The body modifications
Well, here I can only speak for my car because there are no two individual conversions done identically. Everyone express themselves in the EV body work and do it differently. A modern oddly shaped car is more difficult object to work with than a truck where everything is pretty much perpendicular, shapes are square or rectangular. Well, I got tiny CRX to cut the holes in its floor and fit 96 LiIon cells in it among other hardware. By far the most important and complicated part of the body modification is integrating the battery boxes. In fact, this is the only *modification* I have done. Since this is such important part of the vehicle and if not done right can compromise its structural integrity and safety, I'll describe what I've done in greater details. By no means this is the best or the only right solution. Just works for me. Here it goes...
The battery boxes.
Fabricated by Summit Sheet Metal
Cost - $1000
I have two battery boxes. With the exception of the outer shape, they both have identical design, so for convenience we'll talk about singular box here. The discussion applies to both.
The purpose of the box is obviously to contain the batteries, but there is more to it than just have enough storage space. The box should:
- contain all the cells without allowing them to move
- resist impact in case of an accident
- not weaken or otherwise compromise body structural integrity, better if it can reinforce the body
- provide battery cooling (in my case) and ventilation
- be light, strong, compatible with body metal, rust and corrosion proof and easy to work with.
Some of these requirements contradict each other, so a reasonable compromise was required. For instance, the steel box is easy to weld and it's compatible with steel body, but it is heavy and not corrosion proof. On the other hand, aluminum box is light, has excellent anti-corrosion properties, easy to bolt and drill but not as easy to weld without special equipment, not terribly compatible with steel, and more expensive. Plastics are the lightest, but cannot reinforce the body if the large cut outs are made. I did not want to compromise safety, so the plastic boxes were ruled out. Eventually I decided that aluminum will be the best choice. While I could construct the boxes out of flat pieces and aluminum angle bolted or riveted together, I really prefer welded boxes option. Since I don't have TIG welder and expertise in aluminum welding, this also meant I will have to outsource the fabrication to some sheet metal shop.
First, since I know the pain of re-making and upgrading the car and the battery packs (I had several Lead Acid packs in it before), I knew I had to design universal boxes allowing any future upgrades as painlessly as possible. So rather than make the boxes matching exact size of my present pack, I decided to make them just as large as the chassis allow, filling unused space with dummy material, This way I won't regret if I change the type of the battery in future and it would not quite fit - I could not have made the boxes any larger anyway. Whatever boxes size end up to be, I will *then* fill it with as many cells as will fit or until desired pack voltage will be reached. As you will see later these two requirements magically coincided - total 96 cells fit in my boxes, exactly as many as the drive system requires. Lucky me.
The other design consideration was cooling. LiIon battery I use has relatively high internal resistance and thus generates plenty of heat wasting precious Wh carried on board. Air cooling is sufficient, but the layout requires streamlined air flow, good air ducts if required, and and sealing. I use semi-active forced cooling. No fans are deployed, but as I drive oncoming air enters special air scoops directing air through the air channels between individual battery cells and getting exhausted from outlets. All this happens underneath the car, so the body cut seemed necessary. Because I also wanted to keep center of gravity as low as possible, hanging boxes lower than the floor level was the only option.
The idea behind forced cooling. Turn out it works very well and more than adequate: the faster I go - the better cooling, just what the battery requires.
One consideration for inevitable future battery upgrade: I made my battery boxes larger than necessary to just fit 96 LiIon cells in them. I made the boxes as large as the body allowed cutting out all the sheet metal behind the seats to the end of the body. This way I can accommodate larger battery later and if it won't fit I'd have nothing to regret - it was not possible to make the boxes any larger anyway.
Once the strategy was determined, design thought out and good sheet metal fabrication place found, the following was done:
The floor from behind the seats to the rear end of
the car was entirely cut out.
The only tool used to do it was the angle grinder.
The "bridge" across the body where the rear suspension is supported was left alone. Everything else is gone.
The portion of the trunk floor where spare tire use to be.
Clean up and initial measurements.
The rear box will go here.
Overview of the body with maximum cut outs.
The metal bar was placed under body to have the reference how deep the box should be inserted.
Mock up of the front box. Carton pattern cut to actual hole size was used to refine dimensions of actual box.
As they say - measure seven times, cut once.
Trust me, cutting and pasting paper is far easier than aluminum. Very necessary step, and more than one iteration.
Since aluminum cannot be welded directly to steel, the boxes were bolted to the body. To do this it was necessary to create the surfaces to bolt the boxes to. So the steel skirt around the perimeter of each box hole was welded into the body. The skirt is made of 1.2 mm thick sheet steel, thicker than the body metal. Separate pieces cut first, bent to fit the body parts they were about to be welded to, and finally put in place. The skirt essentially forms vertical surfaces around perimeter of each hole (rectangular for the rear box and more odd shaped for the front one). A sheet metal bending and cutting machines were invaluable during whole process. So, the major steps are:
Fit all 4 sides making sure the steel
"box" (for the aluminum box) is strictly rectangular.
Measure, measure, measure. It pays off. Yellow arrow is pointing to the steel skirt (front side of the front box).
