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How do you design a lightweight battery box for an electric vehicle?

Multi-Material structures and Distributed Electronics allow engineers to rethink how Electric Vehicles are designed

7th July 2020

Electric vehicle battery box concept

With the increasing proliferation of zero-emission vehicles being driven by legislation, new technologies are required to meet this demand. The design of lightweight battery boxes is critical for increasing the range and efficiency of Electric Vehicles. Multi-Material structures allow a designer to place the Right Material in the Right Place. This article describes the work completed during the Bedpow project. Bedpow was an Innovate UK funded collaborative project between Ultrawise Innovation, Tribus-D, University of Warwick, Tata Technologies, Transforming Stress, and Far-UK.

why go electric?

Electrification is the current major factor driving technological development in the automotive sector. International treaties, such as the Europeans Union’s 2021 C02 emissions targets, have set out the levying of fines towards automotive companies proportional to the amount of fleet C02 emissions exceeding a target threshold - €95 for each g/km of target exceedance for each car registered. Current projections estimate that Europe’s top 13 car makers are set to miss these targets, resulting in a total of €14.5bn fines being levied. The magnitude of these fines has economically incentivised the automotive sector to become open to change and adopt disruptive technologies, such as Electric Vehicles (EVs).

Table of Automotive OEM fleet Co2 emissions vs target

The Electric Vehicle market is set to expand dramatically in the coming years. In 2018, only 7% of the total light-vehicle sales were EV/Hybrid-Electric Vehicles (HEV). By 2024, it is estimated that between 25-30% of the total light-vehicle market will be EV/HEV. Because of this, there is a great opportunity for companies such as Far-UK to develop technologies which could be adopted by this exploding market.

In current Electric Vehicles the batteries are controlled by separately package control power electronics, where each package requires its own cooling solution and housings. Many of the functions that an EV performs requires Power Electronics (e.g. motor inverters, battery management systems, and battery chargers). Power Electronics produce heat during operation so need to be actively cooled to stop them from overheating. Typically, modern EVs contain separate boxes for the Power electronic systems which are connected throughout the vehicle to the system they control using copper wiring. It is estimated that a typical 200kW EV contains 11kg of copper wiring, along with 26kg of Power electronics in bulky metal enclosures containing aluminium heat sinks.

location of battery pack and packaged power electronics in an electric vehicle


"Distributing the Power Electronics near to the systems which they control, not only reduces the length of cables required but also simplifies vehicle architecture, facilitating easier maintenance."

When designing the packaging for Power Electronics several important factors must be considered; it must provide secure insulation, conduct current, and be electro-magnetically and thermo-mechanically reliable. Incorporation of power electronics within the structure of the electric vehicle (for example, the battery management system incorporated into the structure of the battery pack housing) would save weight, and release space for future EV design and innovation. Distributing the Power Electronics near to the systems which they control, not only reduces the length of cables required but also simplifies vehicle architecture, facilitating easier maintenance. Additional weight and cost savings can be made by combining the cooling of the electronics and the battery using the same heat exchange system.

A new battery box concept would have the following design goals:

  1. Lightweight – Optimised use of materials.

  2. Distributed electronics - Merge the power electronic and battery cooling system.

  3. Modular design – Individual circuits or battery cells can be replaced.

RIght material in the right place

Current EV’s usually have separate heat exchange systems for the battery pack and the power electronics. Lithium-Ion (Li-Ion) batteries are manufactured to work between certain temperatures, so thermal management is important for extending their work life. Cooling systems need to be able to keep the battery pack in the temperature range of about 20-40 degrees Celsius. When Li-Ion batteries are too cold, the internal resistance of the cells increases. This can cause damage during fast charging. Most Li-Ion batteries cannot be fast-charged below 5°C, and no charge below 0°C. High temperatures in Li-Ion batteries can increase thermal aging and may shorten the lifetime. In extreme cases, elevated temperature can cause thermal runaway of the battery.

Because of this, all Electric Vehicles have mechanisms to cool their battery trays. Most battery packs in EV/HEV’s are cooled using an active cooling system. The system can adapt to extreme external temperatures, maintaining the battery temperatures between an optimal temperature window. An exception to this is the Toyota Prius’, in which the batteries are cooled with ambient air passing through the battery pack, therefore this system only works well in mild climates. 

"Weight and cost savings can be made by combining the cooling of the electronics and battery using the same system."

In an active cooling system, fluid is circulated through a material which is in contact with the battery. As the battery gets hot, the heat is transferred into the circulating fluid. The fluid is then cooled by a heat exchanger, for example, the existing radiator in a vehicle (the opposite happens if the batteries need to be heated). A separate system exists for the cooling of an EV’s power electronics which works on the same principles. Weight and cost savings can be made by combining the cooling of the electronics and battery using the same system.

