Good day Victron community. I have a question regarding the BMS. In my case, the JK-BMS for 16x EVE 314Ah batteries. Connected to a Victron Multiplus II 5000VA. This is controlled by a Cerbo GX. Initially, the installation ran a trial without a BMS, and via a dynamic contract with variable pricing, this worked excellently in the Netherlands. Subsequently, the JK-BMS was installed. When the JK-BMS took over control, this caused serious problems and overloading of the Victron Inverter, with currents even reaching 100A! After this, the control by the JK-BMS was disconnected and everything was switched back to the Victron system. The JK-BMS now operates stand-alone, with the sole task of balancing the 16 EVE batteries. My conclusion is that if the Victron functions perfectly, why would you transfer that system to a Smart BMS? The next step is therefore to replace the Smart JK-BMS with a stand-alone balancer, and leave the control solely to the Cerbo GX/Victron inverter. I would like to exchange ideas with someone about this. Kind regards, Peter Wijbenga
What do you mean by “Smart BMS”?
If you’re referring to a Victron Smart BMS, those are only for use with Victron batteries so are not a solution for you.
No matter what, you must have a BMS in the system to (hopefully) prevent catastrophic battery failure, yes.
Hi Justin. Thanks for your reply. No, it is not a Victron BMS, but a JK-BMS. This can be connected to the Cerbo-GX (VE-Bus), and the JK-BMS then needs to be configured (via Bluetooth). JK-BMS are known to be reliable, but I have my doubts about whether such a BMS with an extensive control system is really necessary. I would like to use a BMS that only balances the 16 EVE cells and does not need to communicate with the Cerbo-GX otherwise. The Cerbo-GX and the Multiplus handle everything else perfectly well. Or am I missing insight into how the whole thing functions? As a beginner, that is of course possible; after all, it is fairly complex subject matter. Best regards, Peter
They are at least necessary as the last line of defense. In the BMS you set the maximum cell voltage to a value of 3.6 V or more, but in your Victron system you set the maximum charging voltage to 55.2 V which is only 3.45 V per cell. But as you know the cells of your battery don’t have always the same voltage, and while your battery reaches 55.2 V one or more of the cells may reach already 3.6 V or more. When there is no communication between your Multiplus and your battery, then the Multiplus will happily keep charging with full power and make your BMS stopp charging. With communication the MP can slowly degrease charging power according the state of your cells.
I too have a JK Inverter BMS with a 16-s-battery. I am quite happy that the BMS controls my MP2, but since I am not so happy about the poor SOC calculation of my BMS, I also have a Victron SmartShunt which serves as the battery monitor. So I got the best of two worlds, and together the JK BMS and the SmartShunt act as one of the best BMSs one can imagine.
Hi Tom Berger, thanks for your informative reply. Indeed, if an EVE cell ‘goes off track’ despite a BMS, the system still needs to be able to intervene. So in this case, I should continue using the JK-BMS and restore communication with the Cerbo GX. I probably hadn’t configured something correctly after all. I don’t have that smart-shunt, but it is good to crank up the protection to the maximum. I am going to order the SmartShunt; thanks for your explanation!
For a long time, I wasn’t even aware of just how wildly inaccurate the JK Inverter BMS’s SOC calculation was, simply because I had no reference value to compare it against. However, I did start to wonder when, after several weeks of winter operation without fully charging the battery, the JK BMS reported a solid 40% SOC even though the battery voltage had dropped to just 51 V. Now, the SmartShunt handles the SOC calculation as well as the monitoring of voltage and current; after just a few days without a full charge, a discrepancy of 30% or more in the SOC reading is already apparent. The JK BMS is unable to accurately measure low currents and is sometimes extremely far off the mark in this regard. Yet, the SOC calculation relies on these current measurements. Here is an example of an extreme deviation in current readings. Both devices are measuring the current simultaneously: the JK BMS reports 6.6 A, while the SmartShunt reports 18.2 A:
This incorrect current measurement logically results in grossly inaccurate SOC values after some time. The JK BMS reports 67%, while the SmartShunt shows 31%. The SmartShunt is off by only a very small margin, if at all:
Thanks for this information kr0815 ; it is certainly interesting that I am going through this thoroughly as well. I will get to work on it. Kind regards, Peter
The JK bms regulates charging based on total battery voltage and individual cell voltage, a Victron charger only checks for total battery voltage, that’s not a problem if there is only 1 battery and the cells are all the same… which is not always the case..but an external cell balancer offers a solution in that instance.
By the way, setting up the JK together with a Victron charger is not straightforward! it almost went completely wrong for me too; fortunately I was there and was able to stop charging in time (here too, charging at 100A with already very high cell voltage!)
For short-circuit battery protection you must mount a DC high current fuse on the battery itself like an Adler EF3 fuse.
