How accurate is the internal voltage measurement on the MultiPlus 12/3000/120-16?
Measurement taken directly at the MultiPlus terminals.
Voltage displayed in Cerbo-GX / Menu / Device List / MultiPlus 12-3000 / DC Voltage.
Voltage displayed in Victron Connect. What is the difference in your readings?
I would consider a maximum deviation of 0.02 volts acceptable.
I think you first need to get to grips with the physics of measurements and the permissible errors when measuring DC voltage. That’s the starting point.
Your desired accuracy of 0.02 V at 12 V isn’t always achievable even with professional, high-quality portable test equipment.
Take into account data transmission delays and the quality of your measuring equipment.
Most importantly, the measurement accuracy at the inverter terminals using the inverter’s internal measurement means is more than sufficient. And again, there are processing and data transmission delays before the data reaches the system interface.
So, what exactly is the issue or question you’re trying to address?
Yes, the accuracy of the Multiplus series is limited in resolution. However, it is still sufficiently good to enable a lead acid system to work fine. If you need more accuracy, then add a smart shunt to your system. Expecting +/-20mV accuracy from the Multiplus is not realistic.
Bay the way, SmartShunt accuracy of voltage measurement ± 0.3 %
If voltage = 12V that permissible errors ≈ 36mV
shunt voltage range is 70V dc, 0.3% of 70 is 210mV!!
The shunt does not scale to the applied voltage.
Multiplus is +/- 0.4V typ.
The measurement error of all instruments applies across the entire measurement range. The narrower the actual measurement range and the closer it is to the middle of that range, the higher the accuracy. At the extreme limits of the measurement range, the error is always higher. That’s just physics. However, in the case of the Victron SmartShunt, Victron does not state anywhere in specification that ±0.3% means ±0.3% of full scale 70 V. Temperature can also affect the actual measurement error. That’s just physics. All the best.
Just measured my shunt against BMS.
BMS gives 26.9545V (+/- 32mV), Shunt 27.05V, this is 0.354% of the 27V, but 0.136% of the 70V.
So yes, there are lots of error sources for an A/D converter, and the ‘typical’ accuracy is going to be better than the spec.
I’ve got a Keysight 3458A in my arsenal, and even that has different accuracy across the span of each measurement range. The absolute error always depends on the range span and on where the measured value sits within that range.
From my humble experience 
, having tracked down many voltage differences in the system due to various BMS inconsistencies, I can only say this: by far the vast majority of voltage differences are caused by inadequate wiring and faulty connections in the system.
Secondly, the voltage readings from both the MP2 and the Victron SmartShunt are many times more accurate than those from the majority of Asian BMSs and balancers.
These observations come from a small group of PV owners, which also includes electricians and electrical engineers.
Completely agree! The vast majority of DIY enthusiasts have never carried out the initial measurements of all the circuits (in fact, they don’t even have the equipment to take proper measurements), carried out calibration, or subsequently applied the necessary corrections when commissioning the systems. So there’s really nothing to discuss here, especially when it comes to complaints about the accuracy of MP.
I’m personally just an interested layperson and system operator too. The voltage differences and measurement discrepancies between Victron and the various BMSs (I now operate seven storage blocks) have always been a concern for me. That’s precisely why I measured voltages and current flows with acquaintances who work more closely with this subject professionally. And not just on my own system. May I be blunt? Yes, I’ll just say it…Everything that came from the BMSs in terms of data was rubbish. That’s exactly why I run my system entirely without any communication whatsoever between Victron and the respective BMSs. I’m fully aware that this isn’t mainstream, but I think it will remain that way in the future too.