Tack weld first in one spot, make sure it is perpendicular to the body center line, then weld in few more points.
Then install other three sides. Make "sure" the joining angles are 90 degrees.
Three sides installed and tack welded in. At this point it is still easy to correct any mistakes.
Once that is done, continuous joint welding follows.
Welding side wall.
Where the gap between the skirt and the body metal is too wide, the metal strips were used to patch the gap.
The rear side of the front box, the metal is bent to the shape of the body. No gaps ensure quality weld
The rear box skirt.
This side is completed.
When all the sides are done, angle grinder with sanding disk used to smooth out all the sharp edges and clean the surfaces.
Finally, the skirt was primed to prevent corrosion and prepare for the painting.
As you already know from this sketch, the box has "basement" compartment forming the air intake and exhaust. Since the front front of the front box is wider than the rear, to get approximately the same intake area as exhaust, the scoop in front is hot as high - it is about 20 mm (~3/4") vs. 25mm (1") high. For the rectangular rear box both front and rear have 25mm high intake/exhaust. Since there has to be free air flow between individual cells (they have vertical air channels molded), the battery floor is made of the steel grid, which is laying on the vertical aluminum fins welded to the bottom of the box every 50 mm apart. Granted, the fins are rectangular for the rear box and trapezoidal for the front one. The grid is cut to size and isn't attached to anything. The cells, which will be held down by the box lid, hold the grid in place. On the bottom of the rear compartment there ate series of 1 mm holes to drain the water in case it gets in through the exhaust opening.
Plot of the cell arrangement in actual size is very
useful and can help avoiding costly mistakes.
Once the measurements are done, transfer the carton pattern onto aluminum.
When this is done, verify. Verify. Verify. Verify. Verify. Ve.... ....
Verify once more, and cut out individual parts. Regular jig saw works just fine.
Large straight pieces easily cut by special press.
Other press is used to bend sheet metal. Don't even think to do it by hand.
Cutting steel grid for the floor of each box.
Perfect and clean 90 degree cutouts are accomplished by using this machine.
Front box part. The front and rear walls and side 45 degree patches are gong to be welded in
Once the box dimension is determined, fabricating it is quite straight forward process. Since I opted for welding aluminum, skilled labor is essential for good quality and pleasing look. There are some fine nuances in the design, but the general idea should be clear from the photos below.
This is basically it. The boxes are the only part of the EV didn't make myself, and I'm glad I outsource fabrication to the sheet metal and welding professionals. It should be mentioned that the welding was done using TIG welder, and the guy who did it is very experienced in this kind of work. Cutting, bending and otherwise shaping of the aluminum was done by other friend of mine - a pro in such work. Having large presses and equipment is a must if you want final result look like OEM made. Mine does.
TIG welder with Argon shielding gas.
Assembling the box from separate pre-cut pieces. Tack welding in couple of points and verification of orthogonality and fit.
All look OK, few more weld points and...
...moving on to the next piece. Final weld (double seam) will be done after all the box is assembled like this.
Final continuous seam welding.
About 1.5 mm diameter special rod is used.
You can tell - the pro is welding!
Final result - very uniform beads.
Top view of the rear box ready to be installed. Fins and center partition are visible through the grid.
Side view of the rear box.
Time to put the boxes in the car. Turn
out, it was more convenient to do from underneath because we could jack it up to precise
position with a floor jack. It is nearly impossible to press on the box from top and
hammer it in without damage to the aluminum walls.
unexpected obstacle appeared - the hand brake cables formerly attached to the straps holding gas tank are too short, and adjusted to the maximum length still do not clear space for the front box. We decided to cut the plate holding ends of the cables in half and spread apart both halves as far as possible from each other, so the cables don't start out in parallel but rather extend from the point of attachment at the angle, each toward its wheel. New holes were drilled on the vertical body partition to accommodate this, and the trick worked very well - the cables cleared the space for the box and no functionality of the hand brake impacted.
Installing rear box
Installing front box
The hand brake cables are a bit too short and corners are not cleared. The box cannot get in! Whoops...
The cables start out parallel and then bent toward the sides. What if I make them spread from the very beginning?
Solution - The plate holding ends of each cable was cut in half, mid piece welded in such that the cables directed at the angle.
Final result - the gained length was enough to clear the space for the front box.
Unsuccessful attempt to install the from box from top...
Right side is being jacked up to level it.
The position of the box is determined by how far the intake scoop should stick out under body.
Closer look to the front scoop.
Exhaust of the front box - view from the rear.
Once the position of both boxes was settled, aluminum walls were clamped to the steel skirt. To avoid ugly look of the rear box hanging below rear bumper, it is placed at the angle. It is not as steep as appears on the image, the photo is taken while the rear of the car was on the stands while the front was not.
5.5 mm dia holes were drilled around perimeter and M5 stainless steel screws were used to bolt the boxes to the skirt - about 20 bolts evenly spaced around each box. That's it.
The front box clamped in place.
Side view of the front box. On the very left of the photo spread apart brake cables are visible.
Top view of the front box.
Rear box with the grid placed in it.
Rear box relative position to the body.
Both are clamped and ready to be bolted to the skirt.
Bolted in and ready for the battery.