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mindaugas avatar image
mindaugas asked

SOC diference between Victron Smart Shunt and JK BMS

Maybe someone can help me?

I have JK-BMS plus Victron family - SmartShunt, Multiplus II, Cerbo GX and Smart Solar MPPT 100/50. 24 V system with 210 mAh battery. It been fully launched 1.5 month ago.

Original idea for SOC was to use JK-BMK CanBus, but I been told that it is not compatible with Victron.

Then I ordered Smart shunt.

For few weeks I been adjusting SOC as there was a difference between JK-BMS and smart shunt. Usually JK-BMS had higher SOC than SmartShunt. Few weeks ago I went for vacation and after a week I saw that my SOC reaching only ~70% (based on SmartShunt measures). After coming back I found that SmartShunt has SOC value of ~70%, but JK BMS had 98% and charging was stopped due to cell OVP.

What can be an issue? settings?

BMSSmartShuntSOC
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Joe avatar image Joe commented ·

I have an almost Identical set up, How did you get the JK BMS to work with the victron smart shunt?

When I connect the victron smart shunt positive wire to the battery, the JK BMS will say discharge short circuit protection and turn off.

Did you find this problem?

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mryoshii avatar image mryoshii Joe commented ·

If this happens, you either have a short circuit somewhere, i.e. incorrect wiring. Your shunt is defective or maybe the starting current of your inverters/mppts is too high.

You could measure with a multimeter the resistance between + and - at the smartshunt.

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Joe avatar image Joe mryoshii commented ·

Thanks for the help, it happens with the mppt and inverter disconnected. I've checked the resistance across the shunt and it measures 0.

The system works flawlessly (minus the shunt) when I disconnect the red positive wire from the shunt, I'm not sure what to do

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peter-3 avatar image peter-3 Joe commented ·
I can't work that out , the shunt is connected to the negative side with a little lead fused at 1 amp which goes to the positive side , so if they was a short it would blow the 1 amp fuse ?.

I have a JK BMS inside a 50ah battery and can't see why you would get a short circuit disconnect unless to battery was fully discharged ?.

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3 Answers
Louis van der Walt avatar image
Louis van der Walt answered ·

@Mindaugas this is an important issue that many people struggle to understand. When you compare values from 2 different measurements they will only match if the way they measure the same way.

The Smartshunt does count the power flowing in and out of the battery. That means if you have a battery with capacity 100Ah and use 50Ah then you should be able to add 50Ah back again. Simple, but other things can influence this like your battery cells degrading over time and then your battery capacity reduce with this. So now you can only add 49Ah back. The smartshunt has some clever logic to account for this, but it can only look at the counted values going in and out, as well as the battery voltage so it has only 2 inputs.

The BMS in your battery also count power flowing in and out. Most do have a shunt built in to do this, but it can also look at each individual cell inside your battery as well. So your BMS now has much more inputs to look at like the voltage of each cell.

In your case your one cell has an imbalance to the others. The Smartshunt does not know there is an issue and think you can add more power, while the BMS know there is an issue and any more charge will result in battery protection triggering. The BMS will use this to adjust it's SOC value to show that to more power can be added (thus the State Of Charge is at 100%)

Some BMS/batteries will also have a SOH(State Of Health) value. This will be at 100% when the battery is new and your SOC 100% is at your battery manufactured capacity. When 100% SOC is at a lower capacity the difference between this value and the manufactured capacity is the many driver for the SOH which will show a lower health value.

Some more info if you want to read https://github.com/Louisvdw/dbus-serialbattery/wiki/FAQ


PS. @matt1309 many people incorrectly think that BMS like the JKBMS and JBD are using voltages to estimate the SOC because they see the SOC cell voltage settings inside the BMS. They both use a shunt inside the BMS and those SOC voltage is to adjust for the drift.

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matt1309 avatar image
matt1309 answered ·

Thanks Louis

Good to know, does surprise me given even with my balanced battery I see large relatively differences between jk Bms and shunt (10%). And the fact jk bms shows soc on boot with no 100% charge required like smart shunt. Would've thought if both used a shunt and voltage factors they'd be much closer. Seems a large amount for just cell information in bms. Guess that's why so many folk me included assume it was purely voltage in jk bms.

@Mindaugas have you top balanced your cells, 20% sounds like a large difference if it is just one peaking cell. Otherwise maybe the shunt isn't calibrated/settings input correctly.

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Louis van der Walt avatar image Louis van der Walt commented ·
The Smartshunt is very well calibrated out of the factory. I cannot say that all BMS shipped from China has the same calibration out of the box. At least you can calibrate most, and that does improve it. Also not all BMS will use the most expensive shunt. They normally come in 50mV, 75mV or 100mV versions. If you have a 100A capable BMS then 100A will mean a 50mV drop over the shunt or 2A per 1mV measured, while the more expensive 100mV shunt will be able to measure 1A per 1mV


The starting SOC of a BMS is just the measurement the BMS has been programmed with. Some start at 0% SOC and others at 100% SOC. Most cells that you buy should be around 40% SOC which would have been a better starting value, but if your BMS have editing available you can just set that SOC when your build your battery.

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mryoshii answered ·

I also have a JK-BMS and a Smart Shunt. I only trust the victron values because my bms shows completely different readings. I assume that the measurement of the bms is very inaccurate. Also, the BMS already shows 0% remaining where the smart shunt still shows over 30% remaining.

I have calibrated the bms with 5A input current. Still shows inaccurate values

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Louis van der Walt avatar image Louis van der Walt commented ·
Look at the Min/Max cell voltage graph in VRM and see if your min cell drops below the min cell voltage value you have set up in your BMS. Most likely you have an cell imbalance problem which the BMS can pick up and the Smartshunt can not.
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mryoshii avatar image mryoshii Louis van der Walt commented ·

I have no problem with cell voltage. The BMS measures only more A than really flow.

Even if the BMS thinks that 0Ah are left, the system still keeps running because it is not really empty and the BMS disconnects when the voltage reaches 2.5V per cell.


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larsea-dk avatar image larsea-dk Louis van der Walt commented ·
Hi Louis


Is there any possibility to have the jk bms not forwarding the soc and the victron system will pickup either smartshunt or the built in multiplus SOC instead?

My (and also others I discussed it with) jkbms is not counting charge when multiplus feed in and battery charge around 20A, so the SOC for JKBMS will of course get wrong.

BR, Lars

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