This is a problem observed consistently since the major mppt upgrade from 1.39 to 1.4x.
Mppt smartsolar 100/20 networked with BMV700 (dongle located almost next to the mppt, with a 1.5m extension cable). When significant current is being supplied by the mppt, Victron Connect mostly displays its battery voltage about 0.2 V lower than the BMV, despite it receiving the data from the BMV via the VE smart network . The actual voltage at battery terminals, measured with multimeter, is exactly that reported by the BMV (both on the device itself and via the Victron connect app). The measured voltage at the mppt output terminals is higher, of course, due to voltage drop across the connecting cables (about 1.5m of 13mm2 cable). If I remove the mppt from the network it correctly displays this higher voltage. Rejoining the network returns it to the anomalous readings above.
This is not just a reporting glitch of the app. The mppt apparently “believes” these anomalous readings. As a result, the mppt switches to absorb at a true battery voltage about 0.2 volts ABOVE the setpoint. If only looking at the mppt values using Victron Connect you would never realise this - the apparent battery voltage, as shown in the app, stays rock-steady at the set absorb voltage. Meanwhile, as the current reduces during the absorb stage, the true battery voltage gradually comes back down towards the correct set value by the time it switches to float. (This is almost the opposite of the behaviour before the update, when the mppt would switch to absorb about 0.2 V too low and gradually come up to the set voltage...I'm not sure which is worse.....)
The photo below shows a period of about 7 minutes during bulk charging. The mppt is producing about 18 amps, except for a brief drop at about 4 minutes as a wisp of cloud passed. Initially the caravan fridge is drawing about 3 amps, then cycles off for the next 5 minutes. The BMV showing the correct voltage values is on left, and the inaccurate mppt values are shown on right, displayed simultaneously on two mobile devices. The spike in reported voltage on mppt about a minute after the start of recording, corresponds to the fridge cycling off. That may have briefly disturbed the bluetooth connection, causing the mppt to suddenly “see" its real output terminal voltage (higher of course than the battery voltage) but then over the next few minutes it slowly returned to “seeing” a voltage well below the true battery voltage.
Possibly a glitch in the updated software? A bit annoying as could lead to overcharging of lithium battery.