Mazda MX5 Conversion Blog


18 January 2008: Rear battery box prototyping

Today I finally bit the bullet and for the first time made entirely irreversible changes to the car with the angle grinder!

With 24 cells going in the front battery box, I had to fit 16 cells in the back. Due to the small space for the old fuel tank cavity being too small to fit even half that many, the only other feasible place to put them in the boot - or sunken into the boot, to be precise. Fortunately there's quite a bit of space under the old boot (where the muffler etc used to be), and 8 cells can fit neatly between the heavy chassis rails so I only had to cut the thin sheetmetal

I fabricated the prototype battery box from MDF as usual. It's designed with a flange around the top which will bolt onto the chassis rails which it sits upon on either side. Knowing the size of this, I marked out the square to cut from the boot floor and got busy with the angle grinder for a while.

As you can see in the final pics below, it looks like it's going to fit very neatly and I can have a flush plexiglass cover over the cells for show. The only downside is having a significant amount of weight behind the rear wheels (i.e a long way from the vehicle's centre of gravity) worsens the polar moment of interia, which may be slightly detrimental to high speed cornering, but certainly won't be noticeable for daily driving.

A final word, I've given myself a deadline to get the car finished by: 6th April 2008. The Perth Sun Fair is on this date, and our WAEVA club is planning to have an exhibition of some of our electric cars, so I'm adamant to have mine ready by then! Got a busy couple of months ahead of me..


(1) The battery box prototype, ultimately to be made from 6mm aluminium

(2) The boot with cutting lines marked

(3) A view from beneath the boot floor

(4) An hour later, a gaping hole where the boot used to be..

(5) Battery box dropped into the hole. Bolts will secure it to the chassis rails along the sides.

(6) Box with dummy batteries installed (and one real one, for interest)

(7) A view from below. No part of the box extends below the rear bumper or toe hooks.

(8) Another view from below (looking back).

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