May 2023 – Our Journey to Scotland

Using Victron MultiPlus-II for top balancing LiFePO4 cells

Top balancing is a topic where a lot of people have written about – and now it is my turn …

It is common understanding to use a regular charger when top balancing, and one the one hand set the Charge Voltage Limit (CVL) to cellCount * 3.65V and use a reasonable current and wait for an extended period of time until all cells have reached their cell voltage maximum. And reasonable means to use a current where the BMS balancer keep up with and distribute the Amps across the cells without going into a Overvoltage (OVP) for a single cell.

So, instead of using a charger with a high supported current of at least 20A we now can use our regular Victron MultiPlus-II inverter/charger – with the help of Venus OS.

The reason why we cannot use a Victron MultiPlus-II out of the box as a charger is the fact, that is does not support fixed Amp configurations (only maximums). And after a while at a specific voltage the MultiPlus-II would enter Absorption phase and thereby reducing the current over time and stopp charging after a while altogether.

So with the help of a custom service (or Python script based on the dummyservice) we can create a battery monitor and set a fixed current.

I am not going into details on how to get a Venus OS device (Victron Cerbo GX or Raspberry Pi) up and running. There is plenty of information on the internet. Or have a look at this article where I briefly describe the setup of a Pi for our BYD battery system.

The *service* itself can be run from a shell: /data/VirtualBatteryMonitor/VirtualBatteryMonitor.py (I copied the script into /data to survive a firmware update).

And then the service should appear in the “Device List”:

Our service as a device to support a constant charge current

There are two more configuration entries needed:

Enable our service as “Battery Monitor” (Settings, System setup, Battery Monitor)
Enable DVCC (Settings, DVCC)

Our service configured as a “Battery Monitor”

The actual parameters (charge current and maximum voltage) can be configured via dbus-spy from a shell:

The actually configured values are then shown under “Parameters” of the service (Service, Parameters):

Current configuration set to 5A constant charge current

Note1: There is no need for an actual integration of the BMS with the Venus OS.

Note2: Use at your own risk. Misconfiguring could potentionally harm the BMS, the battery or both.

Note3: Do not leave the script running / the battery charging unattendedly.

#!/usr/bin/env python3

"""
A class to put a simple service on the dbus, according to victron standards, with constantly updating
paths. See example usage below. It is used to generate dummy data for other processes that rely on the
dbus. See files in dbus_vebus_to_pvinverter/test and dbus_vrm/test for other usage examples.

To change a value while testing, without stopping your dummy script and changing its initial value, write
to the dummy data via the dbus. See example.

https://github.com/victronenergy/dbus_vebus_to_pvinverter/tree/master/test
"""
from gi.repository import GLib
import platform
import argparse
import logging
import sys
import os
import dbus
import os

# our own packages
sys.path.insert(1, os.path.join(os.path.dirname(__file__), "../ext/velib_python"))
sys.path.insert(1, "/opt/victronenergy/dbus-systemcalc-py/ext/velib_python")
from vedbus import VeDbusService
from vedbus import VeDbusItemImport

class VirtualBatteryMonitor(object):
    def __init__(
        self,
        servicename,
        deviceinstance,
        paths,
        productname="MultiPlus Charger",
        connection="dbus",
    ):

        try:
            # Connect to the sessionbus. Note that on ccgx we use systembus instead.
            logging.debug("Opening SystemBus ...")
            dbusConn = dbus.SystemBus()
            logging.info("Opening SystemBus SUCCEEDED.")
        except:
            logging.error("Reading system SOC FAILED.")

        logging.debug("Opening dbus '%s' ...", servicename)
        self._dbusservice = VeDbusService(servicename)
        logging.info("Opening dbus '%s' SUCCEEDED.", servicename)
        self._paths = paths

        logging.debug("%s /DeviceInstance = %d" % (servicename, deviceinstance))

