Victron Cerbo GX + SIMP BMS setup

Announcement - Help us fight the BOTS! Please report all spam including stuff in your inbox!


New member
Sep 6, 2021
Hi all,

I'm currently setting up my off-grid system using Victron components, the SIMP BMS and 2 Tesla model S battery modules.
So far everything seems to go well, I just have a few questions to make sure everything is setup the way it is supposed to be.

Basic setup introduction (detailed scheme attached)
- 2 Tesla model S battery modules in parallel (±24V system)
- SIMP BMS (connected to Cerbo GX using BSM CAN)
- Victron SmartShunt (connected to Cerbo GX using VE Direct)

1) Battery percentage reading
- The Cerbo GX is recognising the SIMP BMS and the SmartShunt
- The SIMP serial interface indicates a SOC of 26% (which seems correct since the voltage is currently at 19.85 for both cells)
- The connected SIMP display shows a SOC of 100% and in fact doesn't show any metrics at all (temp 0C, current 0A...)
- The SmartShunt also indicates a SOC of 100%.
- See details attached
- Q1) Should the Cerbo GX use the SIMP or SmartShunt as the battery monitor system (currently set to 'auto' which seems to result in the SIMP)
- Q2) How can I calibrate the SmartShunt and are my settings (attached) correct? I've disconnected the load minus from the shun and pressed "Calibrate" for a 0 current calibration but nothing seems to happen.
- Q3) The Cerbo is now indicating 39.69V for the battery (coming from the SIMP) which is actually not correct since they're in parallel. Using a multimeter I can confirm the the actual voltage is ±19V. Is there any way to change this?

2) SIMP BMS shut-off
I'm using a NG125N MX+OF which is connected to the SIMPs J4 connector pins 2 (GND) and 4 (trip). Is there any way to test if this is working? Can I performe a test shut-off from the SIMP to test this?

3) Cell balancing & other settings
I've set the BMS to ESS mode and it is supposed to be pre-installed for the Tesla modules. Are there any other settings I should double check to make sure everything is safe & sound?

If all above is sorted out next steps will be 1) connect the solar panels to get the batteries charged; 2) turn on the inverter and connect some AC loads.

Any feedback or input is welcome!
Many thanks.


  • A4 Copy 3.jpg
    A4 Copy 3.jpg
    2.9 MB · Views: 85
  • 92B2CA5B-D89E-4C58-A9EE-0101CC97AC0D.png
    349.5 KB · Views: 80
  • 785991A6-59F6-47A8-977A-4C8B209D0586.png
    364.3 KB · Views: 53
  • A9EE9B3B-C86F-45FE-8957-B1349A54F7F8_1_105_c.jpeg
    172.6 KB · Views: 69
  • 059FE914-F4F1-4080-8CC0-AA3573F46148_1_105_c.jpeg
    263 KB · Views: 75
  • 50E533FC-0020-46BC-8CAE-4CFB32580DFD_1_102_o.jpeg
    124.5 KB · Views: 95


Oct 21, 2017
Ben, mr. Widmore
There are 3 ways of measuring SOC in your set-up: 1) your MultiPlus 2) your Smart shunt or 3) the SIMPbms
My take have a go at the Smartshunt, as you have shed out the money already
Read Manual for SIMP how to deactivate voltage based SOC

read the manual of Victron for Smartshunt
use victron-connect:> local to do the correct settings corresponding with size (Ahr) and voltage of your battery, etc

3) SIMPbms is reading, and controlling the battery at cell-level, you need to assign the correct setup of your battery system in the SIMPbms settings
See manual: go to battery settings etc, and take it from there.
( You have now assigned 2 Tesla bricks in series, but I expect you wish to have them read in parallel)

4) when all is set-up correctly, you could lower voltage limit to induce a fault reading, or heat trigger by lowering the max temp, and check if your SE NG125n MX+OFF triggers

5) tough one: most standard settings in firmware will work for your set-up, but the disclaimer in the manual is there for a reason.
I expect you to understand what you're doing,.....

Using an MPPT from Victron: do read Victron manual, use victron-connect to configure to correct max charge voltage
max cell voltage for Tesla bricks = 4.2Vdc per cell group x 6 = 25.2 max
(standard charge voltage setting of MPPT is ~28Vdc when choosing 24V battery, it will kill your Tesla bricks instantly)

kind regards Carel


New member
Sep 6, 2021
Thanks for the quick and detailed reply, Carel! Really useful and appreciated.

I've updated the SIMP with the following settings:

5 - ESS mode :1

Battery settings
7 - Ah Battery Capacity: 210Ah (as I understand from the manual this is the value per module and not in total)
0 - Slave strings in parallel: 2
a - Cells in Series per String: 6

Charge settings
5- Charger Type: Victron/SMA

Current sensor
s - Current Sensor Type : Undefined
2 - Pure Voltage based SOC :0
3 - Current Multiplication :1

And the SmartShunt as follows
The percentage seems to be correct as I change the Ah of the battery if I compare it to the SOC vs Voltage chart in the SIMPs manual.

The display connected SIMP is still not showing correct values (100% SOC and all 0 for other values) but it's actually not so big of a deal as I have the GX touch display. I'm wondering however if there's still an issue with my setup causing this.

Changed the max voltage for the MPPT to 24V - maybe a bit low but should be enough for my needs.


Oct 21, 2017
Ben, did you try swapping rx/tx wires of the Nextion, perhaps not connected correct....
another question did you see Nextion boot up showing SIMP logo??
could it be you did not flash SIMP firmware on Nextion??
as always giving support is a lot of guesswork my end,

best of success with your build



New member
Sep 6, 2021
"7 – Battery Capacity = each group of parallel cells monitored per slave capacity in Ah"
I understand my total capacity is 420Ahr but not sure if I have to enter 210 or 420 in the battery settings. I assume 210 since it's 2 slaves?