How to design a Motorsport Battery in 7 steps
We use batteries every day to power everything from our electric toothbrushes to our cars. But the batteries used in motorsport are a far cry from the AA’s we slot into the back of our TV remotes. So how do you go about designing a motorsport battery? We spoke to leading experts to find out.
1. Low voltage vs high voltage
There are two main types of batteries on modern racecars:
- Low voltage (LV) – a 12V system which powers low voltage electronics and safety systems such as the Battery Management System (BMS), cell monitoring units, isolation monitoring, current sensors and data logging.
- High voltage (HV) – powers the electric motors and inverters whilst storing the energy accumulated during regenerative braking. Also runs the Power Distribution Unit (PDU), the busbar network and the Manual Service Disconnect (MSD).
The motors used in electric and hybrid racing require large amounts of power, which is why a high voltage battery is needed to power them. ‘Broadly speaking if you want to deliver the same amount of power, there are two ways you can do it,’ explains Dr. Billy Wu, Senior Lecturer in Energy and Manufacturing at Imperial College London. ‘Due to Ohms law where power equals the voltage multiplied by the current [P=IV], you can achieve the same power through high voltage, but low current or high current, but low voltage. The problem is, when you push current through wire there is a finite resistance which causes the wire to heat up to the square of the current. So, the heat generated equates to the current squared multiplied by the resistance [P=I2R]. Therefore, to minimise the heat generated and improve efficiency you want to reduce the current, which is why the battery needs to be high voltage.’
2. Power density vs energy density
However, the design, size and layout of these high voltage batteries differs greatly between a hybrid and electric road or racecar. ‘We need to understand what’s important for an ERS battery in F1 and what’s important for a Formula E battery, because they are different beasts,’ highlights Douglas Campling, Chief Motorsport Engineer at Williams Advanced Engineering. ‘The biggest difference is that an ERS battery is discharged and recharged multiple times per lap compared to Formula E which has one single discharge over the entire race. Therefore, in F1 you are looking for the best power density; F1 is all about getting the power required for that boost into the smallest package, so volumetric and gravimetric power densities are key,’ continues Campling. ‘With fully electric, it’s an energy equation. You have to get all the cars to the end of the race and you’ve got a fixed number of joules of energy to do it. Therefore, Formula E is all about getting as much stored energy as possible and using it efficiently, whilst meeting the level of power required by the Championship.’
Consequently, F1 ERS cells have a higher power density (approximately 10-17kW/kg) but lower energy density (approximately 90-120Wh/kg) compared to the Formula E Gen2 battery (approximately 2.2kW/kg and 232Wh/kg respectively). These differences arise from the different chemistries used in F1 and Formula E. ‘But even within the same chemistry you can get some cells that are ‘power’ cells and others that are ‘energy’ cells,’ says Wu. ‘There is always a trade-off between power and energy. As a rough rule of thumb if you want more power you have to take a hit in terms of energy density.’
3. Cell types
The cells not only need to be of the right chemistry, but also the desired size and shape to fit within a racecar. ‘Cells can come in three different formats. Firstly, the cylindrical cell which is absolutely ubiquitous around the world. That’s what’s in your power tools or your cordless vacuum cleaner. They are named after their dimensions, so the most used cell worldwide is the 18650 which is 18mm diameter and 65mm long – so roughly the size of your thumb. That has a positive terminal at one end and a negative terminal at the other,’ explains Campling. ‘Then there is the pouch cell which can come in different dimensions but is essentially a flat set of electrodes in a polymer pouch, as opposed to rolled electrodes in cylindrical cells. The positive and negative tabs can be on one end or opposing ends. Finally, there is the prismatic cell which has a hard casing around flat electrodes that can be of custom dimensions.’
4. Series vs Parallel
Once the type of cell has been chosen, the next question is how many? This can be calculated initially by dividing the required usable energy of the whole battery pack by the usable energy of an individual cell. Next, the required system voltage, continuous power output and required charge rate must be accounted for. Then the required usable energy of the battery is influenced by the type and number of motors on the vehicle as well as the expected battery degradation.
