Inverter doesn't stop charge after over voltage protection of BMS

[mobot95]

Hi,

I've recently installed a Voltronic VMIII 5kw Inverter with an Yilink battery 7.2kw LifePo4.
Battery Mode: LIB

There is one cell unbalanced, so the battery cannot reach the full charge voltage of 52.5V (it stays at 51.6/51.8) and the BMS trigger an OVP (at 3.7V)
After that, inverter stop the charging for a few seconds but then it's enabled again generating a loop.
In the battery display I can see lot of OVP alarms because of this.

If I set Battery Mode: USER with same BULK/FLOAT voltages the loop doesn't occur: In this way charging it's disabled until the over-voltaged cell goes down at 3.4V (as configured in BMS (Cell OVP Release using PBMSTools software)

Why are there these problems when setting up a communication protocol? shouldn't it be more secure than USER mode?

 

[BentleyJ]

One would think so but as you found out, closed loop comms is not necessarily more sophisticated than simple voltage control.
Is that a 15 cell battery? You need to float charge at a lower voltage that doesn't cause the OVP for 24 hrs to give the BMS time to balance the cells. It may even take longer.

 

[mobot95]

it's a 15 cell battery, i'm trying to balance it using float/bulk to 52.5 with USER mode. In this way, when the cell goes in OVP it seems that does balance even on discharge (there is BL label on PBMSTools in the corresponding cell)

As attachments, curves of charge/discharge of pack and cell voltages (at first peaks, it was charging at 30A and then i've limited at 2A trying to extend the balancing time)


My question is: Why inverter continues to charge after OVP in closed loop comms? Where could be the problem? It's the inverter that query the BMS or otherwise? There is some technical documentation about it?

 

[Andy0826]

Yes, the discharge action of the battery depends on the inverter, but you can set a lower float voltage for the battery (such as 51.6V), so that the battery reaches full charge first, rather than OVP

 

[JBertok]

Individual cell voltage data isn't passed along to the inverter in closed loop, the invert would not know the cells are out of balance. State of Charge (SoC), temperature, current flow direction and amount, and static setpoints like bulk and float voltage preferences, and faults like over temperature, overcurrent, low voltage, etc. As far as your inverter knows, the battery isn't fully charged and it keeps "disappearing" when the BMS shuts it down.
The quickest resolution is opening the battery and precisely top-balancing the individual cells. The process is tedious, requires an appropriate voltage-controllable DC charger, and it's risky due to the dangers of shorting the busbars on top of the cells (100s of amps potential), and the dangers of possible uncontrolled overcharging. A high cell can be brought down with a resistive load directly connected to the cell terminals too. The alternative to manual top-balancing is allowing the BMS to balance the cells, which it can only do if the battery is being maintained with an external steady voltage around the rated float voltage, and the BMS thinks at least one cell is topped-off. (BMS auto-calibration and balancing strategies vary from one manufacturer to another.)
If reaching the float voltage with external input is not possible without triggering shutdown, then try to hold the entire battery at the highest possible voltage (up to float voltage) it will take without triggering the cell overvolt. You might find that at that point there is very little current flow into the battery; it could even be milliamps. However, this is what the BMS needs for the balancing process. Use the BMS app or battery LCD to monitor individual cell voltages to confirm the BMS is balancing.
For future longevity and cell health, it is necessary to allow the battery to spend a few hours every day at 100% SoC so that the BMS can do its capacity calibrations and cell balancing. (A reputable well-built battery with properly matched cells requires less balancing, a lower grade or quality battery will have less precisely-matched cells and require more time to maintain balance.) Any lithium battery that is never allowed to float at a full charge is going to develop voltage discrepancies between cells and the overall available capacity will be reduced when those cells trigger a Cell undervolt/overvolt in the BMS and cause unexpected protective shutdown.
If you can reach a point where the cells are within a few millivolts of each other and the BMS is showing 100% SoC, you shouldn't have to go through this whole thing again as long as you give the battery some float time each day to maintain balance itself.