Refrigerator Battery Backup: Part 4

My refrigerator battery backup project is finally complete and in service. I’ll wrap up the last phase of the build and talk about the next steps – no point just staying at version 1.0, after all.

Low-voltage Electrics

I finished up part 3 with the details on the AC portion of the build, in which I emphasized the need for neat, safe wiring – no sense building a life-safety system that ends up burning down the house. But just as important is getting the low-voltage wiring right. Car and marine batteries are not to be taken lightly, even though they’re “only” 12 volts. It’s the current that kills, and while you may not be electrocuted by a car battery, there’s enough current in those to easily start a fire, or cause a horrific burn (warning: NSFW link. NSFL actually – I’ll never wear my wedding band again while working on a battery.)

Refrigerator battery backupWith inverters, you’ve also got to have a plan for moving LOTS of electrons around. Lots of electrons means high current, and that means big, fat, short wires – the fatter and shorter, the better. With that in mind, I rearranged things a bit inside the drawer; I flipped the inverter around so the control panel faces right, exposing the battery terminals on the left. This allowed for shorter wire runs. For the main power runs to the inverter, I just used the wire that came with the inverter, after snipping off the nice battery clamps. The wire has no markings, but looks to be about 4 gauge. The wire connecting the two batteries in parallel is 6 gauge stranded. The lugs on the battery terminals are Thomas and Betts copper connectors for 2-8 gauge stranded wire; I picked them up at Home Depot and previously covered some caveats to using them. The fuse in the positive conductor is something I reused from my first battery bank build. I’m a little unhappy that I couldn’t find a place to secure it, but given that there’s not a lot of vibration like there would be in the mobile applications this is designed for, I figure it’ll be OK.

The small wires on the left battery go to the charger. I was surprised that a charger rated at 10 Amps per battery would have such thin wires, but I figure the engineers at NOCO know what they’re doing. Each charging circuit has a 15 Amp inline blade fuse, which is a nice touch. The left battery has a twisted pair of 16 gauge wire that goes to the front panel voltmeter.

With everything connected, I gave the system one last function test, and everything was fine. I lugged it upstairs, minus the 90 pounds of batteries, of course, and got it all configured. The little fridge now sits at the perfect height – no more stooping to get things out of the main compartment. I tested it by unplugging the main power cord; four minutes and 48 seconds later, the fridge started back up on inverter power. Not too shabby.

Next Steps

Refrigerator battery bank
The “finished” product.

I’m pretty sure that I’ve never actually finished a build. Rather, I tend to just put the current prototype into production while looking for ways to improve it. That said, there are a couple of problems with version 1.0. The first is the control panel. The voltmeter is obnoxiously bright, and at night it lights up the laundry room with a ghastly green glow. My daughter Grace’s room is right next to the laundry room, and the glow was keeping her up, so I had to disconnect the meter. A simple solution may be to add a momentary push button switch to the power line to the meter – just push the button to get a reading. That’s a little unsatisfying, since I wanted to be able to check the voltage every time I passed by. I might have to shop for a different meter, or I could go whole hog and design an Arduino control panel with flat-panel display, adding in temperature monitoring and logging and remote notification by email when the system switches to inverter. The mind boggles at the possibilities.

For now, though, the system is in operation and has met all its design goals so far. Ginger’s precious insulin supply is now one step further away from accidental loss, so I’m happy with it. And with a Christmas Eve ice storm a real possibility, we may get a chance to test it in anger soon.

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