        # Create the management objects, as specified in the ccgx dbus-api document
        self._dbusservice.add_path("/Mgmt/ProcessName", __file__)
        self._dbusservice.add_path("/Mgmt/ProcessVersion", "Unkown version, and running on Python " + platform.python_version())
        self._dbusservice.add_path("/Mgmt/Connection", connection)

        # Create the mandatory objects
        self._dbusservice.add_path("/DeviceInstance", deviceinstance)
        self._dbusservice.add_path("/ProductId", 0)
        self._dbusservice.add_path("/ProductName", productname)
        self._dbusservice.add_path("/FirmwareVersion", 0)
        self._dbusservice.add_path("/HardwareVersion", 0)
        self._dbusservice.add_path("/Connected", 1)

        # Create all the objects that we want to export to the dbus
        self._dbusservice.add_path('/Dc/0/Voltage', 3.4 * 16, writeable=True)
        self._dbusservice.add_path('/Dc/0/Current', 5, writeable=True)
        self._dbusservice.add_path('/Dc/0/Power', 3.4 * 16 * 2, writeable=True)
        self._dbusservice.add_path('/Dc/0/Temperature', 15, writeable=True)
        self._dbusservice.add_path('/Dc/0/MidVoltage', None)
        self._dbusservice.add_path('/Dc/0/MidVoltageDeviation', None)
        self._dbusservice.add_path('/ConsumedAmphours', 123, writeable=True)
        self._dbusservice.add_path('/Soc', 75, writeable=True)
        self._dbusservice.add_path('/TimeToGo', None)
        self._dbusservice.add_path('/Info/MaxChargeCurrent', 5, writeable=True)
        self._dbusservice.add_path('/Info/MaxDischargeCurrent', 0, writeable=True)
        self._dbusservice.add_path('/Info/MaxChargeVoltage', 3.65 * 16, writeable=True)

        self._dbusservice.add_path('/Info/BatteryLowVoltage', 2.75 * 16, writeable=True)
        self._dbusservice.add_path('/Info/ChargeRequest', False, writeable=True)
        self._dbusservice.add_path('/Alarms/LowVoltage', 0, writeable=True)
        self._dbusservice.add_path('/Alarms/HighVoltage', 0, writeable=True)
        self._dbusservice.add_path('/Alarms/LowSoc', 0, writeable=True)
        self._dbusservice.add_path('/Alarms/HighCurrent', 0, writeable=True)
        self._dbusservice.add_path('/Alarms/LowCellVoltage', 0, writeable=True)
        self._dbusservice.add_path('/Alarms/LowTemperature', 0, writeable=True)
        self._dbusservice.add_path('/Alarms/HighTemperature', 0, writeable=True)

        self._dbusservice.add_path('/Capacity', 156, writeable=True)
        self._dbusservice.add_path('/CustomName', "Virtual Battery Monitor (%/V/W)", writeable=True)
        self._dbusservice.add_path('/InstalledCapacity', 280, writeable=True)

        self._dbusservice.add_path('/System/MaxCellTemperature', 15, writeable=True)
        self._dbusservice.add_path('/System/MaxCellVoltage', 3.4, writeable=True)
        self._dbusservice.add_path('/System/MaxTemperatureCellId', "C5", writeable=True)
        self._dbusservice.add_path('/System/MaxVoltageCellId', "C2", writeable=True)
        self._dbusservice.add_path('/System/MinCellTemperature', 15, writeable=True)
        self._dbusservice.add_path('/System/MinCellVoltage', 3.4, writeable=True)
        self._dbusservice.add_path('/System/MinTemperatureCellId', "C6", writeable=True)
        self._dbusservice.add_path('/System/MinVoltageCellId', "C3", writeable=True)
        self._dbusservice.add_path('/System/NrOfCellsPerBattery', 16, writeable=True)
        self._dbusservice.add_path('/System/NrOfModulesBlockingCharge', 0, writeable=True)
        self._dbusservice.add_path('/System/NrOfModulesBlockingDischarge', 0, writeable=True)
        self._dbusservice.add_path('/System/NrOfModulesOffline', 0, writeable=True)
        self._dbusservice.add_path('/System/NrOfModulesOnline', 1, writeable=True)
        self._dbusservice.add_path('/System/Temperature1', 15, writeable=True)
        self._dbusservice.add_path('/System/Temperature2', 15, writeable=True)
        self._dbusservice.add_path('/System/Temperature3', 0)
        self._dbusservice.add_path('/System/Temperature4', 0)