The supporting structure should not stop the heat from flowing from the batteries/power electronics, causing them to overheat and malfunction. To produce a structure which would maximise the heat flow from the electronics and battery into the cooling system, a test method and test rig were designed to measure the thermal conductivity of different composite structures. Thermal conductivity is a measure of how well a material conducts heat, so the greater this property, the greater the heat flow through it.

A bespoke machined copper block was manufactured, which could house a 40W cartridge heater. The cartridge heater increases in temperature when a current is applied through it, which in turn heats the copper block. The copper block simulates the battery or the power electronics during operation. On top of the copper block (25x25mm contact area) a test sample can be placed, and a water-cooling block is placed on top of the sample – this simulates a cooling loop which would be found in the majority of EV’s. The greater the thermal conductivity of the sample, the more heat can flow through it, and hence, the lower the maximum temperature the copper block will reach. 

layout of test set up used in heat flow experiment

The temperature in and the temperature out of the water flowing through the water block is measured. This is used to calculate the heat flow into the water, and hence the heat flow through the sample. The greater the heat flow into the water, the greater the thermal conductivity of the sample. A sample with a poor thermal conductivity will have a greater ‘thermal resistance’, reducing the heat flow from the block to the water, resulting in a higher block (or electronic/battery) temperature. 

This method was applied to multiple composite samples to determine the optimal composition of the structure, ideally a sample with the greatest heat flow to weight ratio. The results were plotted against each other to compare structures. The better the structure, the greater the Average heat transferred bar (Blue), and the smaller the Max block temperature bar (yellow).

A purely Aluminium 5754 sample performs drastically better than a Carbon Fibre reinforced plastic (CPRF) sample. This is due to the difference between the two materials thermal conductivity, 200W/mK for Aluminium vs 0.5W/mK for CFRP. However, Aluminium is much heavier than CFRP, 2.7g/cm3 vs ~1.8g/cm3. This equates to almost 9g difference between the two 75x65x2mm samples, or a 42% weight saving.

bar chart of average heat transferred and block max temperature for Aluminium 5754 and CFRP

An ideal structure consists of Aluminium for its high thermal conductivity, and CFRP for its lightweight and its high strength. We can produce a CFRP structure and embed Aluminium at areas where high thermal conductivity is required i.e. areas in contact with the electronics/batteries. This ensures we have the right materials in the right place.

CFRP samples were produced with aluminium inserted through the thickness. The area of aluminium is equal to the top of the copper heater block. In other words, 100% of the top of the heat block was in contact with aluminium, and the rest is CFRP.

The chart shows a nominal decrease in performance between a purely aluminium sample (left) and a CFRP with an aluminium insert sample (right). The weight saving between these two samples is 7.65g, or a 30% weight saving. Therefore, any aluminium that is not in contact with the heat source is unnecessary and causes additional weight.

bar chart of average heat transferred and block max temperature for Aluminium 5754 and CFRP with aluminium insert

By varying the amount of Aluminium used we can determine an optimal ratio between heat transfer and the percentage of aluminium in the structure.

Graph plotting the percentage of aluminium in contract with the heat source area vs heat tranfers and max block temperature
  • CFRP Sample with 25x25mm square of embedded aluminium at the centre

     

     

     

     

     

         100%

  • CFRP Sample with 17.5x17.5mm square of embedded aluminium at the centre

     

     

     

     

     

    50%

  • CFRP Sample with 12.5x12.5mm square of embedded aluminium at the centre

     

     

     

     

     

    25%

  • CFRP Sample with 8.5x8.5mm square of embedded aluminium at the centre

     

     

     

     

     

    12.5%

It can be seen that the heat transfer (blue) increases linearly, and the Max block temperature (yellow) decreases with the percentage of aluminium in contact with the heat source. The percentage of aluminium that could be chosen would depend on the cooling requirements of the electronics.

The issue with this approach is that the Aluminium is located in the centre of the heat block, the perimeter is in contact with CFRP. This is not an efficient use of the contact area. The same amount of Aluminium can be ‘spread’ out across the heat contact area by using aluminium rods. 12x 4mm diameter rods is equivalent to the same amount of aluminium as there is a square which is 25% of the heat contact area – i.e there is no increase in weight

Graph plotting the percentage of aluminium in contract with the heat source area vs heat tranfers and max block temperature with aluminium rods added
  • CFRP Sample with 12 4mm aluminium rods embedded at the centre
  • CFRP Sample with 12 4mm aluminium rods embedded at the centre with a 25x25mm square displayed

     

     

     

         25x25mm

  • CFRP Sample with 12.5x12.5mm aluminium embedded at the centre

     

     

     

     

    Equivalent Aluminium

The graph shows that when ‘spreading’ the same amount of aluminium over a larger area in contact with the heat block, greater thermal performance can be achieved (the bold data points are the aluminium rod inserts). This shows that not only optimising the amount of material is necessary, but also the location of that material is critical.  

