question

@Mark Cooper, there are a number of factors here: first, the BMV calculates SOC as a function of current through the shunt, not a function of battery voltage, so it's important when first connecting the BMV to make sure that your batteries are fully charged first (since the BMV defaults to 100% SOC upon first connection) or to -after connection- fully charge your batteries and then perform a manual sync to 100%.

If you connected the BMV to a partially-discharged battery bank and did not manually synchronize the SOC, then the SOC reading will be off until the BMV completes an automatic synchronization.

Another thing to look at is to make sure that you set the total Ah capacity of your bank correctly in the BMV setup; if you accidentally set the Ah capacity too high, for instance, the SOC reading won't be accurate.

The final thing that I would look at is to make sure that you don't have anything other than the BMV shunt connected to your battery NEGs. Any charge source or load that is connected straight to the battery bypasses the shunt, which means the BMV can't "see" it, which means that your SOC won't be accurate.

A very common mistake there is leaving the chassis ground cable connected straight to the battery... don't do that. Exactly one thing can be connected to the NEG terminal of your battery bank, and that is the "battery" side of the shunt; all other system NEGs including chassis ground, loads, and charge sources, must all be connected to the "loads and chargers" side of the shunt.