Originally Posted by sabahtom
I'm such a novice at this that the best thing seems to be the most simple option, so if that is fibreglass with a steel frame and no ventilation I'll do that (I need to check the cost of it here). Then I can get it on the road and see how things work and save up for a round of upgrades.
I am not an expert in composite structures so I am feeling my way through this. And now that I am almost done with the rear battery box there are things I would do differently. One mistake I made was I removed too much material from the car so I have a hole in the rear deck of the car that is several inches larger than the outside dimensions of the battery box that must be bridged with something. It would have been better if this was the steel of the car.
For the battery box itself I used the high density blue foam insulation material that can be found commonly in the construction industry. This is 1/2" thick material with a moisture barrier bonded on both sides. I removed the moisture barrier and then cut the pieces to size with a hobby knife or utility knife and a straightedge. I was concerned that this material would not be strong enough to support the batteries but even bare it supports the weight of a single layer of my 100 AH cells without deforming. Since the box was to be supported from a lip around the top I covered the floor area with wax paper and with the box upside down glassed the outside of the box first with enough excess cloth so that there is a 4 inch lip on the floor. I used alternating layers of 7.3 ounce (per sq yard) unidirectional cloth, two in each direction. Then another layer for each direction of vertical side of 8.8 oz bidirectional cloth. So the outside verticals all have three layers of cloth and the outside bottom has six layers of cloth. I then reinforced the outside vertical corners with a bias glass tape. Then I turned it over and did the same thing on the inside, extending the glass over the lip that was formed when glassing the outside of the box. This ends up with approximately 1/8" thickness of glass on the lip of the box. What I would do differently next time is use only the bidirectional cloth since it drapes better over corners. I would probably go to four layers of cloth on each side to increase the stiffness that would be lost by using the bidirectional cloth instead of the uni.
I am guessing it weighs less than an equivalent strength aluminum box and it is insulated. I can turn it upside down and stand on it. I have used it as a stool (although not very comfortable.)
If this sounds like a lot of work, it is. It is messy, a little smelly and itchy. It is not cheap either. Probably similar in cost to having someone cut, bend and weld aluminum. And it certainly took longer. There were a couple of points where I thought about throwing it all away and paying someone to do the sheet metal work. Would I do it this way again? I thought I knew what I was getting into when I started. I had done some smaller work with the materials before. In my situation I would do it this way again. A lot of people would choose some other way.