This experiment shows that combining multiple materials with different thermal conductivities can increase the heat flow through a structure, maintaining the operational temperature of the electronics. High thermally conductive materials only need to be in contact with the heat source, which allows freedom in material choice in the surrounding structure.

This methodology can be applied to the distributed electronic battery case, high thermally conductive materials can be surrounded by composites to produce structures which can maintain electronics and batteries at a safe temperature whilst remaining lightweight and strong.

How to make power electronics smaller?

Miniaturisation of the power electronics is necessary to integrate them into the battery case structure. Far-UK was part of a collaborative Innovate UK project named Bedpow, which aimed to miniaturise these power electronic systems, packaging them in a high thermally conductive compound, providing lightweight heat sinking and EMI shielding. The developed Power Electronic circuit is 35-40% smaller and lighter than a typical existing counterpart, but also more efficient. 

The miniaturisation of the Power electronics was due to a novel method to embed electronics, called the ‘lift-off’ approach, developed by Tribus-D. This method embeds power and control electronics at a package level rather than at a circuit board level. A sintered metal layer is deposited on a rigid metallic plate, which with selective processing forms the conducting tracks of the Printed Circuit Board (PCB). Electronic components are connected to these tracks and encapsulated before the circuit is ‘lifted off’ the rigid metallic plate, leaving the tracks exposed underneath for connection to external inputs and outputs.  

step by step process of the silver sinter lift-off approach

The advantage of this, apart from the inherent miniaturisation, is that the thermal dissipation can be improved close to the heat-generating devices (e.g. power MOSFET and resistors) by attaching a thermally conductive backplane (e.g. copper) and using a ceramic filled encapsulant. The thermally conductive backplane can also act as a local EMI shield (Electromagnetic Interference), reducing problems with noise. The ceramic filled encapsulant can increase the thermal conductivity by 2-3X compared to an unfilled polymer encapsulant. When miniaturising electronics, thermal management is the number one priority. As a rule of thumb, the smaller you make an electronic component, the faster its temperature increases. Therefore, by increasing the thermal conductivity of the surrounding material, the heat can flow more easily from the electronic component, reducing the risk that it will break due to overheating [Patent Pending]

  • miniaturised power electronic with connector and pcb added side view
  • miniaturised power electronic with connector and pcb added
  • size comparison between miniaturised embedded circuit, conventionally sized power electronics, and miniaturised power electronic with connector and pcb
  • Embedded circuit made using lift-off method

    A new battery box concept

    Each of the modules in the pictures contains one cell and one miniaturised balancing circuit, manufactured using the lift-off methodology. Both the cell and the circuit are removable, so can be replaced. The outer shell is Carbon Fibre infused with a thermoplastic resin to improve component recyclability reducing weight without losing on structural strength. Aluminium rods are embedded through the structure to the channel, in which a water-cooling pipe sits. The heat produced by the circuit travels through the embedded aluminium rods into the fluid flowing through the cooling pipe. The pipe is also in contact with the battery cell, combining the cooling systems for the battery and circuits into one. Connections and wiring between the electronics and the battery are integrated into the structure, reducing package space. The battery and electronics are secured by additively manufactured housings.

    This singular module can by multiplied as required.

    single module of battery concept
    • new battery pack concept manufactured
    • new battery pack concept manufactured end view
    • new battery pack concept manufactured base view
    • new battery pack concept manufactured top view
    • new battery pack concept manufactured power electronic view
    • new battery pack concept manufactured electronic packaging view
    • new battery pack concept manufactured side view

      The new battery pack concept incorporates the power electronics into the structure, distributing them near the systems they control. This saves weight, and release space for future EV design and innovation. Distributing the Power Electronics near to the systems which they control, not only reduces the length of cables required but also simplifies vehicle architecture, facilitating easier maintenance. Additional weight and cost savings have been made by combining the cooling of the electronics and the battery using the same heat exchange system. This is facilitated by a Multi-Material structure, which locates the Right Material in the Right Place. By rethinking existing practices and adopting new technologies, car manufacturers will be able to adapt to legislation to develop the next generation of zero-emission electric vehicles.

      Joseph Dobson - Development Engineer at Far-UK

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