Battery Box & Battery Cooling

How to Secure, Mount and Cool 400V / 70kWh worth of batteries. 

Probably the most time consuming part of the design so far has been the battery box. Lets take a look at the enclosure, the mounting systems and cooling plates.

Enclosure

All the battery modules have to be housed in a enclosure that will protect the battery's from the elements, it doesn't have to be a completely sealed unit although that does have its advantages if you can do that, however then it the isn't usually a serviceable item. This is one personal gripe I have with most modern day cars, electronics, household items. So I would like to have this potentially be a serviceable item if it ever needed to be. 
I need to integrate the cooling plates but more on that later. My aim was to get as many batteries in the standard GD euro chassis as possible with the minimum modifications. This way Andy at GD shouldn't need to make any huge modifications when it comes to his jig. The standard front area, I'm refraining from using the word "Engine Bay", will need to be widened in a few spots to allow for 4 modules mounted length-ways next to each other. There will then be 2 layers of 4 modules down the tunnel, I was happy with this as it allows for nearly 100kg of the 350kg battery pack to be mounted in the tunnel, a very low central place for weight distribution. The other 20 modules will me mounted in the "not an engine bay - bay". The BMS actually fits in nicely in the tunnel too. 

The enclosure its self will be laser cut and folded 2mm aluminium. This has been a long but very satisfying process, lots of hours spent on CAD looking at different ways to mount the modules and keep them safe and secure. Im happy to say as of last week the order was placed so in around 4 weeks I should be receiving all the laser cut and folded material, its about to get very real! 

Modules in the chassis without the enclosure.

Modules in the chassis with the enclosure and the contactor unit.

Mounting System

With 350Kg of batteries they need to be firmly mounted, its not like an engine that might weigh 250KG but is a solid lump that can be mounted from 2 points.

I have 28 individual modules that all need to be mounted, my plan is to try and get the whole pack to be one structural unit. So I needed to find a way to be able to stack the modules without damaging them. 
My solution was to design and get manufactured small spacers that will transmit the load through the mounting screws so not to actually put load on the modules themselves. It wouldn't have been economical to have these made in one piece with the 95mm long M8 bolts so I will just weld them to the standard bolts.

Spacers made to support the cooling plates above. 

Here you can see the spacers supporting the cooling plates for the next layer while ensuring no load is placed on the top of the battery modules. 

On the base of the battery enclosure there will be a 25mm box section frame that will support the weight of the modules and then bolt into the chassis. 
This will then have a 2mm aluminium plate on the bottom of it to help with Aerodynamics under the car. 

Battery support frame

I was concerned that the top of the pack might be unstable and also as the original engine mounts also would have provided some torsional stiffness across the chassis I'd like to reinstate something to replicate that.

I have to be careful I'm not just adding additional weight everywhere I can, so I took my inspiration form a strut brace style mount and came up with the following to be made from 4mm steel. There will be bent arms that come from the chassis over and ontop of the battery box, then a plate that the top of all the modules will bolt and connects the 2 arms. This should secure the top of the pack and also tie in the 2 sides of the chassis. 

Cooling Plates

Cooling, and actually heating but more on that another time, is a hot topic when it comes to building an EV. In theory you don't need to liquid cool the batteries, however this would seriously limit the potential output of your pack. OEM manufacturers normally have very intricate plates sandwiched between the cells, this is the ideal way however in low volumes not economically viable. 
In the self built EV market there a a good number of cooling plates you can buy that have cross drilled holes throughout the plate and then water is pumped through these. Now these are an off the shelf solution so a positive there, however only normally cool a few modules at a time. I have 28 modules...

So what can I get manufactured at a reasonable cost?
I could get a plate and have it milled out to have the cooling tracks in, I would need a 2,000mm x 800mm plate to be machined, that just feels expensive before I even ask for quotes. 

I can get aluminium laser cut fairly easily, however this is only 2D shape so would need to come up with an interesting design.
That's where my 3 layer plan came into action. 



A solid bottom layer of 2mm aluminium, a mid plate that has all the cooling tracks in (4mm), and another solid top plate (2mm). This will then allow me to cool all the cells on each later by pumping the coolant round, I can connect all the layers in series or parallel. As each module will have thermistors on the cells I'll be able to monitor the temperature of each cell and check the cooling is working correctly, I think I'll start with them hooked up in series and if this works then leave it. My concern with having the layers in series will be the difference in the temperature between the first and last module in the system, but I can spend hours calculating it or just go for it and monitor what happens. 

Underneath each battery module there will also be a polymer thermal pad, this will help with the fact I am not directly on the cells and rely on convection of the heat to get the maximum cooling effect. 

A quick mock up to convince myself of this 3 layer idea. 

Cooling tracks in the mid plate

BusBars

The modules are all fairly well shoehorned into the pack so there is not much space between them. There is not enough space to have cables connecting all the modules. Also every crimped joint is a potential failure. So I need to look at busbars, I know I've overdone it on my bus bars but I have gone for a 3x28mm minimum cross section giving an area of 84mm², the main battery cables are 70mm². So my bus bars are well over the requirements, even more so as that 70mm² for the battery cables does not account for all the air between the strands, the bus bars are of course a solid 84mm² copper. 

There are 28 individual designs of bus bars for the battery pack and also the contactor unit, I have ordered these to be laser cut, I will then bend them myself and get the non contact areas coated in an electrical insulator for safety. 

The busbars are an extremely efficient way of linking together all the modules in the pack as well as the connections to all the contactors. Having solid connections like this should hopefully give the reliability and feel of an OEM car that I am after. 

All the busbars needed the 3D model, a 2D flat DXF for the laser cutting and then a bending drawing listing the bend angle and direction to be manufactured. It was a long process to get it all sorted but done now.

One of the 28 Busbar designs needed for the battery pack

Battery Pack in the Euro Chassis

Removable Battery pack

Modules within the battery pack