Full disclosure: 15 years of power system experience but Victron Noob here.
Have new 48V “off grid” system with LFP batteries. Typically use for the weekend and then it sits idle for a few weeks at a time. Have MPPT voltages (absorption, Float, and Equalization) set to a moderate 54.4V. My understanding was that charging would TERMINATE once battery voltage voltage hits 54.4V (duh!).
Every morning, when the array gets illuminated, the system wakes up and enters bulk charge making up for the slight overnight drain of the batteries but then it continues beyond 54.4V and has been slowly “walking up” the battery each day, here is a 4 day snap shot:
I do understand now, from the manual(s) that there can be conflicts in terms of what controls the charging when there are multiple chargers so it is possible that perhaps the charge is being governed by the settings in the Multiplus II. HOWEVER, I have not made any changes to the factory default values and settings. The default is lead acid with Absorption voltage of 57.6V and Float Voltage of 55.2V. So if the Multiplus II settings were controlling the charge… wouldn’t the battery have been charged to 57.6V? Is there some way the “4 stage adaptive charging” would have elected only to charge to the Float Voltage of 55.2V?
Another possibility is that the LFP battery has an internal BMS and the BMS blocks the charging current, leading to a small overshoot of the voltage on the DC side. It might do this because of cell imbalance, one cell reaches the protection voltage.
A possibility is that on the final stage of the charging, the evolution of battery voltage is quite steep and quick.
And I’ve observed that Victron likes to use for smoothing out the measured voltages single-pole recursive low pass filters.
But the coefficients for those filters are quite relaxed and many times such overshots can be observed.
Considering the fact that you already have a (very) moderate voltage set (3.4V per cell), a 55.2V (3.45V per cell) will not hurt at all the batteries.
Also, like other above have said, after long times at such low voltages, the cells can be very unballanced and therefore the overshots. Hence the initial statement.
Equalization should be disabled for lifepo4. If you click on the charger tile in the GX device can you see if the charger is under external control? My Multiplus controls the charger and the voltage is not the same due to cable length etc. You should download VEConfig and set up the multiplus and check what the settings in there are. All settings should be the same incase you lose communication.
OK, thank you. The MPPT is getting temperature from the Smart battery sense so I have just now disabled the temperature compensation. However, before I disabled it, I noted that it was set to -64.8mV/degC. The weather has actually been quite warm at my location so it seems to me that not having this disabled then the charging would have terminated at lower voltage, not higher voltage? Either way, I have disabled it, we’ll see if that changes things.
The multiplus temp sensor is NOT connected and I don’t currently have the ability to adjust settings on the multiplus but will work that in a separate thread.
The batteries do have BMS and a CAN connection but are not on the victron approved list. I thought I’d take a baby step approach before attempting to connect the battery BMS to the CAN ports on the Cerbo.
It is best to choose the lithium battery profile, then edit the voltages to suit your battery specifically.
Then save it as a custom preset.
The lithium profile add the low temp cut off to work with the smart battery sense. But on the other hand if you have a smart network and a GX, there could also be that contributing to weird behaviour.
Thank all of you for all the help and suggestions! As you noted, the voltage is modest so I don’t have a safety issue or have to worry about damaging the batteries but I have my homework to do!
I will see if I can figure out how to read the BMS data and verify that all the cells are uniform. (note: I don’t think this is root cause because I have 2 batteries in parallel but it is obviously good practice to verify battery health).
I will resolve my VEConfigure issue (on a separate thread) and edit the Multiplus II settings appropriately.
After the responses I both disabled temperature compensation and I adjusted the voltage set points on my MPPT down to 54.10V. I adjusted the MPPT voltages because I wanted to determine if the MPPT or the defaults in the Multiplus II were controlling the charge. As you can see in the plot below, charge behavior was “normal” the minute I disabled temperature compensation. I confess I still don’t understand why the MPPT would have been charging the battery above the voltage set point given that the voltage compensation is a negative number per degrees C and the batteries were relatively warm, in the 20C range. I can save this mystery for another day… but any explanations would be welcome (I could open Victron Connect app and look at the MPPT and it does provide good and valid temperature telemetry so I don’t think the temp sensor is a valid culprit).
Really appreciate all the input helping me get to the bottom of this! Thank you so much.
