What about batteries and battery enclosures?

Choosing batteries

As described in an article on using second hand Tesla batteries, the only real advantage of second hand batteries is price. The disadvantages are:

• unknown history of the battery - may mean that it will fail earlier or perform poorly compared with a new battery

• the Tesla battery packs are designed to fit beneath the floor of a Tesla vehicle - and may not fit within the confined space of your converted vehicle

• the Tesla battery packs need cooling - which adds expense and complexity to your conversion

• the Tesla battery packs come with proprietary battery management systems, which need expertise to deal with in a conversion project

The alternative to used Tesla battery packs is to purchase new prismatic (large rectangular) cell batteries. The advantages of using new batteries are:

• you can choose from a wide variety of cell sizes to optimize the battery pack for the space and weight limitations of your vehicle

• the new batteries are covered by a longer warranty than the second hand Tesla batteries

• the new batteries usually have the LiFePO4 chemistry, which is environmentally less damaging that the old Tesla battery packs

• the LiFePO4 batteries do not need cooling if charged gently (Level 1 & 2 charging), and are more tolerant of being discharged than the old Tesla batteries

The disadvantages of the LiFePO4 batteries are:

• more expensive than Tesla batteries ($/kwh) - though may end up cheaper in the long run as they are less likely to fail

• the batteries swell when charged - so need specially designed battery enclosures that are able to keep them under pressure of approximately 300kgf to control the swelling otherwise the battery will degrade more rapidly. 

Battery Enclosure Design

The battery enclosure for a used Tesla battery is complex because it needs to accommodate the cooling fluid circulating through the batteries - so needs to include pumps and plumbing.

The battery enclosure for prismatic LiFePO4 cells needs to be designed with elastic spacers and adjustable clamps to keep the batteries under compression throughout the charge/discharge cycle.

The battery enclosures also need to meet the Australian design rules (known as VSB14) for electric vehicles - for example, the battery enclosure must be design to withstand the forces of 20x the weight of the batteries in the event of a head-on collision, and 15x the weight of the batteries in the event of a side collision. The design and mounting of the battery enclosures should be specified by an engineer to ensure the vehicle can be registered.