# === All code below is to simply run it from the commandline for debugging purposes ===

# It will created a dbus service called com.victronenergy.pvinverter.output.
# To try this on commandline, start this program in one terminal, and try these commands
# from another terminal:
# dbus com.victronenergy.pvinverter.output
# dbus com.victronenergy.pvinverter.output /Ac/Energy/Forward GetValue
# dbus com.victronenergy.pvinverter.output /Ac/Energy/Forward SetValue %20
#
# Above examples use this dbus client: http://code.google.com/p/dbus-tools/wiki/DBusCli
# See their manual to explain the % in %20


def main():
    logging.basicConfig(level=logging.DEBUG)

    from dbus.mainloop.glib import DBusGMainLoop

    # Have a mainloop, so we can send/receive asynchronous calls to and from dbus
    DBusGMainLoop(set_as_default=True)

    pvac_output = VirtualBatteryMonitor(
        servicename="com.victronenergy.battery.VirtualBatteryMonitor.ttyO1",
        deviceinstance=0,
        paths={
            "/Ac/Energy/Forward": {"initial": 0, "update": 1},
            "/Position": {"initial": 0, "update": 0},
            "/Nonupdatingvalue/UseForTestingWritesForExample": {"initial": None},
            "/DbusInvalid": {"initial": None},
        },
    )

    logging.info(
        "Connected to dbus, and switching over to GLib.MainLoop() (= event based)"
    )
    mainloop = GLib.MainLoop()
    mainloop.run()


if __name__ == "__main__":
    main()

The script is available here.

For my use case, this really helps as now I have a powerful charging (3 * Victron MultiPlus-II 48/5000/70-32 in parallel) that can charge the battery initally with 140A+ and later with smaller and smaller currents until all cells have reached their maximum voltage.

Maybe you find this useful, too.

Changing the cabling of a Victron MultiPlus Compact 12/1600/70-16 from 35mm2 to 2* 35mm2

In this article I describe what I did to install 2 pairs of 35mm2 cable to a Victron MultiPlus Compact 12/1600/70-16.

Out of the box the MultiPlus comes preconfigured with a pair of 1m 35mm2 (2AWG) welding cables (and as a side note: with unusually thin M8 cable lugs).

When connected to a 12V battery based on 4s Eve LF280K cells, the maximum current drawn can go beyond the recommended 0.5C rating – especially when the cell voltage decrease under the nominal 3.2V or the total cable length is longer than 1m. Using a larger inverter or smaller cells will make things even worse.

And when we look at Victron’s Recommended battery cables document, we see that they are recommending 50mm2 for currents up to 150A anyway (for cable lengths of up to 5m).

Victron recommended battery cable sizes, copyright https://www.victronenergy.com/upload/documents/BatteryCables.pdf

For a 1m cable the theoretical voltage drop is within their recommended range of 0.259V. But they explicity state that resistance leading to additional voltage drop due to contacts is not calculated into the recommended cables size.

Already a cable size of 2m will lead to over 3% and 0.3V voltage drop when the cell voltage is only 4* 2.6V = 10.4V (and by default raise a “Low Voltage Alarm” on the MultiPlus). And even a cell voltage of 4* 2.75V = 11V is close to 3% and over the recommended threshold (of course, calculation is based on full inverter load of 1600VA). Besides Victron explicitly recommends a voltage drop of under 2.5% in their Wiring Unlimited document.

