With a DC coupled battery (where the PV panels and battery storage only have one inverter) then to stop the zappi draining the battery during normal (ECO+) solar charging you need to set a small Export Margin.
This setting is found on Master Device on the menu Other Settings -> Advanced -> x x x x x -> Supply Grid -> Network -> Export Margin
Set the Export Margin to 50W and the battery should take priority over the zappi or eddi. (In some cases a setting of 100W or 150W is needed). Your myenergi devices will now wait until there is a small export before starting to divert power to the car.
To charge the car using a low rate tariff you then either need to set a Schedule in the myenergi online account (myaccount.myenergi.com) or via the app.
Note - during the scheduled boost the connected load (i.e. the EV or hot water heater) will take power from any source available, so unless the battery is turned off or also set to charge the car will still drain the battery.