Custom battery holder

For many years I faced the same problem as many other Lotus owners: Keeping the battery alive during winter. To be honest, I have not been very successful in this - a trickle charger was no option due to lack of a power outlet in my garage... Also the battery kill switch didn't do the trick completely: the self-discharge of a conventional battery was just too much. In addition, I don't drive enough... For everyday life and going to work I'm in the comfortable position to be able to do most of the trips either on foot, by bike or public transport - so on one hand the Elise is not abused for shopping runs, but on the other hand she is condemned to sit in the garage for prolonged periods....
A different soultion had to be found. Somewhere I once read about somebody who installed a light LiFePo battery on the wheelarch by using a 3d printed support structure. Well, that looks like a promising approach!

Mechanical Design

Instead of a printed supporting structure, I opted for a different design. The battery itself should be mounted in an aluminium cage. Underneath, 2 bent sheetmetal brackets support the weight of the battery and cage and form the interface to the curved wheelarch surface. In addition, a small panel for a battery kill switch should be featured. The CAD model is shown in the picture below.

CAD model of the battery holder assembly
Fig. 1, CAD model of the battery holder assembly

To check if the design is stable enough (especially the narrow bend of the 2 support brackets), I did a FEA in Fusion360 using the included NASTRAN solver:
screenshot of the FEA results
Fig. 2, screenshot of the FEA results

The maximum stress is around 8MPa with a load of 4kg (twice the weight of the actual battery!). So there is enough margin and the design can be used safely. I prepared the necessary drawings and models and sent them to a company for manufacturing. For finishing I chose a golden anodisation.

Finalisation

After having received the finished parts, I assembled the pieces. The individual components are joined by bonding (using an aerospace standard process and adhesive). The battery is installed and fixed with a strap. The actual fixation of the cage on the wheelarch is done with SCOTCH EXTREME mounting tapes (each strip holds 5kgs), this makes the complete assembly removable but still securely attached in the car.

assembled battery holder, front bottom view
Fig. 3, Pictures of the assembled battery before installation in the car
Of course no Lotus modification is complete without the compulsory weight check ;-):
old battery weight new battery weight
Fig. 4, Comparison between old and new battery weight

I will leave this figure uncommented.... the weight reduction should be obvious.

Results, experiences

This setup is now in use since more than a year - without any problems. The mounting tape is more than strong enough and nothing has vibrated itself loose. As a positive side effect, the available space in the boot is bigger (which can easily lead to compensate the achieved weight saving....). The battery itself is very strong and I never had problems starting the car. I don't notice any self discharge and the capacity, albeit smaller than the original, is large enough to sustain for one week (I didn't try more than that).
Finally I ordered some new carpet on the internet which I used to cover the complete boot (including the gap left by the original battery).