So why is Victron fitting the inverters with only 35mm2 cables? Especially since they are using welding cables that are only rated up to 60°C. I do not know.

But I do know, how I can fit an additional 35mm2 pair into the inverter and minimise a potential heating problem.

Adding a second pair makes particular sense at least in my case, as I am using a JK-BMS and Eve cells that both come with two M6 terminals per connection point. So running two cable pairs to battery and BMS saves me from using a bulkier and stiffer 70mm2 cable that I would have to split at the BMS and main positive cell anyway. And with that, I can still use the Anderson SB175 connectors with regular housing and 1383 wire contacts and without having to resort to 2/0 housings and 1328G1 wire contacts.

The inverter comes with 30mm holes in the front panel where the supplied cable is fitted with an M25x15mm cable gland (side note: why are they using IP68 glands when the whole inverter is only rated at IP21). Eland H07RN-F 35mm2 cable has a diameter of 14.6mm, so actually two of these cables do not fit through the holes at once.

But as the cable lugs are actually that long that they stick out of the chassis the required diameter is 2* 12.5mm = 25mm which is just the size of the hole. When wrapped in heat shrink we need some more space. And certainly we want a little bit of head room, so the cables do not scratch against the metal when moving.

Klauke 105R8 M8 35mm2 dimensions, https://www.klauke.com/kr/en/compression-cables-lugs-to-din-cu-4935

So, the M25 holes had to be enlarged slightly to make space for the double cable lugs as seen on the picture below. I used a Hilti GDG 6-A22 grinder for this. I covered the inverter to prevent metal splices and dust getting inside (board, circuity) of it. And I added extra insulation around the cable lugs to prevent them cutting into the metal.

Bottom side of MultiPlus with enlarged holes and extra cable insulation

Mounting the cables to the connection points is done with two Klauke M8 35mm2 DIN 46235 compression cable lugs (back to back).

Note: the compression cable lugs from Weidmüller will not fit, as their connection plate is too long.

Instead of the factory supplied washers, spring locks and nuts, I use M8 serrated washers and lock nuts. As the negative connection point (which is directly under the 250A MEGA fuse) is around 2mm higher than with only one cable lug, I added an additional (copper) washer under the fuse terminal to make more space.

I could cover the original bolts with insulation tape to prevent accidental contact with the chassis when squeezed (but this is something that could have happened even before).

Victron MultiPlus Compact 12/1600/70-16 with dual 35mm2 battery cables

Now we have a 2* 35mm2 = 70mm2 connection to our battery as seen below.

2* 35mm2 connection between battery and inverter

So the voltage drop over the whole cable (1.5m from invert to battery with 70mm2) should be around 114mV:

So as it seems, the main difference between the larger and the smaller cable is the power loss (17.22W vs 34.44W at full power or 15W vs 30W at 0.5C). So all in all we save nearly 0.84% battery capacity per cycle with the thicker cables (which is 5000Wh over the whole cell life) – probably less than we spend on the cables and lugs, the labour and the time to do the calculation and writing up the article …

Building a battery case for a 4s Eve LF280K configuration

Based on our Eve 8s design, I made a sketch for a 4s 12.8V battery, which I could later connect to a Victron MultiPlus Compact 12/1600/70-16:

Wooden case for the 4s Eve LF280K battery

Again, this battery has a wooden inner case and sits inside a utz 400mm x 300mm x 325mm RAKO container.

There are a few differences however:

There is no space for fuses inside the container
(so it is more like a traditional battery);
all bus bars are “bent” and not straight
(we need three 35mm2 pairs, so six cables altogether);
main positive and main negative are on the opposite sides of the cells;
I use a JK-BMS B2A8s20P without soldered cables but with dual M6 threads
(so I can use 35mm2 cables all the way).

utz RAKO 400mm x 300mm x 325mm container with wooden frame

To cut the plywood in an efficient way, I used a web site called cutlistoptimizer which gave me this result:

Cutting suggestion by https://cutlistoptimizer.com/

For this build, I planed all the boards after cutting, before putting in the cells. With this, I hoped to minimise the chance of any particles on the board damaging the cell insulation.

