Victron MultiPlus and LiFePO4 SoC value

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boringdice asked Jan 6, '23

Victron MultiPlus and LiFePO4 SoC value

Hello. Recently I kicked up some simple system for home usage with one 12v 150ah LiFePO4 battery and MultiPlus 12/1600/70-16, also added VRM portal connectivity. The battery has its own bms system without any cable connection capabilities, just bluetooth and a separate app for it. The battery description says that the max charging current - 75A and max voltage is 14.6. My MultiPlus config looks like this:

A few hours after charging the battery to 100%. I have this picture on the VRM portal.

First question. Why does MultiPlus indicate battery current consumption?

Second question. When I leave a battery in this state without charging and using it (only grid power), after a few days VRM shows the battery SoC on something around 80%, but BMS app shows 99%. As I understand the battery voltage moving slowly to 13.6 - 13.8V (full charge regular voltage) which is the normal behaviour for LiFePO4 battery, but MultiPlus consider this transition also as discharging, because the "State of charge when Bulk finished" is 95% is based on charging voltage. Do I need some additional sensors for the battery like a smart shunt to fix this issue, because I can't find any related setting for MultiPlus battery monitoring related to that?

battery charging Lithium Battery SOC

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bbmz answered · Jan 07, 2023 at 03:18 AM

If you don‘t use the battery and therefore only small currents (e.g. for the BMS itself) occour neither the 99% of the BMS nor the 80-ish % of the VRM are accurate (nor the xx % of an addition SmartShunt). The measuremen-errors at these low currents are much more prominent on these „low-load“ scenarios then in „heavy-load“ configurations.

The flat voltage-curve of LiFePo4 unfortunately does not allow a (good) voltage-based SoC-calculation.

From my point of view the only thing that helps in your situation is a recalibration of (both) SoCs via a regularly dond discharge/charge-cycle („calibration-cycle“).

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Answer 1 · 2023-01-06T18:21:08Z

Alexandra answered · Jan 06, 2023 at 06:21 PM

@BoringDice

Yes to a shunt; unless you are happy with your local monitoring solution? It really does come down to how each device calculates/derives or considers SOC. The inverter only really measures voltage not amps.

And in theory a battery at float is at a lesser capacity and so technically at a lower SOC, since it is discharging down to the float as you said.

But that is spliting hairs so to speak.

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