Refrigerator Battery Backup: Part 3

We continue with what is quickly turning into one of my favorite builds ever. Today we’ll tackle the electrical design and build.

A word of warning: again, I’m really comfortable working with electricity, but that doesn’t mean you are or should act like it if you’re not. Either skip it altogether or get a qualified electrician to wire this up for you – he’d probably get a kick out of a project like this. Don’t forget my disclaimer when reading about projects like this.

Electrical Design

You’ll recall that the main goal of this project is to power a small fridge off a battery bank and inverter in the case of a power failure. The batteries are to be kept charged and conditioned at all times, and the inverter needs to be sized to handle the start-up surge of the fridge compressor. The battery bank also needs to be sized to get a decent run time between charges, and the charger needs to be able to quickly top off the batteries when power is supplied by a generator, to keep the amount of fuel burned to a minimum. Lesser but still important design goals are silent operation, automatic switchover to the inverter after power loss, and neat, safe wiring in a movable drawer to allow maintenance.

I’ve already tested the batteries and inverter for ability to start the fridge and for run time, so I decided I’d better test my automatic switchover. I want the fridge to run on grid power whenever it’s available, then switch to the inverter automatically. In addition, I want a time delay before switching to inverter power if main power drops while the compressor is running, or if the power outage is one of the many temporary blips we have. These goals point to the need for a time-delay relay, and I spent a long time sourcing something that would do the job and be affordable. I settled on a Dayton 5WMK0 that I picked up for $74 from Grainger with my company’s discount. Pricey, yes, and something whipped up with an Arduino and some code would have been both cheaper and cooler, but this is a life safety appliance, and sometimes you just have to pay your money for an engineered off-the-shelf solution.

Once I got all the parts together, I decided to function test everything, just to make sure my idea would work:

For the schematically inclined, check out the circuit diagram.

Electrical Build

Refrigerator battery backup
Not the flux capacitor!

With the design tested, I turned my attention to turning the prototype into a finished product. I like my projects to look professional – nice, neat wiring is a must. The heart of the project is the time-delay relay, so I mounted the socket for it in a 4″ square handy box. Next to that on a nipple I mounted a single-gang handy box for the outlet that the charger will plug into. All the wires coming from the connections on the back panel pass through the smurf tube into the back of the big box and a neatly routed and terminated with crimp-on spade terminals. The power cord that acts as input from the inverter is secured to a cable clamp coming out the bottom. Everything is mechanically and electrically secure.

IMG_20141207_140959_587After double checking the wiring, I cut a rectangular hole in a blank switch cover plate and closed up the boxes. I mounted and connected the charger, threw the batteries back in the tray, connected the inverter, and performed one last function check to make sure I got the wiring right. Everything worked exactly as planned.

Next Steps

All that’s left is to apply some sort of finish to the case – I’m thinking gloss black enamel on the outside, with a flat black interior. I also need to fabricate a small control panel for the two pilot lights and the DC voltmeter I’ll be adding. I’d like that to be a small stainless steel plate held to the the drawer front with stainless cap screws – stainless and gloss look pretty bitching together. Only trouble is that I need to find somewhere to procure the metal. That’s a task for another day, though.

Almost done. I’ve got to wrap this up soon – there are Christmas preps afoot that need my attention.

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