And for the small board at the short side of the case, I did also use 20mm plywood, but planed it several times until it I could just slide it in.

This is how the wooden case looks with the cells and insulation boards (shown in red):

Top view: battery cells with depicted insulation boards (shown in red)

Note: when using a JK-BMS I found it important to have the main negative connection point on the upper left (or lower right). Only with this orientation it was (relatively) easy to connect the cell to the BMS.

It needed some fiddling to get the main negative cable pair to the BMS and the main positive cable pair out of the frame, as we can see from the image above.

The connection to the individual cells are fed through WAGO 221-2411 2 conductor inline splicing connectors. The holes into the top board were done with a forstner bit and a jigsaw. This version of the BMS can be fixed with four screws to the board (so no need for wire straps as with the 24s version).

Again, instead of a display I just used the pluggable power button that is connected to the display port of the BMS to power on and off the device.

In the end, I added Anderson SB175 connectors and 1383 (2AWG) contacts to both pairs and connected them to the inverter.

Some more details

All 35mm2 cables are Eland H07RN-F flexible rubber cable;
compression cable lugs are Klauke M6 35mm2 DIN46235;
cell contacts were secured with M6 serrated washers and M6 16mm steel bolts;
BMS threads B- and P- were secured with M6 lock nuts to M6 16mm steel bolts (with the bolt upside down);
cell wires from the BMS were fitted with uninsulated ferrules;
cell wires on the positive cell poles were fitted with ring lugs and a 2.5mm2 hookup wire;
I added handles to the SB175 connectors to facilitate disconnecting the cable pairs;
I added dust covers to the SB175 connectors;
all compression cable lugs and the SB175 were crimped with a Hilti NUN54-22;
all cable lug connections and Sb175 were heat shrinked;
I added 2*35mm2 cable pairs with SB175 connectors to the inverter by replacing the existing 35mm2 welding cable with M8 lugs (you still need M8 lugs on the inverter positive and negative terminals).

Things to improve next time

Mount the SB175 connectors to the outside of the container
With this the lid can be closed and the cables and BMS are better protected against pulling;
add 3A inline fuses to the cell wires;
use 45° angled cable lugs for main positive and main negative to make it easier to get the wires routed outside the container;
feed an additional wire pair for the voltage sensor from the main positive outside the container to be able to connect it to the inverter (but I am not too sure about this, as I think the voltage drop on the 2*35mm2 connection is neglectable – it might better to add a temperatur sensor to the main positive):
maybe add protective wire sleeves to the SB175 connectors (but they are quite expensive):
add a Victron MK-3 USB-C interface with RJ-45 cable into the case (to be able to restrict AC power on the inverter).

What did it cost?

Cost calculation for the 4s battery including case and inverter

Summary

This case is not as complete as the 8s version – due to its form factor. Neither the inverter has an RCBO nor the battery has a DC MCB. This has to be added separately (incurring additional cost and space). As written above, the 4s version is more like a traditional battery. However, the form factor is quite compelling; 3.5kWh in 400mm x 300mm x 325mm case. Especially in combination with the compact edition of the Victron MultiPlus. And the cost (as always without labour) is very reasonable, as well.

The inverter delivers 1200W constant power – in my opinion, enough for a small and mobile electricity build. Runnig a Krups Inissia Nespresso machine is not a problem, and boiling water with our 1000W immersion heater neither. Worst case, you could also run a 300W infrared panel heater for more than 11 hours.

One drawback of the inverter is probably the relatively small charger. With 70A at 12V it can only charge the battery with around 840W. This is certainly not the problem of the battery which would support charging up to 1344W.