As our water consumption merely exists of use water for eating, drinking and personal hygiene. The 6 liter 24VDC 400W Elgena boiler is installed directly under the kitchen sink. This small boiler will mainly be operated over the solar energy surplus or can be manually overridden in case it is needed. A larger boiler would make sense if three persons want to utilise hot water for a shower directly after one another, which in our case almost never happens. With a heating time to >70°C of just 30-35 minutes, we are not wasting energy on heating water that will ultimately not be used anyway.
The battery state is constantly monitored by the installed Victron BMS 702. Standard these are programmed in such way that an alarm contact closes when the battery State of charge (SOC) drops below a certain set value (factory value 50%).
Using BMV State of charge [Victron Energy]
The mode of this relay however can also be inverted. Meaning that it closes once the SOC reaches a certain programmed high threshold value. Indicating the battery is reaching it’s full state. The SOC value also triggers the MPPT algorithm to provide a lower amperage output than the solar panels potentially could deliver at that point in time. As it regulates from bulk phase to absorption and finally float as it reaches increasing SOC values.
-The whole control is ‘automagically’ over the SOC of the energy system
The inverted alarm signal is used to switch a high amperage NC relay. In case the boilers thermostat now commands to heat up the water. It will create an increase of the power demand from the system. Signalling the MPPT to optimise its output heatIng up the water to the preset temperature. In this way we are fully harvesting the solar energy and not straining the battery unnecessary with unloading cycles to make up hot water. Once the demand decreases again the MPPT wil resume to the required phase again depending on the programmed SOC open and close values.
When we drive the truck, the 55A alternator is coupled to the living unit via a Citrix-CT 120 smart relay. Once the starter battery is topped off again, it senses the increased voltage level and automatically connects the living unit battery in parallel.
Now again the same will happen automatically for the power supply to the boiler. The BMV will see the increase in SOC (if not already there). And the whole play of signals and relay switching happens again, or can be bypassed when switched from auto to manual on the control pannel.