question

@highlandbishop

Since you have a battery of unknown condition and unkown charge history, that was 105AH new, it would be prudent to assume (especially if it is more tha one year old) that there no longer 105AH of capacity there.

Set your dischrage floor to 50% as you really should not be drawing it under that. 12v is basically discharged on lead acid. Yes you can discharge to a power voltage but generally you are damaging it at that point.

SOC is not determined by voltage but rather by how many amps over a specific time hence the term Amp Hour or AH.

On lead acid, if it was not charged fully through the correct length of absorption at the coreect voltaged charge to reverse the chemistry, then has not had a good start point and also will not have the full AH of charge. Hence the weird voltage for 85%. This combined with the fact that the second hand battery does not have the full capacity of 105Ah means it is is going to be interesting to work out the true SOC of your battery.

Q1. Having the charged detection voltage lower and charge detection time and tail current not set correctly means the SOC might synchronise too early.

What charge voltages are you using on the mppt? And how is it compared to the shubt settings? Maybe if you share some of them we can help you tweak it to get it to work better.

Under a 1Amp load your battery voltage should not drop that much if the battery is in good condition.

SOC is calculated from current in/out of the battery. Make sure ALL connections besides the battery negative are on the system side of the shunt so it sees all charging and discharging currents.

There is a good explanation of that mechanism in the manual:

https://www.victronenergy.com/upload/documents/SmartShunt/9172-Manual_BMV_and_SmartShunt-pdf-en.pdf

The "charged" voltage of the shunt is one part of the trigger to reset SOC to 100%. The other part is the tail current. SOC resets to 100% when battery voltage is above the "charged" voltage and when the current into the battery is less than the tail current.

As you say, battery voltage is a poor indicator of battery state of charge unless the battery is in a true resting state for some time. which is generally not possible in an RV. For this reason, a battery monitor that bases state of charge on current in and out of the battery is needed.

SOC should not be dropping unless a load is detected by the shunt. One common problem is that the shunt is connected backwards. That would mean a charging current would be counted as a discharge and visa versa.

You have a very high charge efficiency factor, 85% more normal. For an old lead acid this may be too high.

Normal absorption is 14.4V

Shunt will only show state of charge change if it detects current. Showing SOC drop while charging with small load indicates load greater than charge current, or a wiring fault, like charge current bypassing the shunt. This usually from extra connections to battery neg, or a reverse wired shunt.There should only be a single neg connection to the battery, the shunt. All other negs to the load side of the shunt