Do you see what the whole problem is with BMS data? When, for example, you’re building a battery pack yourself, it’s important to take empirical voltage readings for the entire cell string before the BMS, rather than after the BMS and at the battery terminals. You can then enter those figures as correction values in many BMS units. For example, the JK-BMS, which I’m not particularly fond of but find acceptable because of its rather poor and inaccurate coulomb counting for SOC. But if you apply the appropriate calibrations, including current calibration under different loads, you can actually work with it quite happily. On my own setup, for example, the SOC drift over 7–10 days is no more than 5%. The voltage matches the readings from the MP’s built-in measurement facilities and the data from the BMS, taking into account the resistance of the busbars and cables. So you could say everything is within acceptable tolerances. But how many people like me are actually willing to spend the time taking measurements and tuning everything? Exactly, not many. So it’s easier to say that Victron is useless, JK is absolute rubbish, and everyone’s a charlatan, there’s no truth in this world 
Is that really the truth… just BMS calibration? I don’t want to, and won’t, get into a BMS discussion now. But when it comes to determining the SOC in particular, it’s a fact that very small charging or discharging currents aren’t recorded at all. I don’t know whether that’s explicitly still the case with the very latest JK models, but the older ones had a glaring weakness there, just like all the other commercially available models. Especially when the charging and discharging currents are distributed across several blocks, this eventually becomes a problem. I see it every day.
This isn’t directly related to the topic, but there’s also a weakness here in shunt or MP2 SOC estimation. The standby consumption of the balancer, BMS, display or any other installed equipment isn’t taken into account. Rough calibration is possible via efficiency, but standby consumption as a function of time isn’t.
Just as an aside. The topic is MP2 measurement accuracy.
The issue of measurement accuracy and SOC calculation is certainly nothing new. Over short periods, especially if your system reaches RCV and goes into absorption every day, or at least once every 2 - 3 days, it’s not really a problem at all. The built-in measurements in the MP are accurate enough, and the BMS can also be calibrated to a reasonable level of accuracy.
The problem starts to show up over longer periods, and especially in parallel battery systems, where RCV and absorption may not be reached for 7 - 10 days or even longer. In this case, the SmartShunt is a lifesaver, although there will still be some margin of error (but it’s acceptable). And yes, very small currents are extremely difficult to measure accurately.
That’s just the reality. 
One brief comment, and then let’s leave the subject here. In winter, I regularly go several days without calibrating the shunt. Since I use DESS very intensively and all its calculations are based on a highly accurate SOC determination, I find it a great shame that Victron does not really offer a solution for the issue of “accounting for standby consumption”. In the consumption calculations in DESS, it is carried through to additional decimal places, but this source of error remains. But that is specific to my situation and is not generally applicable. I have already had discussions about this topic as well.
Otherwise, I completely agree: in my view too, Victron’s measurements are good enough. Many other factors have a much more significant influence.
As typed in a post above you can calibrate the current and voltage of a JK bms…is it possible to calibrate or adjust the current and voltage reading of a Victron Shunt, RS or MP inverter?
I own a DC voltage meter with 0.02% / 1 microvolt accuracy.. it’s realy funny to measure actually voltage over less then 1 centimeter of an thick copper busbar, but I can at least accurate measure the battery’s 54,xx voltage.
You can and must calibrate the shunt.
I had a go around with Victron on a Multiplus 3kva 24v unit not agreeing with ANY of the other Victron gear I have, and compared to a Fluke 87V..
I squawked about it to the local dealer who offered to just swap it out. I said sure, after you test it FIRST. The spare unit had similar issues. They sent it in for repair, and let me know when it came back, at which point I traded.
Voltage readings were MUCH closer.
Thread is here:
The battery voltage measurement of my Multiplus 12/3000(120v) is off by 0.30 volts. No, it’s not a wiring issue. A fluke multimeter confirms the discrepancy. I don’t like it but i deal with it by setting the absorption/float setpoints to 0.30 volts lower than I actually want. The Cerbo reports the voltage as measured by the smartshunt so that’s accurate at least.
Let’s close this topic with the following point, along with a clear example.
Calibrate your BMSs and shunts, and also check the accuracy of your measuring instruments against reference equipment, and then everyone will be happy. There’ll be no complaints about Victron, BMS manufacturers, or manufacturers of ready-made batteries.
The main thing is to approach these matters with the motto: “There’s always room for improvement”, and it doesn’t matter how long it takes
