LI-ION BATTERIES: SIZE DOES MATTER

The idea of having a seriously flammable substance on board with only a few thin sheets of metal between it and the occupants would be enough to kill the idea stone dead, if we were to launch petrol- or diesel-powered vehicles today. Fuel tanks have been located on top of – as well as beside engines, and even directly above the front occupant’s legs. Cool….

A whole raft of knowledge ensured that fuel tanks were progressively re-located into safe zones such as just ahead of or between the rear wheels, and such a thing as fuel system integrity – no leaks in the event of a crash – were core objectives as much as occupant safety. There were many serious incidents along the way, but we need to remember an empty or near empty fuel tank is far more dangerous than a full tank.

Enter the lithium-ion (Li-Ion) battery

Not only has the battery a store of electric energy – it also has combustible materials. As long as the vehicle does not develop a serious electrical issue or the battery is severely damaged, as many battery/electric vehicle (BEV) and plug-in hybrid electric vehicle (PHEV) owners know, the only worrying thing to happen is a slight reduction in charge capacity as the battery gets older.

However, what happens if the vehicle is impacted to the extent the battery is damaged?

In 2011 NHTSA performed a side pole impact on a first-generation Chevrolet Volt, and the test result was good. The wreck was carefully stored – just in case further analysis was required. Some three and a half weeks later, the battery ignited – and did so again, and again with more encores than an Australian television series. It was a forewarning.

GM admitted that in their private testing the battery pack was ‘discharged’ yet in the public test it had been subject to impact whilst charged. When the test agency asked GM about the equipment, and would it be available to support the newly delivered Volt, the answer was… no. Subsequently, the special discharge tool did see the light of day, and very few Chevrolet Volts suffered from a fire caused by the battery.

More to the point, in the real world, a Li-Ion battery will not have the electrical energy discharged during impact or even some time after impact. Safe isolation – the default in the event of severe impact – is not the same as zero stored energy. Oh – and this applies to everything from e-scooters through hybrids to full electric vehicles.

The event

What happens if the battery damage is enough to initiate thermal runaway? The story starts at point of impact, where the cooling system – especially if it uses a combination of coolant and refrigerant typically used to manage heat rejection of larger battery packs – could be damaged. In effect the battery may not have the ability to regulate internal heat rejection. If the battery is internally damaged, if internal shorting takes place it will draw on the remaining stored energy, which then may result in chemical reactions taking place. From then on, a major thermal event will occur, each cell in thermal runaway initiating the failure of adjacent cells:

Thermal run away generates…

Gases and heat which …

Vent – the cloud will contain toxic heavy metal particles, and…

A white vapour cloud (may explode or burn) leading to…

A fire burning at 1 000°C or more.

If a battery does catch fire, it will burn for days. Water is not required to put it out, but to cool the battery core, so reducing the chemical reaction process. Given the internal chemicals are chosen for their performance, it may come as no surprise they are hazardous. The fumes that come off the burning Li-Ion batteries are highly toxic, and in enclosed spaces, dangerous.

So how does everyone remain safe from point of impact to completion of repair? Everyone who handles the vehicle with an Li-Ion battery needs to recognise the potential risks and prepare for the worst. It is notoriously difficult to find out what is going on inside the battery pack since the control modules which could report the status including internal hot spots or shorts, may also be damaged. So, a simpler approach would be:

1. Ensure that all staff who come into contact with the vehicle have a level of high voltage system training, and those who don’t are kept away. That’s the first protection.

2. Place the vehicle in a dedicated zone whilst in storage. That’s the second protection.

3. When the vehicle is undergoing repair – and if the battery pack is removed – place the pack into a dedicated storage zone. That’s the third protection.

Magic numbers

Have you noticed what’s missing? The amount of space recommended by the vehicle manufacturers.

We do know if a thermal event does occur, it can happen at the scene of the accident through to days or even weeks afterwards. If the thermal event does take place and the damaged vehicle is parked at a conventional distance from other vehicles, the fire will burn out surrounding vehicles and nearby buildings.

Vehicle manufacturers do not give consistent guidance on this point, but it’s still worth checking out. As a rough guide the magic distance range from 5 m to as much as 10 m from surrounding vehicles and buildings. There are not too many businesses that can devote that much space to storage of a single vehicle – let alone a small fleet. This is a rapidly evolving situation, where present ‘solutions’ are not solutions at all. Meanwhile, sales of vehicles equipped with larger Li-Ion batteries continue to grow steadily, especially in Europe and North America.

How will this mess be solved? Well, the good news is the battery technology is also evolving quickly, and the hope is to find less toxic chemicals to use in such batteries. Make no mistake, the ‘old’ chemistries are in use now and have been for years, so the issue outlined above will be with us for years to come.

Keep gathering information

The first defence is to seek information – what vehicles are you likely to want to repair, and what is the status of the training in the team? Not everyone needs to be trained but everyone who is likely to come into contact with such a vehicle needs to know enough to get the right people working on it – in effect, a minimum level for nearly all colleagues, and a few specialists. Investment in this must be in the context of return, so no need to do this if the business strategy is to specialise in repairing vehicles that don’t have high voltage systems.

The next defence is to plan. Assuming the business is already or wants to repair vehicles with high voltage systems, think about how the workshop can be protected in the event of fire, and extend that thinking to the external storage area. Then, think again about return on investment – can the business make a living with less ‘repair stock’ on hand, and are there ways to clean up the internal workflow, and so improve rate of throughput?

As always, think about the next steps before shelling out any cash.

By Andrew Marsh