I am looking to upgrade the solar controller on my boat and would like to add a 12v immersion heater to use any excess solar to produce hot water once the batteries are charged. I've read other posts and the challenge with using relays controlled by the MPPT controller to switch the load on and off on the battery side is sizing the 12v heater load so that it doesn't exceed the solar's excess charging capacity and micro-cycle the batteries.
Hot water is the last remaining item that we currently can't generate without shore power or running the engine. We plan to cruise and even if it takes 2-3 days to heat our 8 gallon water tank it would still be worth the effort.
I was wondering if it's feasible to instead of placing the relay on the battery side of the MPPT controller, to put it on the solar array side to divert some but not all of the solar array to a separate PWM load controller attached to the immersion heater.
So when the batteries need charging the programmable relay is off and all of the solar panels are feeding to the Smart Solar controller. Once the Smart Solar controller reaches float stage, it turns the programmable switch on which activates a DPDT relay that switches the main 400 watt collection of panels over to the PWM load controller directly attached to the immersion heater (no battery) while 100 watts of solar remain attached to the MPPT controller to maintain the float voltage.
If the load on the MPPT controller increases and it can't maintain float, the switch would turn off and add the 400 watts back to the MPPT controller. I'm thinking that the Smart Solar controller would 'see' the drop in available wattage as if a cloud moved over a portion of the array and adjust based on the remaining wattage.
Would this work as an alternative to looking at another brand of MPPT controller that has an integrated auxiliary output? I'd really like to build my system out using Victron.
System details:
4x 100w solar panels (12v parallel) on DPDT relay
2x 50w solar panels (12v parallel) directly attached to Smart Solar
Smart Solar controller with programmable switch
PWM load controller (TBD)
50A DPDT relay with outputs to Smart Solar and PWM controller
300w 12v immersion water heater
4x Trojan T-125 6v FLA house batteries (series + parallel, 12v, 480AH)
