When setup correctly the BMV should be considered as the master as it accurately measures all current flowing into & out of the battery - it then uses this continuous data in conjunction with the BMV setup parameters to calculate & track SOC %.

In case any of the BMV setup parameters do not match the reality of your battery bank (such as an old/degraded battery bank only providing 60% of its initial capacity), then inaccuracy between the BMV calculated SOC % & reality will result.

Any minor SOC calculation inaccuracy will also cause SOC % to 'drift' & the error to worsen progressively over time - to minimise errors compiling over time, the battery needs to be fully recharged regularly, so that the BMV can also synchronise to the 'true' 100% SOC before any errors get out of hand (every week or two at a minimum).

Also, if there is any kind of wiring error to the BMV shunt, that will also cause serious errors with the BMV data - ONLY the battery should be connected to the battery side of the shunt & ALL loads/charge sources must be connected to the other side of the shunt. All connections must also be made to the M10 bolts and NOT to the small screws securing the PCB board to the shunt end blocks.

The Multi CAN be used in cases where there is NO BMV in the system, but it does NOT have a shunt to accurately measure DC loads - it back calculates / estimates this from AC output / input & some other data sources.

Current flow from a MPPT solar charge source can be factored into the Multi SOC calculation, but the system needs to be setup to do so (refer to battery monitor section in the CCGX instruction manual). Any other 'unknown' DC load or charge source will NOT be considered in the Multi SOC calculation & will lead to significant errors - accordingly the Multi battery monitor function can NOT be used in these setup configurations (but a BMV can as it measures all current flow regardless of source).

The Multi SOC calculation is also setup to NOT go over a particular SOC% (I think the default is 80 or 85%, but this can be configured in VE. Configure), depending on which stage of the charge phase the Multi is currently in (this is to avoid the Multi SOC % calculation potentially indicating an unrealistic 100% SOC, when the charger is still in 'bulk' phase).

So in summary, I recommend to;

1- Use the BMV as the ONLY source for SOC % & disable the Multi battery monitor / SOC % calculation (to avoid confusion & more questions).

2- Check that the current BMV setup parameters are correct (to the best of your knowledge) & ensure that your BMV is regularly synchronised (to prevent excessive SOC % drift).

3- If the BMV SOC % doesn't seem to add up, then investigate a bit further and correct the real issue / BMV setup parameters as required to eliminate/minimise any error (this may take some days of trial & error to fine tune).

Just ensure that if you are referring to battery voltage as a sanity check for SOC %, then you need to let the battery to rest for ~4h+ (ie. overnight) with NO loads or charging to allow the voltage to stabilise at the true 'resting' voltage (If you have been running a large load then it will artificially drag the battery voltage down, well below the true 'resting' voltage for any give SOC).

Thanks @Mark . Appreciate.

I think indeed my initial set up of the BMV was not exactly a real 100% SOC. I will try to find how to reset the BMV when i am as close to 100% .

However, i know the bmv is the key to soc. My color control is set as the bmv being the battery monitor. So i hardly understand why is the soc shown on the color control is different from the one shown on the bmv.

using Multiplus, the options in the above image under "system setup" only shows battery moniter but the rest such as synchronize VE. BUS SOC, Solar current to improve, Solar charger controller are not available in my system any advice