It may be far from the most high-profile component in an electric vehicle, but the battery box performs a key safety and lightweighting role. Polyurethane can help.
Slung underneath the floorpan, it’s a hard life for the humble battery box on an electric or hybrid car. To survive, the box has to be tough enough to deal with damage from the road or to survive a side impact crash. Safety concerns meant that relatively heavy metal parts were normally used. But pultrusions using glass fibre and polyurethane matrix materials are gaining traction.
Galen Greene, marketing manager at Covestro, explained the attraction of battery boxes. ‘In the beginning, we looked at where else PU could be used, and at industry trends. E-mobility is quickly rising to the top.’
For Galen, the key question was whether polyurethane pultrusion offers strength benefits for electric vehicle battery packs. ‘We carried out a feasibility study where we looked at pultrusion in several different components,’ he said. ‘We looked at using polyurethane pultrusion in multiple locations, including the steel box which holds the batteries, the inner struts which add further structural strength and the bottom of the box. The last of these is the winning part so far.’
Irina Bolshakova, market manager automotive and transportation, Huntsman EMAI, added that pultrusion could be used for long, flat components, such as under-floor battery holders or the centre console. ‘That’s where pultrusion can be used as good technology,’she said.
Good volume
Covestro has carried out work to find the best combination of metal and composite components for the structural elements of the boxes. ‘There is the potential to use polyurethane pultrusions as the outer box,’ he said. ‘In our evaluation, there was a slight detriment in crash performance by changing all components, including side walls, inner struts and bottom, to pultrusion. However, this design showed a weight benefit.’
But he said that while there is a weight advantage, it would need to be implemented in an actual car. ‘For this initial design, our evaluations found the best crash performance came from making the bottom section out of the material,’ he said. ‘This offered a significant crash performance benefit over a benchmark battery pack made from traditional materials.’
In a crash, the car provides front and back crash protection, but the box itself protects the battery from side impacts. Here, the uni-directional strength of pultrusions comes to the fore. ‘In this design, the pultrusion plates are 1.5 cm thick, and around 30 cm wide… seven panels side-by-side would equal the length of the passenger section where the batteries sit between the wheels,’ he said. ‘This aligns the fibre across the car so that it provides strength against side impacts.’
Mechanical fixing
The panels are mounted to the vehicle using bolts, and adhesive is used to seal the panels together and keep the weather out. In the Covestro design, the adhesive could also be a structural component, but its main function is to seal the box.
If an OEM is developing an electric vehicle from a blank piece of paper, they can position battery packs and motors in the ideal spot at the design stage. However, when converting an existing gasoline or diesel model to hybrid or electric power, batteries and motors have to fit in an existing chassis that was designed to house an internal combustion engine. These designs often will not accept a large battery pack and ancillaries. Manufacturers focusing on hybrids will face an additional challenge when trying to fit in the additional drive train. Many OEMs offer most models with either single or dual motors and different battery capacities, dependent on the performance and range requirements of their customers.
Stop the rattle
The battery box is the outer covering for a series of smaller internal boxes which house individual batteries. Usually, these compartments are separated with struts.
Batteries are attached to the vehicle in a number of ways, they can be fastened to the struts, directly to the floorpan or they can be attached to the pultruded panels, explained Galen.
Esther Quintanilla, global & EMEAI mobility market segment leader at Dow Polyurethanes, said her company is interested in protecting battery components from vibrations, and the passengers from noise. This can involve innovative polyurethane foams.
‘It is important to protect the battery packs from shocks and vibration to ensure that they perform well for a long time, she said. ‘Foams used in these applications have to be temperature resistant, and have good resilience to protect the batteries.’
One of the issues with batteries is the thermal run-away effect, she added. ‘This is where batteries get very hot quickly when power is drawn from them, compared to a conventional combustion engine which gets gradually warmer over time,’ she said. ‘The temperature rises very quickly, almost in a step rather than any ramping action and this can cause a thermal shock.’
OEMs are already introducing new flammability requirements for battery insulation applications, for example the PV 3357 from Volkswagen. ‘Encapsulating the battery with thermally resistant foam helps to make the structure more stable,’ she said. ‘The foam used can be very similar to hood liners, and this battery packing application has grown tremendously over the past years. The foam is ultra-light, and can be produced similarly to a slabstock, or in a box controlling same acoustics and performance distribution very carefully all around the block.’
Cushion the blow
Densities can range down to 12-13g/l, Quintanilla said, but thin sheets of higher density foam may be needed to protect batteries in narrow gaps. Here, foams can be up to 300kg/m3 in density.
This is confirmed by Evonik, which has a range of Tergostab surfactants aimed at producers making insulation foams between 200 and 800kg/m3. Global marketing manager James Paul said that polyurethane foam can do more than insulate batteries from vibration.
‘Batteries have an optimal temperature, and if they get too hot or too cold it can notably impair performance,’ he said. ‘In regions such as Scandinavia, range can be notably lower than in warmer climates. This is made worse by the drain on batteries when occupants turn on the heat. In contrast, with a combustion engine, heat is a by-product which can be used to maintain a comfortable cabin climate. A team within our Advanced Polyurethane PL is working on solutions to insulate battery packs to help maximise the range of an EV.
Quintanilla added that it is very important that the foam has the same properties in all directions in all parts of the block. ‘Although polyether or polyester polyols can be used, our formulations are based on polyether and they are usually MDI foams,’ she said. ‘OEMs don’t care if it’s TDI or MDI, but the foams need to perform following the OEM’s critical requirements for powertrain. Dow’s focus here is through its Voratron range of materials for battery packing, she added.
Why are materials companies interested in battery covers? Apart from the light weight and safety implications, it has a lot to do with volumes. As Covestro’s Greene said: ‘In the battery concept we have developed, there are kilogrammes of polyurethane. This could become the largest PU application in the car. Looking at projections for e-mobility, the maths quickly shows it can be a very attractive new market.’
If battery boxes today, why not whole floorpans tomorrow? This could be some way off, said Bolshakova. ‘Floorplans today are shaped to help with aerodynamics and safety performance, using different thicknesses and complex designs which contribute to strength and NVH performance,’ she said.
