Refrigerator Battery Backup: Part 2

In part 1 of this series, I covered the design goals for a refrigerator battery backup for our small insulin fridge. In this post, the build continues with more casework, some cable management, and getting ready to install the electrics.

Casework, Day 2

I remember my college and grad school days, living in dorms and tiny apartments and being frustrated that I didn’t have access to my dad’s big shop and garage to work on projects at need. Looking back on the past few years, I now realize that I’ve been feeling the same sense of frustration because my shop has been a pig sty. Having my shop usable again has been really great – I feel relaxed and confident when I’m working on projects and getting stuff done. Early New Year’s Resolution for 2015: keep the shop clean and ready for work at all times. Plus maybe start making a little man-cave down there. You know – for resale value.

Still, I procrastinated all day yesterday because I didn’t want to face the task of digging a 4’x8′ sheet of 3/4″ MDF out of storage and onto the saw horses so I could start butchering it up into smaller cuts. So I fiddled about with more cleaning, and I helped Grace build a new indoor hutch for her bunny. It came out great, and she did most of the work herself after a little instruction on how to use the chop saw. Apparently it’s only hot glue guns that have it in for her, since she wrapped up the project with no significant injuries.

Refrigerator battery backup - rear case
Rear of case with recess for electrics

When I finally ran out of excuses and got the MDF processed, I realized that I made a design error on the cabinet. My intention was to have back panel connections for the fridge and for grid power, but the cables sticking out the back would push the cabinet too far out away from the wall. So I designed a recessed area at the top of the rear panel where the electrical boxes will go, to keep the cables neat and allow the cabinet to be push up more or less flush with the wall. But the recess interfered with the top of the batteries, so I had to slide them forward in the battery tray. They’re up against the circuit box now, and everything appears to fit. And I don’t know why the 1/2″ MDF I used for the panel is green. That’s just the way it came from Home Depot.

With the back panel figured out, I was able to get the case finished pretty quickly. Pocket screw joinery is great for this kind of work – you can dry fit everything, get it all squared up, and test for fit and function before committing to glue-up. I took advantage of that – the first top I cut wasn’t exactly square, so I had to remake it. Plus I found that I needed to add some supports to keep the top from sagging under the weight of the fridge, and I was able to quickly mock up the pieces and check for interference with the drawer.

Refrigerator battery backup - drawer slides and cable management
Drawer mounted, with cable management

Mounting the drawer came next. With batteries topping out at 90 pounds, I didn’t want to risk using standard drawer slides, which all have a weight limit of 110 pounds. So I ordered Knape and Voght heavy-duty full-extension slides rated at 200 pounds. Caution: heavy-duty slides require 3/4″ clearance per slide as opposed to the 1/2″ for regular slides, so plan accordingly.

Cable Management

Refrigerator battery backup - cable managementMost of the electrics for this project will necessarily move on the drawer, but there still will be stationary connections on that recessed panel. I need to get power to and from that circuit box in a safe manner, so cable management is in order. I toyed around with a couple of ideas until I came up with the solution shown here. That blue plastic tube is flexible conduit, better known to electricians as “Smurf tube” for the obvious reason. My approach to getting power to and from the drawer is to use a length of Smurf tube to connect two junction boxes – one fixed to the cabinet wall, and one to the circuit box. The tube is bent into a U shape and follows the drawer as it’s being pulled out. I’ll feed power to the stationary box to the back panel through another Smurf tube attached to the underside of the cabinet top. With stranded 14-ga THHN wire in the tubing, there should be enough flex for the number of times the drawer will be moved, and everything will stay nice and safe – no cables getting pinched and starting a fire.

Next Steps

On to the electrical design. I’ve already done a little testing to make sure the whole thing is going to work, and I have a little video of that. But the day job beckons, so I’ll leave that till next time.

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