Thanks for this information FrisianStar; indeed, a fuse has been incorporated into the battery line. I’m going to look into this whole mather thoroughly, reconnect the CAN cable, and make a cautious fresh start. I’ll let you all know later how things go and what issues I encounter. Kind regards, Peter
Always use a smartshunt for measuring SoC. DESS predictions will work better too. Let the BMS do the balancing and guarding the battery. If charge rates are to high, you can create a Nodered script that gradually lowers the charge rate by lowering DVCC Amps when SoC > 95%.
This also gives your battery more time to balance.
that is exactly what dbus-seriallbattery does, down to a level of each cingle cell voltage
Dear all, the Smart-Shunt has arrived, and I will provide an update once it is installed.
The Victron Smart-BMS is only compatible with a Victron battery. You can also calibrate the JK-BMS current sensor; this will ensure that the SOC calculation matches more closely with that of the Victron Smart Shunt. Under NO CIRCUMSTANCES should you omit the JK-BMS, as this will result in a FIRE caused by overcharging the weakest cell in your 16S pack; that cell with the lowest capacity will then be charged well above 3.65V and will become VERY HOT. Setting the correct capacity for the cells also helps a great deal; for new cells, this is usually around 5 per cent more than the supplier specifies.
No, this is definitely not true. My JK BMS is current calibrated, and it reports high currents almost perfectly:
But at low currents the JK BMS is wrong almost all the time, but in a very unpredictable manner. Sometimes it’s almost right, and sometimes it#s totally wrong:
The current measuremnt of the JK Inverter BMS is not usable, and therefore depending on charge- and discharge currents the SOC from the JK BMS can be very far from reality. This can be a risc for the battery, i.e. when a user want’s to hold the battery between 40 and 50% SOC, which in reality can be a real SOC of less than 10%.
Remarkable that you have such hugh soc difference.
With my Victron RS6000, jk bms v19, diy batteries and the Victron Smart shunt I have never seen more difference then 3% at +/-50% soc, last week it was 91% vs 90% for the smart shunt.
Check however the measured total voltage by the jk bms, one of mine was way off, but is easy to calibrate using an accurate dc voltmeter.
It’s completely explainable: I have 5.4 kWp solar modules but I only have a MP2-3000 which according to German grid code has a maximum power of 2.4 kW, plus I have an old 0.6 kW micro-inverter. To be able to feed all solar surplus into the grid I have a rather big 15 kWh battery, and i discharge the battery with 0.5 kW all over the night. That is a fairly small current, but because the JK BMS is off by a large factor when measuring such small currents, it naturally calculates an incorrect SOC based on those erroneous figures. Over only one full charge cycle I get 35 to 40% SOC difference between the JK BMS and the SmartShunt. And just to say it again: the SOC caclulation of the SmartShunt is correct.
Before, when I didn’t have the SmartShunt, I noticed the wrong SOC calculations of the JK BMS when in winter my battery did’nt reach 100% for weeks, but the JK BMS reportet 45% SOC at a voltage of just 51 V (which probably is something between 5 and 10% SOC). But now I ise a NodeRed flow to automatically discharge my battery over night so much, that it get’s full not before at 5 or 6 pm. This NodeRed calculation depends on month, daytime, solar forecast, and a reliable SOC. I would not be able to use that with the wrong SOC caclulations from teh JK BMS.
When you carge and discharge your battery with high currents only, you will barely see any big discrepances at all. But most of the time most people have small currents.
The voltage shown by my JK BMS is always right. The small voltage difference which can be seen in my screenshots is a result of the battery being 5 meters away from the MP2 and Smartsolars and the SmartShunt. When the MP2 discharges the battery at night, then the JK BMS shows a voltage of 0.2 to 0.4 V over that of the SmartShunt, and when the Smartsolars charge the battery in daytime, the voltage of the near SmartShunt is 0.2 to 0.4 V over that of the JK BMS.
The current measurement of the JK-BMS is certainly not perfect, but neither is that of the SmartShunt. Both SOC readings start to drift after a few cycles; I’ve installed around 40 JK-BMS units and 8 SmartShunts myself. Without manual correction, none of the 48 SOC readings are accurate after a month. Some JK-BMS units remain fairly consistent with the SmartShunt, whilst others are significantly off the mark. Manually resetting them monthly or charging them to 100 per cent brings them back into line for a while. There’s not really any risk involved; whether fully charged or fully discharged, it’s the battery voltage that ultimately determines the control, not the SOC!
Have a JK BMS and a Smartshunt.
Also find the SOC difference very prominent when they are not fully charged per day.
Smartshunt provides the SOC - whith re-syncing every few months as one must do.
JK BMS, as per screen below, gives the data I need to monitor using HomeAssistant on a Rpi connected to the JK BMS.
3.45v is my 100% SOC.
Balancing starts at 3.4v.
Float is set at 3.4v - to give ample time to top balance.
This is the screen I rely on to make sure the battery is happy.
The Delta my “warning” - must never exceed 0.085v. Ideal is 0.008v when fully charged.
As you can see, the Smartshunt SOC is close as can be at this time.
Normally the SOC’'s are close as can be.
Best of all worlds.