Building the "Aquarium"

When we showed Paul's car off at the La Jolla Independent BMW owners gathering in June, 2021 I realized that no one knew what they were looking at. Electric conversions are still relatively rare and when we popped the hood to show the big aluminum "oven" that holds most of the Tesla batteries people asked questions like "is there a motor in there"?

The front battery box with clear panels, wiring, and LED lighting

Working on a clear plastic skin for the battery box "fish tank"

The "oven" in Paul's car

This got me thinking. It sure would be nice if people could actually see those batteries. Very few people know what a Tesla battery looks like and they're impressive with all their thousands of little laptop cells. I made my battery box out of steel (because it was cheap and easy and I figured it only needs to last until battery technology improves enough that I switch to some not-yet-invented higher density cells) so, while it needs to be weatherproof, the skin is not structural. I had started on an aluminum skin but after the show-and-tell I decided to switch to a clear acrylic version instead. A see-through box, showing all the guts in their high-voltage glory should be super cool.

I dub thee "The Aquarium."

The first idea was to use aluminum

CAD (Cardboard-Aided-Design) was used to plan the cuts

I don't have much experience working with acrylic so I had a lot to learn. Fortunately, I had recently cut some sheets for one of the art installations at my daughter's gallery so I knew the stuff cuts well on my table saw. But my box is far from square and requires a lot of tricky cuts so I had to figure out how to do that.

If I had gone the aluminum route, it would've look something like this

Normally, when you cut plastic with a jig saw you get a cut where the plastic fuses together again behind this blade. The secret to avoiding this is to use a small toothed metal blade, turn off oscillation on the saw, set the saw to its slowest speed, and go slowly so that you don't build up excessive heat. You can also spray the blade with water to cool it but I found that unnecessary.

The cut melts together if you go too fast

I wanted the edges of my box to align perfectly. That's hard to do with measuring and cutting. An alternative method is to do what wood workers do with laminates and veneers: use a trim router with a follower bearing bit. I wasn't sure how this would work on plastic but it turns out to be great.

I also wanted the edges to be polished. This can be done in two ways, or a combination of both. First, sand the cut marks off the edges, then polish with a buffing wheel and compound and/or melt the edges with a MAP-gas torch. This sounds crazy but it works well.

It looks nuts but it works

Top battery extension box prior to weld-up and edge polishing

Top battery extension box test fit

I needed to attach the plastic to the box so I needed fasteners. I decided on small M4 stainless steel flat-head screwed threaded into stainless steel rivet-nuts. This required drilling over 100 holes into the rails of the box, mushrooming the rivnuts into the holes (I broke one mandrel in the process), accurately marking where the rivnuts meet the acrylic, drilling and countersinking holes in the plastic, and attaching.

I made locating "dowel pins" out of sharpened m4 bolts and threaded them into the rivnuts to mark on the plastic where the holes needed to be drilled. Drilling plastic can be a challenge with normal twist drill bits -- the flutes of the bit grab and shatter the plastic. To address this, you can either grind the flutes away to a sharp point or buy bits specifically designed for plastic. I chose the latter.

Countersinking also turned out to require a special tool. A normal fluted countersink bit leaves a rough hole in the plastic. You need a zero-flute bit to get clean holes. I also needed to make sure I got the depth of each hole exactly right so that the bolts ends up flush with the surface so I used the drill press to set a reliable stop. Then I used the MAP-gas torch (quickly) on each hole to polish the cut. Be quick though. If you hold it too long it melts and enlarges the hole. Repeat 130 times.

Zero Flute countersink bits provide a clean finish.

Normal countersinks have only 2 flutes and really tear up the plastic. Compare to lower left with zero-flute bit.

Hitting the hole with the torch (right) polishes up the finish

Some parts of the box require affixing two pieces together. For this, I used Weld-On 3 acrylic plastic cement. It's strange stuff. As thin as water and just as clear, you apply it with a hypodermic-style needle to the joint and it is drawn in via capillary action. The glue actually melts the plastic, "welding" the two pieces together.

It took a long time to fit all the pieces together. I added a thin adhesive weatherstrip to the box edges and backed the acrylic with black vinyl to provide clean lines.

This is access hole in the back panel for connecting the BMS, thermistor expansion module, and high voltage traction wiring. Of course, it gets a waterproof cover.

The top battery is longer than the box is wide so it requires a special extension box to weatherproof it

The extension box successfully welded onto the side panel, with just enough room for coolant hoses

Time to drop it into the car for a test fit

Super tight fit. The iBooster literally touches the side of the box.

It fits well underneath, sitting on the frame rails, original motor mount brackets, and front subframe. The lower two batteries sit where the oil pan would normally be.