This project builds a solar powered dehumidifier solution for a boat stored in Florida. The dehumidifier can be run without any land based power through an inverter running off the house battery bank. The solution will manage the battery discharge and solar charging to ensure the house batteries are not abused.
12 volt deep cycle battery bank
120 volt inverter tied to house bank
Relay and modified extension cord ( hot wire run across the relay), or a switchable inverter, where the on/off switch is run across the relay.
Arduino, small breadboard, a few resistors, jumper wires, leds and 9v power supply
High level design
The project will use an 120v inverter that runs off the house 12volt battery bank, and an arduino to measure battery voltage using a resistor ladder to step the voltage down for the arduino, and control a relay that will power the dehumidifier from the inverter, or turn on the inverter if your invertor can be wired with an external on/off switch, when the battery voltage is strong.
The inverter itself is a power drain, so if you can make the inverter switchable, you can simply run the load off the inverter directly and use the small relay to close the on/off switch for the inverter. The load could also be a small AC unit if you do not want the heat a dehumidifier puts out.
The voltage trigger parameters can be changed in the code, but i have set the relay to fire when the battery voltage hit 13.3 which means we are being charged by the solar panels, hence the sun is shining and the batteries are fairly well charged. Once the relay is fired, the code will keep it on until the battery voltage hits our lower trigger, in this case, I have set it to 12.5 volts so when the batteries have been drawn down to roughly 75% charge. This way the batteries will not get run down too far, and the relay will not fire again until the batteries have been recharged somewhat. As a way to ensure the batteries get fully charged, every five days, the code will enter a loop for two days where the relay will not fire no matter what the voltage is. This way, hopefully once a week, the batteries are 100% charged.
In my deployment, our battery bank is 600amp hours, and the dehumidifier draws 30 amps from the batteries when running through the inverter, so without sun, we should be able to run close to 5 hours if the batteries are at 100% when we start. There will be some contribution from the sun as we do not fire the relay unless the batteries are at 13.3v showing they are being charged. The 450 watts of solar panels contribute 22 to 25 amps in full sun, so overall, we should be able to run the dehumidifier 3 hours for every 5 hours of sunshine we see.
Given the closed state of the boat, with little airflow, and the quart an hour dehumidifier rating, the running of the dehumidifier a couple of hours each day should result in two gallons or so of moisture being taken out of the boat each week. This should have a very strong impact on the humidity levels and hence mold growth inside the boat over the storage season (the summer months in our case).
Although not needed in the design, I also wanted to be able to track how often the relay was firing, so i put together 8 leds in a row on my breadboard and will light the leds to show the binary display of the cycle count. This gives the ability to see a count up to 255, and since it is not practical for the relay to fire more than once a day, given the recharge needed to hit the high voltage trigger level after a dehumidifier run, and given the two days off each week, this allows a count that will cover a full year.
Here is a wiring diagram. The relay can be used to turn the inverter on to power the load, or switch the load on by closing the Hot wire of a live AC circuit. The 8 LED’s provide a Binary count of the number of times the relay has been closed (the load cycles).