Selectively turn off appliances as battery runs down

Hi all!

I've got a question for all you experienced Permies out there.

First off I plan to convert a chest freezer to a fridge using the method described here,

https://permies.com/t/31282/Energy-eficient-Chest-Fridge-Kwh

I am also really interested in sizing my batteries much smaller than a conventional solar system, since I am on a tight budget. One avenue I am exploring is lining the fridge with 2L pop bottles filled with water, then setting the thermostat to ~ 0 C, so that when there is plenty of sun, the bottles will freeze, and act as a thermal storage. Then if there is no sun, the bottles will melt, and keep the fridge nice and cool for hopefully a few cloudy days. One challenge with this is how to keep my food from freezing, while allowing the pop bottles to freeze. I'm hoping that placing the thermostat near the bottles, and insulating the gap between the food and the bottles, will solve this issue. We will have to see.

Anywho, my real question is how prevent the fridge from clicking on if there is a few cloudy days, while still allowing things like my water pump and lights to function normally. Ideally I would somehow like to have two inverters (or DC power lines), one that is live if there is any usable charge at all in the batteries, and the other that only works if the batteries have lots of charge in them. The fridge could then be wired to the latter, so that in the case of a few cloudy days, it would not come on, instead relying on the ice to keep itself cool, and allowing the other loads to use the battery. Then once the sun comes back out again, the battery first recharges itself, then the fridge would click on, refreezing the ice and 'charging' it's thermal storage once more.

Anyone had experience with something like this before? I would like as much as possible to keep costs down, as the whole point of this system is to save me $$$ on batteries. What would be really cool is to be able to schedule different appliances in terms of their importance. Communications could be used all the time, even if it means damaging the batteries, water pumps and lights would be next, fridge next, then wall outlets for laptops and washing machine.

Thanks so much!

You could Install  voltage controlled relays between the inverter and the various loads. Cost of the relays is around $5 to $20 per circuit.

Joseph Lofthouse wrote:
You could Install  voltage controlled relays between the inverter and the various loads.

Joseph, are you saying that there are relays that will shut off various loads when they sense below-defined-threshold current coming in from the sources (PV array, wind, etc)?  That would be huge for certain appliances!

John: I suppose that they are designed to be connected to a battery, and not directly to a stand-alone source but yes, the devices are readily available. A search for "Voltage Controlled Relay" returned more results than I wanted to sort through. Some in the under $20 price range control AC/DC loads up to 10 Amps and 600 Volts, and had variable set points on the DC sensing circuit.

Inverters are not 100% efficient. They get more efficient as they reach their max output so you should size your inverters with care if pinching every last watt is important to you (as it is for me). More electronic cut outs and sensors and whatnot are just going to be more electrical draws.
Solar living has three main components:
1. lower your power consumption
2. increase your power production
3. increase your power storage.

for your fridge, I wouldn't complicate things with relays and voltage sensors and automatic cut outs. Increase the efficiency of the fridge with better and more insulation while making sure that the fridge can dump out the heat it makes through the coils on the back. Put your fridge on a timer so it only comes on during daylight (charging) hours and don't open the door outside of those times.

If batteries are unaffordable, lower your standards and run the batteries that someone else thought no good anymore and test them frequently. Keep the ones that work well, recycle the others. 'They' say that batteries are only good for a short number of years but this is not really true. With care a good battery can last as long as twenty years without too much loss of overall capacity. More importantly, a battery that is only holding 3/4 of it's original capacity is still a viable battery and rich folks will throw them away as soon as they are no longer 'perfect'. You will still get some valuable use out of them.

What has been used before is a dc load controller to turn on smaller dc loads.  I've often thought of using one to control an ac relay. Most of the modern mppt chargers have an auxiliary relay built into them. We set a desired voltage on the battery bank and it turns on a 12 volt load for us.  We use it for the vent fan.  You could also use it to turn on something like this:
https://www.amazon.com/gp/aw/d/B00PQ7Y8ZU/ref=mp_s_a_1_5?ie=UTF8&qid=1496804366&sr=8-5&pi=AC_SX236_SY340_FMwebp_QL65&keywords=solid+state+relay&dpPl=1&dpID=41dNO-vXk9L&ref=plSrch
That way your inverter remains on but you automatically turn on extra loads based on state of charge of the battery.  It could also be used to run ac dump loads such as electric water heaters and air conditioners. Or ac pumps charging up a large pressure tank...
Best regards, David Baillie