question

matthias-nagel avatar image
matthias-nagel asked

Does a Smart Shunt or battery monitor become unnecessary for LiFePO4 with BMS?

I am currently planning a system with three MPPTs, one Phoenix charger, a Cerbo GX and a Smart Shunt (or BMV-712 Smart) with an external battery temperature sensor. The battery is a lead-acid battery.

Maybe, in some future the lead-acid battery will be replaced by a LiFePO4 battery. From my understanding LiFePO4 batteries require a dedicated BMS which is either integrated into the battery (most vendors) or is external to the battery (e.g. Victron's own LiFePO4 plus Lynx Smart BMS or smallBMS). However, in all cases the BMS must be compatible with the Cerbo GX. The BMS communicates with the Cerbo GX and reports the internal cell status, SoC, temperature and requests the optimal charging voltage/current from the Cerbo GX.

Does this imply that the Smart Shunt (or BMV-712) becomes unnecessary? IMHO, the BMS should be able to report all the values (and more) which the Smart Shunt does report. Is there anything I miss?


Addendum on Sep 23rd 2023 due to answers up to now

Reading the answers I confuses me even more.

Until a few weeks ago, I had been thinking that lead-acid and LiFePO4 batteries were basically the same, i.e. "dumb" energy storages. Of course, I was aware of the fact that they are using different cell chemistry, that due to that difference in cell chemistry both have their specific advantages and disadvantages and that they require different charging strategies (i.e. different bulk, absorption, floating voltages, different charging current, etc.). However, I had been under the impression, that one could simply throw out a lead-acid battery, drop in a LiFePO4 battery, adjust the configuration of the Cerbo to the new battery according to the specifications of the battery and that would be it.

Then I learned that LiFePO4 batteries are sensible snowflakes which require their own tailor-made BMS which is either integral part of the battery or must be added externally. Without a BMS LiFePO4 batteries would die, explode or do other unpleasant things.

I also learned that the Cerbo GX stops being the controller of the charging process, but that the BMS takes over (of course, assuming that the BMS and the Cerbo are compatible). That's why I conjectures that a SmartShunt (or BMV-712) becomes unnecessary, because the BMS takes precedence anyway.

Now, the answers up to now are basically saying that one should keep the SmartShunt in place, because the built-in BMS of most batteries are less reliable and more inaccurate than a SmartShunt. That's confusing. I thought the whole purpose of a BMS was to cherish the poor LiFePO4 in the first place. Why do I need a BMS if not for that reason?! Moreover, if (most) BMS are that bad should I even connect them to the Cerbo GX at all or just keep them disconnected and let the Cerbo GX continue to use the SmartShunt?

cerbo gxBMV Battery MonitorLithium BatteryBMSSmartShunt
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5 Answers
dariusdiy avatar image
dariusdiy answered ·

From my experience the BMS is not as accurate as a Smart Shunt, besides this some Chinese BMS's have very weird behaviours in the GX device.

Also to be mentioned is that the SoC reported by the SmartShunt is wayyyy more precise and granular then the 3 different brand of BMS that I have.

Personally I would not exclude the SmartShunt from the setup, but I don't see why a setup wouldn't work just with a CAN connected BMS to the GX device.

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Paul Simon avatar image
Paul Simon answered ·

It really depends on the type of BMS you are going to use. I have a LiFePo Battery with a cheap chinese BMS that provides a voltage, current and SOC reading. But I still upgraded to a Smart Shunt since the calorimetric SOC on the BMS was very inaccurate. No wonder as it uses a few tiny SMD resistors as measuring shunts.

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pwfarnell avatar image
pwfarnell answered ·

The Victron Lynx BMS incorporates a shunt so you do not need a SmartShunt or BMV. However, the other Victron BMSs still need a SmartShunt or BMV as they do not include SOC measurement.

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Sarah avatar image
Sarah answered ·

If you get a smart shunt now and later decide to upgrade to a Lynx BMS which has its own shunt you can re-use the shunt to accurately measure the dc loads.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

You are mixing the functions of a BMV/SmartShunt (battery monitor) and BMS (battery management system).

A BMV/SmartShunt just measures the voltage and current of the battery and shows/calculates the SOC, nothing more, it doesn't controll anything.

The BMS monitors each cell, manages the cell balancing and protects the cells from overcharging/discharging by sending signals to the system (to the GX device) and/or shuts down the battery. Almost all BMS also (try to) calculate the SOC but in most cases that's not very accurate and drifts away over time, in that cases a additional BMV/SmartShunt will improve the SOC calculation.

5 comments
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matthias-nagel avatar image matthias-nagel commented ·

How does a Cerbo GX react if both a compatible BMS which reports the SoC and a SmartShunt is present? Does the Cerbo use the BMS or the SmartShunt as the source for the SoC? Which one takes precedence?

I know that a SmartShunt/BMV alone can only monitor the battery and does not control anything, however the combination of SmartShunt/BMV and Cerbo GX can control. Or more precisely, a Cerbo GX controls the system based on the readings which it reveives from the SmartShunt/BMV. Hence, I was wondering why this does not suffice or vice versa, if I have a BMS which can do a lot more (i.e. measure each primary cell individually), isn't enough.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ matthias-nagel commented ·

You select one in the settings. -> look into the manual

https://www.victronenergy.com/media/pg/Cerbo_GX/en/configuration.html#UUID-cc6001e7-c9ac-0a9e-37e3-75bfdff69699

The only thing that a GX device will control in combination with a BMV/SmartShunti is:

- the charging current if DVCC is enabled

- the ESS discharge SOC

Both will NOT protect the battery from overcharge/discharge.

You will ALWAYS need a BMS for lithium batteries. The BMV/SmartShunt is optional if the BMS doesn't measure the SOC at all or not accurate.

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matthias-nagel avatar image matthias-nagel Matthias Lange - DE ♦ commented ·
You select one in the settings. -> look into the manual.

Thanks for the link. Much appreciated.

The only thing that a GX device will control in combination with a BMV/SmartShunti is: […] Both will NOT protect the battery from overcharge/discharge.

Why not? I mean, of course only measuring won't do it. But the GX has a relais output which can be programmed to switch when the SoC drops below a certain level. In combination with a VE Battery Protect or 3rd party contactor, this can disconnect the load.

If the SmartShunt/BMV is able to measure the SoC more accurately than most integrated BMS, this should protect against discharge.

Same for overcharging. If the Cerbo GX detects 100% SoC (via the SmartShunt) it should throttle the chargers (e.g. MPPT, Phoenix), doesn't it?

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ matthias-nagel commented ·

The BMV/SmartShunt is only measuring the whole battery voltage not the individual cells.

Charging is based on voltage not on the SOC. The charger will not stop charging just because the SOC hits 100%. The charger stops charging based on it's charging algorithm.

The BMS stopps charging/discharging as soon as a single cell voltage goes to high or to low. Especially while charging it often happens that one singe cell goes up very quickly (but for the charger the battery voltage looks good) and the BMS will reduce the charging current via DVCC or stops charging completely.

You ALWAYS need a BMS for lithium batteries.

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Fideri avatar image Fideri commented ·
Are all values reported to GX visible on VRM? Eg temperature and cell values.
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