Please note that this chapter only covers the mechanical make of the battery boxes. Their circuitry is described in the chapterdistribution_boxes.
TUEV demanded that the traction batteries be safely enclosed in robust boxes (vs. the alternative approach of strapping them together with belts). One might question whether it is really the best choice to have such rigid structures in the car, that unavoidably affect the car's crash performance.
I have tried to dimension and arrange the battery boxes in a way that sufficient “crumple” space is left before and after them. See also my safety considerations in the chapter "auxiliary frames" - "battery box frame".
The boxes were made by Marco Lorey in accordance to my drawings. Here he is doing some last welds before handing them over to me.
The material of the front and rear box is steel - only the large cover of the rear box is aluminium. Steel at one hand makes life easier since it can be welded easily. On the other hand however, there is really a significant weight difference between steel and aluminimum constructions, and á posteriori I think that one should invest any required effort to save weight.
To start with, my drawing of the front traction battery box.
Finding the correct position within the engine compartment was not easy. Here (the view is upwards from underneath the car's nose), a welding magnet holds a wrench extension against the hood to verify the available space when the hood is closed. On the second photo, the traction battery box is placed on top of the motor, with pieces of wood to ensure sufficient distance. Only with the traction battery box in its correct position, I could start making the battery box auxiliary frame.
Front traction battery box resting on the auxiliary frame with the mounting brackets provisionally tacked on, front and back side.
After determining the mounting position and preparing the auxiliary frame, I welded on the mounting brackets and added enforcement rails. Behind the holes near the top edge, I welded nut bolts for easy mounting of the top lid. Right picture: detail photo of the welds on a mounting bracket.
The front traction battery box after getting sand-blasted, coated with primer and black paint. The edges were sealed with silicone (which turned out not to stick so well on the paint …). The cells will leave some space to the two smaller sidewalls. The wooden blocks are intended to fill out this gap. The brackets above will hold M8 nuts (welded to rectangular sheet metal bits and glued in with silicone) to fix the top frame.
The cells are surrounded by foam insulation, as a thermal insulation and to keep them aligned. Note that the space is actually not sufficient for effective thermal insulation.
Front traction battery box finished. The cells are pressed down by an aluminium top frame, to avoid that the cells can move against each other (which would particularly put stress on their terminals) and as a safety measure to prevent that they fall out in inappreciable situations. The photo also shows the cell modules of the BMS and the EVDisplay “sender board”, which will be discussed in the chapter “distribution boxes”.
TUEV demanded a metal cover for the battery box: The benefit over a plastic cover is sure debatable - the metal cover cannot be opened easily in case of a fire, it is prone to make the battery box explode and it may also cause short-circuits when it touches the cell terminals. Anyhow, in some cases it is best not to contradict…
The cover is made of an Aluminium frame and sheet metal. Inside, it got some thermal insulation and a sheet of foam plastic:
Top view of the front battery box cover:
Let's again start with a drawing:
To make place for the rear traction battery box, the spare wheel well had to leave …
Towards the opening, the scoops in the sheet metal were cut open, hammered flat and welded up again, so as to create a more-or-less even surface.
Butt straps to connect to the remainders of the spare wheel well at the rear side of the box, and enforcement rails at the bottom.
The brims of the traction battery box were cut out where the sheet metal remained elevated. The sheet metal sticking out on top is a “firewall” between the battery compartment and the cage of the traction battery charger. On this photo, the traction battery box is already sand-blasted and coated with primer.
Bottom view of the rear end: The battery box is bolted to the remainder of the spare tyre well (via welded-on butt straps as shown above).
Mouting frame on top of the rear traction battery box. It enforces the construction and its vertical surfaces act as guiding rails and mounts for the cover. The mounting frame is bolted to the car's frame via M8 bolts. Since the lower surface is not even, I had to improvise stepped washers so that the heads of the bolts rest on a horizontal surface.
Note that on below picture, the mounting frame still extends nearly to the backseat. I have later shortened it to only align with the battery box, mainly because of crash safety concerns.
The M8 bolts are inserted from below and go through weld-on nuts on the mounting frame. Their open ends, visible in the right half of below picture, will act as supports for a metal grid that holds down the lithium battery cells. Note that on the picture, there are yet provisional lead-acid batteries installed!
The cover of the rear traction battery box is made of aluminium. Here, it is being ground - for cosmetic reasons.
The front plate holding the isolation test button, the pre-charge circuit and the maintenance switch is being bolted to the mounting frame. It stays on the mounting frame permanently, while the rest of the cover is easily detachable.
Mounting frame with front plate and battery box cover. Behind, the cage that goes over the traction battery charger and the cover of the rear distribution box. The rails on top of the cover are for enforcement and, together with the bolts sticking out to the sides and the mounting brackets, for holding the active subwoofer.
The inner side of the battery box cover again got thermal isolation and protective foam plastic:
The rear traction battery box again has an aluminium top frame, to press down the cells and to secure them against falling out. There is only little space left if you want to keep a safe distance to the cells' terminals, so making the top frame turned into quite a tedious job. Here, the rear part of it:
Rear traction battery box finished, with cells inserted, aluminimum top frame and BMS cell modules in place.
Since i was concerned that the cells might not be held tight enough by the top frame, I later fitted some pieces of wood between cells and frame.