Designing the rear battery box

I’ve made quite some progress in designing the front battery box, therefore now it’s time to also work on the rear box. Earlier I made a prototype and did some designing and after further developing that design I knew it had to fit. Only keeping the original tow bar would be a challenge. Another original tow bar for the wagon that I bought did not give me more space unfortunately Well in worst case scenario I’ll have to modify or even build one.

In the back I need to install 5 modules, a stack of 3 modules and another one of 2 weighing 125 kg together.

5 modules achter

I sketched the box itself but not yet decided on how to mount the modules. Having a self supportive box will become quite heavy given the side. Therefore I decided to explore the possibility of a supporting frame.

battery frames

My idea was to have a slot in the first and last frame in which I could slide the module. I bought some 15x15x2 square tube and used my mill to add the slot.

sleuf frezen

sleuf erin

Fits nice and tight onto the module.

draagstang op module

I measured how far I could push it on and what the overall width would become and continued digitally. The challenge is whether I will be able to actually tilt the modules in. Added a small strip to level the mounting rail.

frames met 1 module

On top of that there will be a 40x15x2 aluminium tube which then is the basis for the rear module.

frame met bevestigingsbuis

I added props that lock the aluminium tube sideways and provide additional overall stiffness. After the module another strip and a top strip.


Tightening the modules is still an open question. Current idea is to use M6 and weld nuts.


The lenght I need to mount two modules and the aluminium tube is bigger than the available room to move in a bolt. I see two options:

1. Socket bolt
In the support above drill a hole that allows a socket bolt through combined with an open weld nut. Disadvantage: the hole needs to be 10 mm which weakens the tube.

10mm gat

PS. the hole on the right hand side is 6 mm and is intended to the module just above.

2. Use a threaded end
In the support above add a 6 mm hole which allows a M6 threaded and through which then can be fastened in a closed weld nut using two nuts. Advantage is: less weakening of the tube. Disadvantage: more screwed connections and nuts to handle (with the risk of dropping one).

boutgaten 6mm

Update: it will become option 2

It does requires some extra attention when securing everything but in the end the frame is stronger.

draadeind oplossing

So with all modules, strips and supporting tubes it will look like this:

gevuld met modules

After that I used the ‘sheet meta’ function in Onshape to add the outside.

sheet metal skin

Technically this are three parts which can be fabricated by welding and bending. I can generate drawings and have the panels laser cut.

drawing sheet metal part

If designed the parts in such way that I can bend them on my own press brake.

After that I assembled everything in context with the car and tow bar. Seems to exactly work out with the original tow bar.

incontext trekhaak en auto

The box (made of 1 mm steel) weighs 20 kgs which includes the frame.

gewicht achterste

I’m not sure what kind of material I want for the outer box. Stainless steel is the best option but also the most expensive and most difficult to produce.

I did start working on the frame a bit already.

lassen middenframe

To be continued!

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OldVolvo is a classic Volvo hobby blog by Lars Rengersen.

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