Hi, the dealer talk about discharging below 65% is rubbish. There are a number of facts you must consider with your batteries and shunt settings. You may be expecting too much from the shunt, its readings with AGM batteries are a 'best guess'. Fine tuning may help.
Battery capacity.
You are assuming the batteries each have 100 Ah capacity. This is true when the average 100 Ah battery is in good health, correctly fully charged, at 25deg C and with a load of 5 amps. If conditions vary then the capacity under those conditions will be different . Thus in real life conditions the SOC readings may differ from your expectations because the effective battery capacity has changed.
Variables also occur on charging, expecially with solar , where conditions vary. Thus an early sync to 100% may occur, or no sync at all.
The net result of variables is that the SOC may not be fully accurate especially when charging, with AGM batteries charge efficiency can vary significantly.
Battery charging voltages.
Windy nation offer 3 types of AGM battery, each type has slightly different charging voltages. These are printed on the battery case. And discussed in the data manual sheet, available on the site,
Assuming your battery is type NSAP12, the manual suggests, absorbtion volts 14.1 to 14.7 and long term float 13.5. The printing on the battery suggests 14.4 to 15 volts charge and 13.5 to 13.8 float. I suggest 14.5 volts absorbtion and 13.7 float for a solar charger. Long term on a AC charger drop the float to 13.5.
On the Victron charger set up these user values in expert mode with adaptive absorbtion period or absorbtion period set to 3 hours.
You may need to vary the absorbtion period to ensure the batteries are fully charged.
Smart Shunt Settings
The shunt syncs to 100% when the following is true,
Charge voltage exceeds 14.2 volts, (0.2 or 0.3 below charge setting on solar controller),
And the current into the battery has fallen below tail curent, set to 1%, ( 2 amps for 200 Ah battery).
And the above conditions have existed for charge detection time set to 3 minutes.
Tail current can effect sync especially with variable solar conditions or aged batteries.
Charge efficiency is not easy to set as it varies depending on depth of discharge, battery state of health, charge current, charge voltage.
For AGM start with 90% and Increase or decrease with experience. This has most effect on SOC under charge conditions.
Peukert value, for AGM set to 1.10 for a starting point. Increase or decrease in steps of 0.05. This has most effect under discharge conditions.
Knowing the battery is full so as to manually set 100% SOC.
With AGM batteries you can consider the battery 100% charged when the current under absorbtion volts, falls to less than 1% of capacity, 1 amp per 100Ah, ( or for aged batteries where the current falls to a low value and stops decreasing).
Note this current is under the absorbtion voltage applied to the battery, and where the possible available current is greater.
Summary.
Monitoring AGM batteries in a cyclic application with a modest solar charge source with variable loading, will never be completly accurate.
With fine tuning of the shunt and charger it's possible to get within a few%, however long term drift may occur and ocasional manual sync to 100% will be needed when you are absolutely sure the batteries are fully charged.
Mike