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.
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.
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.
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.
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).
I will raise this old topic, just my question is somehow related with the discussion here.
The current setup is simple - LiFePo4 battery pack (16S, 105Ah) connected to a DC bus and then to the Multi. The Multi is with currently operating as standalone charger, I only set the battery type and parameters in the charger section and set charge current limit (no assistants, dvcc, and other fancy tricks for now).
The Multi is seeing around 0.2V lower voltage than the actual voltage on the DC bus and battery terminals, even on the Multi DC terminals itself - I can measure the same voltage at each of the three points. This is no matter whether there is a load or it is idle.
I is also seen while charging, the actual voltages for each parameters are 0.2V higher than the Multi configured setpoints.
I cannot find a solid reason for this voltage reading difference.
Interestingly the MPPT (SmartSolar 150/35) sees the voltage as it is on the bus, the same is seen by the BMS and my multimeter no matter where I measure (battery terminals, DC bus, Multi battery terminals or MPPT battery terminals), only the multi sees the voltage-0.2V.
After I upgraded to the latest firmware (497), it is the same.
Is there a configuration option that can cause this?
An 0.2V calibration difference is not uncommon. It is unlikely to really be a problem, and if it is, you can play around with SVS (shared voltage sense) which will tell the Multi the voltage the battery sees, and it will compensate for that 0.2V.
Also, while the Multi is under load, it is perfectly normal for the voltage to measure low. It always measures higher on the charger side, lower on the discharge side, with the battery in the middle. Also depends how it is wired. Some people wire the MPPT to the second set of terminals on the Multi, causing interesting voltage deltas.
There is no configuration option and the calibration is done in the factory and cannot be messed with. Well, at least not without a LOT of effort…
Thanks for the answer. The above observations are done at idle. The MPPT is not charging, since there are no PVs yet. I simply gave the MPPT as an example that it reads the voltage that is seen by the bms and the multimeter, and only the multi is the device seeing different voltage even when I measure it on its batt terminals which is no different than the readings at each end of the DC wiring.
All devices are connected to busbars for the pos and neg terminals, using copper wires with sufficient cross section (35mm2 for the battery and Multi, 10mm2 for the MPPT to the buses). Anyways, this should not affect readings at idle.
Under small load there is no change of the mentioned difference the Multi sees (around 0.2-0.3V). I will check again what is happening under heavy load, but as far as I remember it was slightly higher which is probably normal.
I should check what I can do with the SVS.Or I will end up simply tuning the values in the config having them with -0.2V compared to the real ones if there is no other way.
In an ESS system (I assume you don’t have that configured yet), the solar chargers sync their voltage to the Multi anyway, so the solar chargers will then actually mirror that 0.2V low. It really isn’t a big issue, it is just a calibration difference of about half a percent.