Voltage discrepancies - Multiplus-II

Hi All,

Seeing some voltage discrepancies on my setup

Not saying this is the issue but seems that since the latest firmware update, things are not kosher…???

In this pic, BMV-702 shows 54.08v, Multi shows 54.42v, BMS show 54.4v

In addition to the above, I normally run ESS “Optimised without Battery Life” but when charging, system goes from Bulk to Absorb way earlier than normal and BMV voltage is nowhere near my set charge volts.

Once in absorb, according to BMV, charge volts sits well below set charge volts. To get anywhere near set charge voltage, I have to switch to “Keep Batteries Charged” and adjust charge current in DVCC.

Any ideas appreciated thanks

I have changed settings so that Multi is monitoring the batt

When set to “Optimised without battery life”, charge volts sit below the set bulk/float volts

When set to “Keep batt charged”, voltage rises above set bulk/absorb - have to adjust charge current in DVCC to maintain near set voltages

Where the voltage is measured can make a difference.
There can be a considerable current on the cabling between the battery and the inverter and therefore a voltage rise/drop depending on whether the Multi is inverting or charging.
I think you should establish which devices are reading accurately with a calibrated meter if it bothers you.
Some BMS’s can be calibrated.

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There is also something called SVS (shared voltage sense) in Victron systems. If enabled, the reading from the battery monitor (the BMV in your case) will be passed on to the Multi. The Multi will then take note of the discrepancy between the two, and adjust it’s own voltage reading to be closer to that of the battery.

The same is done for the solar chargers.

In an ESS system, that solar chargers are always calibrated to read closer to the Multi in order to improve some operations that rely on it.

To me it sounds as if the Multi is measuring higher than the BMV. That could happen if the Multi is charging at high current levels (and indicates a high impedance node in your cabling).

I agree with Phil here: Get a good multimeter in and measure on the clamps/bolts/terminals of the Multi/BMV to make sure it’s a calibration issue.

Thank you for your thoughts Phil, Plonkster

Will dive in with the multi-meter…

@SolarSmurf you did not mention which BMS/Battery you have and if it is linked up with the Multi.

If you just have the MP II it’s voltage sense will be used.
If you add a BMV it will override the MP II.
If your BMS/Battery is connected it will in most cases override the BMV and the MP II.

You can set which one you want to be used as The One. So once your multimeter show you who is boss, you can set it there.

System Setup >> Battery Monitor
image

Hi @Louisvdw

Battery is DIY with a DALY BMS, not connected yet :slight_smile:

Will do some more digging tomorrow during charge cycle

Thanks

And then there’s this that happens randomly :worried: I call it the BMS dance. BMV is before BMS

Started a few weeks ago, just random - My money is on the BMS. While this is going on, I cannot connect to BMS so BT is shutdown. Reset BMS and all’s fine till the next time. When BMS comes back online, Temp is Ok (15deg), Cell diff is ok, no BMS Alarms, actual Batt volts is as per BMV

I have a spare BMS but I have left things as they are - want it to fail permanently. Trying to do warranty claim with a random issue like this could be challenging?

Yeah, it looks like the BMS disconnected. When that happens, the Multi (and solar chargers, if you have those, doesn’t look like it) maintain some kind of voltage level on the DC bus, but it’s prone to ripple because it has only the capacity of the capacitors on the DC bus to work with. So then you tend to get that squigly.

Edit: Hey, @TheTerribleTriplet , time to tell your DC ripples story again! :stuck_out_tongue:

@plonkster het so gewag vir daai een :stuck_out_tongue:

TTT had the same problem @SolarSmurf . Get rid of that Dali BMS and get the one that TTT is importing now. At what volts are you charging at the moment with the MMPPT

Yip, agreed. As for MPPT’s, have 2 but at 3am, they not doing much which raises the question about where that voltage is coming from?

@Gman - I did see the post about TTT’s ripple issue but this happens during discharge so don’t believe it’s the same issue

Definitely considering that!

The Multi doesn’t know the battery disconnected, so it is still trying to push a little current into what it thinks is the battery, and attempting to regulate it, poorly.

Solar chargers do a much better job. They can hold the DC-bus fairly stable when there is sun, to the extent that the inverter can actually operate and make some power… but because of the insufficient storage on the DC-bus, it creates a DC-ripple. This shows up on discharge usually, but it can technically also happen on charge (from the AC side).

Ahh, thanks for the insights :+1: