Only Guessing, Multiplus 12/1600/70 failed to update firmware 4.89 at 40% "writing firmware". Is this the latest firmware or latest VRM?
asked
Multiplus Fails Firmware Update with latest version VRM
Make sure that the DMC is disconnected and if a VE-BMS is connected then unplug this.
basically ONLT the Multiplus and the GX device is to be connected to the VE-Bus when doing the upgrade
Hopefully this has already resolved but I found I also needed AC power to be present (as well as battery) to complete the upgrade of a MultiPlus II. This is in addition to Paul’s advice above.
