but I'm getting a little tired of messing with this!

--Scott

[C.Martin]

It’s been a long time...but I remember Victron saying they calculated time remaining off of 50%... I guess if you want SOC to display Zero when you are actually at 50%...then setting 95AH is fine... I would recommend setting what’s actually in the batteries.

Victron just uses the setting to trigger a learning reference. The charge goes way up...then when it sees the drop...it allows time for the surface charge to dwindle and sets it’s 100% point. 100% of your batteries...estimated. If your batteries show a lower voltage than you would expect...two things could be causing this.

Contacts for the sense wires, shunt wires, terminals... if they aren’t clean...the resistance will slew the voltage.

Or, your batteries are weak. They don’t have enough plate to sustain the voltage you are looking for. It is not uncommon for batteries to sulphate and loose available surface...or plating to eventually dissolve...

Google voltage curves for Lead Acid batteries. They make them that show expected voltage based on charging or discharging based on c/? Rates...you can see the amps on the BMV, divide that into the 190...to find the C rating...and compare your voltage to the graph. There may be some temperature compensation needed.., if your batteries don’t follow that curve...they may have lost some capacity. A perfect 12v battery fully charged and rested unconnected for 2hrs might settle in just above 12.6 volts...maybe 12.7 volts. Do the same test to a weak battery...it may show 12.4v. And as you put it under load during use...it will drop to the equivalent of 50% SOC voltage...while your battery monitor still reads 70%... That’s if you used 190aH vs 95AH. You have set it so the batteries should, obviously have MORE capacity than 95%....so your SOC numbers will be at a lower than expected figure after amps are removed...at some point your voltage reading will cross the curve.

If it hasn’t been too long...an equalization charge may help restore partial capacity...but if left sulphated for a long period...it isn’t very effective.

The solar being in the mix confuses the bmv. Your solar converter could be putting out 14+ volts of bulk charge...then a cloud rolls in...the voltage drops to your less than float number..and the bmv says...aH ha...100%. It’s a maddening thing but not much you can do about it.

I have the BMV, and BMK...so I compare them from time to time to learn more about what they are reading.

[twinboat]

You use the full battery bank capacity. Using half, will have you reading the wrong remaining capacity. If your down 25%, your down 50%. How does that make sense.

[srellis]

I don't think I'm explaining this right. I get a nice full charge. The attached image is from two hours after I came off the charger this morning, and that's after 24 hours or so at the charger's "float" level of 13.24 volts. The batteries are less than a year old, have only been mildly abused once (too-low discharge), and perform well.

So I'll do what I just did, charge fully and unplug, and there will be that fine high voltage, and then it will slooooowlly drop to a level that tells me I need to charge (which I usually do at 12.4 or so). Now, this DROP is no mystery--there are things in the camper that draw some power even when the disconnect is disconnected (including the Victron itself). I understand that. What I don't understand is that while the VOLTAGE slowly drops, the Victron continues to list the capacity as 100%. The voltage is not dropping because my connections are slowly failing, or because there's some slowly-increasing load causing the voltage drop. There is a steady-state slow drain that leads, in two or three weeks, to dead batteries if I don't charge.

Again, that's no mystery: there's stuff using power. But why doesn't the Victron see that use?

Yeah, okay, so there's no photo attached; posting it someplace I can get a URL is beyond my flu-addled brain this morning. But it still says 12.77 volts, 2:40 after coming off the charger. (And yes I have solar but I'm under a carport and it's cloudy out--I'm not getting any solar gain.)

So this is my current theory: I did the "zero current calibration" in the state the camper is in now: disconnect off but obviously actual current drain not zero. It's making sense to me that if I pulled the battery cable and then did that calibration, it would then register the drain that happens all the time. Except--crap--if I do that, there will be no power to the unit to do the calibration. Sigh.

[C.Martin]

I wonder if the discharge amperage is below the threshold set to register the drain...and I’d just triple check to make sure there isn’t a ground wire connected directly to the battery bank instead of the load side of the shunt..thereby avoiding detection.

[twinboat]

Originally Posted by C.Martin

I wonder if the discharge amperage is below the threshold set to register the drain...and I’d just triple check to make sure there isn’t a ground wire connected directly to the battery bank instead of the load side of the shunt..thereby avoiding detection.

You may be on to something with the un detected draw, due to a ground on the wrong side of the shunt.

There should be NO wires on the battery side of the shunt or the negetive battery itself, except the one cable.

[srellis]

For the record, I believe I solved this. It appears that the issue was that the Victron was actually seeing the "storage state" of the camper (battery disconnect disconnected, but some draw still happening from gas detectors, whatever) as zero current. And it was doing that because I had told it to, wanting to see nice neat ZERO numbers on the display in storage but not considering that that was false information.

First thought was just to do a reset to factory parameters, but that didn't fix it. Ended up pulling all the wires off of the load side of the shunt and THEN doing the "Zero Current Calibration."

I believe that did it. Thanks for the ideas!

[babock]

If your battery capacity is 190AH, that is what you put in...not 90AH.


Also, the zero current calibration is done with no current flowing. That means ALL loads are turned off including CO detectors.

[srellis]

The latter is exactly what I just described doing.

As to the former, if, as I've gathered, the system only uses that Ah info to do math, then whether one sets it up to show a battery at a level such that it needs to be charged reads "50%" or "0%" (in my case, whether one sets the capacity at 190 Ah or 95 Ah) would appear to be a matter of personal preference. If that number has some other effect on the system, I'll change it back.

Thanks!

Originally Posted by babock

If your battery capacity is 190AH, that is what you put in...not 90AH.


Also, the zero current calibration is done with no current flowing. That means ALL loads are turned off including CO detectors.

[twinboat]

Originally Posted by srellis

The latter is exactly what I just described doing.

As to the former, if, as I've gathered, the system only uses that Ah info to do math, then whether one sets it up to show a battery at a level such that it needs to be charged reads "50%" or "0%" (in my case, whether one sets the capacity at 190 Ah or 95 Ah) would appear to be a matter of personal preference. If that number has some other effect on the system, I'll change it back.

Thanks!

If you happen to go a bit below 0%, while setting it at 1/2 the battery capacity, does it record negetive numbers ?

I would think that would set thing out if wack.

[babock]

Originally Posted by srellis

If that number has some other effect on the system, I'll change it back.

Thanks!

It affects your tail current setting. Also if you put in 1/2 of your total capacity and you go below 0%, you basically have no idea where you are at.

[srellis]

Originally Posted by babock

It affects your tail current setting. Also if you put in 1/2 of your total capacity and you go below 0%, you basically have no idea where you are at.

Thank you. That's useful. I'll change it back.