Refrigerator Battery Backup

November 29, wake-up temperature: 9°. I’m no stranger to working outdoors in the cold, but it takes a while to acclimate, and it usually doesn’t get this cold and snowy until well into January. Time for some long-neglected indoor projects.

As you may recall, I built a pretty sweet portable battery backup unit a while back. Built into a large mobile tool chest, it was a good attempt at integrating all the pieces needed to store electrons and turn them into AC power in a grid-down scenario. As integrated as the build was, it suffered from a few issues. First, the charger was really noisy, and leaving it plugged in and charging all the time was a source of family friction. Second, it was “portable” in the sense that cars are portable – yes, the wheel around nicely, but getting them up and down stairs can be problematic. Finally, it didn’t work, at least not for its intended mission of powering a fridge to keep things cold without having to run the generator full time. The inverter I chose was just too small for the start-up surge of even a small refrigerator compressor, which was the main use case for the finished product. Disappointing, to say the least. Time for a new design.

Electrical Design

Like this, but full of insulin.

My new build seeks to solve these problems while leveraging what I learned from my first build. To keep the project focused, I decided to eliminate the portability factor and build a dedicated battery bank designed to power just our backup fridge. With that in mind, the form factor started to take shape in my head. I pictured a pedestal upon which the mini-fridge could sit, kind of like the pedestals offered for front-load washers and dryers these days. The pedestal would have a drawer with a tray for the batteries, charger and inverter, so that everything could be reached for easy service. As a bonus, the pedestal would reduce the stooping needed to access the fridge.

I had a couple of other design goals. First, I wanted to make sure it would work. The current inverter obviously needed to be replaced, so I did a little Googling and settled on a Cobra CPI 1000 that I picked up on Amazon for $75. I did a test last summer to make sure everything would work, and it was a go – the inverter easily handled the start-up surge, and the batteries kept the fridge running for more than 14 hours without budging the voltmeter, even on an 80° day.

With batteries and inverter sorted, I turned my attention to the charger. The Scumacher SC-10030A smart charger I was using was all wrong for the job – too many controls that reset to default when the power was removed, difficult to read labels, and a bulky case that set the control panel at an awkward angle in the tool box I was using. Plus, it buzzed like a bee hive when in operation. I needed something a little better than consumer grade electronics for this job – something sealed, quiet, and with a minimum of controls. At Brother Harold’s suggestion, I turned my attention to the marine battery charger market, and eventually settled on a NOCO Genius GEN2 charger. It has two 10A banks for a total of 20A output, so it should be able to charge nearly as quickly as the Schumacher, but do so silently and without any need to fiddle with switches.

Another design goal is automated operation. I want this bank to be able to switch the fridge over to inverter power automatically if the grid power fails. That’s actually a little more complicated than it sounds, because if the power fails while the compressor is on, switching immediately to the inverter won’t work – you need to let the compressor sit idle for five minutes or so before reapplying power. So I’ll need to fiddle with some sort of time-delay relay here, one that can sense the loss of grid power and switch the fridge to the inverter five minutes later. I can either buy an off-the-shelf timer relay, or mess with a timer chip and roll my own. I’ll likely suck up the $70 for the COTS parts and call it a day. After all, this is a life-safety application, so spending some bucks on an engineered part is probably justifiable.

I want to add a few other bells and whistles too. One is a digital voltmeter, to monitor the health of the batteries. I’ll mount that to the front drawer so it’s visible at all times. I’ll also add a pilot light powered off the inverter; since I intend to leave it running all the time, the light will ensure me that the inverter is alive and well. I’ll also add a pilot light for the grid power, just for fun.

Casework Design

I’m not going to get away with an off-the-shelf cabinet for this project, so I’m going to have to roll my own. It’s going to have to be mighty sturdy – the batteries alone weigh 45 pounds a piece. Put all that weight on a flimsily built drawer and you’d be asking for trouble. I decided to go with 3/4″ plywood for the drawer, heavy duty 220 lb capacity drawer slides, and a case made of medium-density fiberboard (MDF). To make the drawer extra sturdy, I planned on setting the plywood bottom into dadoes cut into the drawer sides. For quick assembly, I went with pocket screw joinery – not fine joinery by any means, but perfectly suited to a project like this. For overall dimensions, I went with a box about 24″ square and 15″ tall, giving me plenty of room for the guts without raising the mini-fridge too high.

The Build

Unfortunately, I had to spend four solid days digging my way through the garbage-filled remains of my once beautiful woodshop before I could start work on this project. I took almost 3/4 of a ton of crap to the transfer station, and I still have more to go. But I managed to excavate far enough to see the floor and use my machines for the build. It was great to get back to making sawdust again – I haven’t built a legit workshop project in almost nine years. Sad but true.

Refrigerator Battery Backup
Cabinet drawer with battery tray.

I started my build with the drawer. It really only needs to be a tray, so I made it only 3″ tall. It’s exactly as wide as the two batteries side-by-side, and I added a partition to keep them in the back of the tray. I figure that 90 pounds of ballast is best kept as close to the rear of the case as possible so it doesn’t tip when the drawer is open.

Refrigerator Battery Backup
Drawer with all the goodies.

In keeping with my goal of making the electronics accessible and visible, plus adhering to the unstated but nonetheless important goal of tidy and safe wiring, I decided to build a circuit box. The top of the box will act as a mounting platform for the inverter, and the side of the box will provide a handy spot for mounting the charger. I made a slot at the lower end of the side panel so the charger wires can be tucked into the circuit box. The inside of the circuit box has plenty of room for the time delay relay and other doo-dads.

My intention was to mount a finish drawer front made of MDF once the drawer is mounted in the cabinet. But looking at the picture, I may want to replace the front of the drawer with a piece of plywood that’s as tall as the circuit box, to give the drawer front some support. And of course, the drawer will be painted matte black – it’s just how I roll.

Next Steps

Tomorrow I’ll work on the cabinet body, and maybe tune up that drawer front. After that, I’m a little stuck – the drawer slides I ordered won’t be here for another couple of days, and I still need to round up that time-delay relay, a decent voltmeter, and the electrical odds and ends that’ll take me half a dozen trips to Lowe’s and Radio Shack before I’m satisfied with the parts. I tend to get a little anal about these projects.

I’ll post more as the build progresses.

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