question

Yes, it's technically possible (with an additional external relay - rated to the load) and you have a few options to use for control, you could select to engage the dump load based on the charger going into float, high panel voltage or high battery voltage - see image below;

However, that is quite a course method of control and not really ideal. There is no way to combine conditions or of knowing if there is enough 'spare' PV power available to fully power the dump load without drawing on the battery.

So with this type of very simple on/off control method, it's vital that you don't go overboard with the size (power) of the dump load/heater - It's better to go smaller (lower power), so that there is sufficient PV power available to fully cover the dump load more often.

An ideal dump load control system/strategy would include the ability to modulate the power provided to the dump load to suit the 'spare' PV available at the time.

@Mark Many thanks for the answer , that is what I wanted to hear was planning to connect to a 300w 12 water heater element through an external relay . will have 1050 W solar . How would I modulate the power as suggested ? Sorry if dumb question.

Simon - I have same initial question as your's. I also want to use 300w immersion for dump load - did you ever get the solution and working?

tks Guy

If you have a BMV712, there is an approach to close a relay based on a battery State of Charge condition, which seems a bit better than switching based on voltage level: https://community.victronenergy.com/questions/22862/bmv-712-relay-switch-condition-charge-status-soc-p.html