The number of cells in series defines the battery voltage. ‘Each cell has a voltage range. Fully charged it’s usually around 4.2V and the lower limit is typically 2.5V to 2.7V, but each cell is different,’ says Campling. ‘You have a range of voltages from the powertrain, and particularly in Formula E, where you’re looking for the highest efficiency, you need to be quite specific about the required voltage range of the battery pack. If you connected all the cells in series you sum up the voltage of each cell, so you would reach the upper voltage limit of the battery pack before achieving the required usable energy. Therefore, typically you have to connect a number of cells in parallel to adjust capability without changing output voltage. In this way, you design a battery pack which meets the usable energy target as well as the output voltage target. It is important to maximise the operating voltage, because the higher the voltage, the lower the current, which results in smaller busbars and wiring and therefore significant weight savings.’
There are many different ways of connecting the cells and modules to meet both the required usable energy and output voltage targets. ‘There is a trade-off between using a small cell with a low amount of energy content vs a larger cell with high energy content,’ explains Anthony Law, Head of Motorsport Batteries at McLaren Applied Technologies. ‘For example, the cylindrical 18650 cells give you much more flexibility in terms of the space, number and how many you can connect in parallel within the battery pack. You can squeeze these types of cells into more unusual spaces, making it a smaller unit to package. Whereas the larger pouch types are a bit more limited. So there is a trade off between the additional weight associated with the individual cell type and how they are all connected together, the number of busbars and connections vs the flexibility in design.’
5. Temperature vs Performance
The next consideration is temperature. The hotter the battery, the faster it will degrade. But reduce the temperature too much and the battery loses performance. ‘F1, takes their batteries way above the spec sheet recommendations in terms of temperature, which allows them to get a lot more performance out of the battery but then it degrades quicker,’ says Campling. ‘That is absolutely fine in F1 because they can refresh the batteries multiple times a season, but that is not acceptable in Formula E because those teams need a battery to last for a full season. Therefore, the temperature limits are strictly defined based upon outputs from degradation modelling and testing.’
There are two ways in which a battery can degrade:
- Cycle ageing– the charge of a battery becomes less and less the more you use it over time.
- Calendar ageing– the lithium-ion cells lose capacity even when they are not in use which is extremely difficult to model and predict.
In F1, calendar aging is not a threat, but for the Gen2 Formula E battery which needs to last for two seasons, it is a series factor. In addition, there is also the risk of catastrophic failures such as thermal runaway.
‘You can get thermal runaway when your battery goes over approximately 80 or 90degC. This triggers chemical reactions within the battery which are exothermic and so release a large amount of heat,’ explains Wu. ‘You end up in this loop where the exothermic reactions generate heat which then fuels the thermal runaway even further. Often, the battery then catches fire because the electrolytes used in batteries are flammable. It effectively becomes a mini-flame thrower.’ The delicate balance between battery performance and temperature is what makes battery design one of the hardest tasks facing today’s engineers.
Battery degradation can be reduced in two ways. ‘You can either use less of its available capacity and/or keep the battery within an optimal temperature window,’ explains Paul McNamara, Technical Director of Williams Advanced Engineering. ‘If your battery range goes from 5% capacity all the way up to 95% of total capacity, it will degrade more than if you go from 10% to 90%. Then you have a trade-off between using more cooling which will increase weight vs packaging more cells to increase the capacity.’
There are essentially three ways to cool a battery: 1) Air cooling 2) Two phase cooling 3) Single phase coolants (liquid cooling)
In motorsport, single phase coolants such as liquid cooling are mostly used. ‘The question is whether you flow that liquid through a coldplate or whether you do flooded cooling,’ highlights Law. ‘With flooded cooling you use a dielectric fluid, because these are non-conductive so you are taking advantage of the fact that this fluid can be in direct contact with the cells. So you essentially fill the battery up with dielectric fluid. The benefit of this is that you avoid any additional thermal resistance that you can get when using a coldplate. A coldplate is normally a metallic heat sink, with a cooling fluid flowing inside it, which is attached to the bottom of the cells via a thermal adhesive. For this, you would ideally use water as it is a much more efficient cooling medium than dielectric fluid and it is less dense and therefore of lighter weight. Further weight savings come from the fact that you are not filling every nook and cranny of the battery and today’s coldplate’s can now be made with extremely thin walls, yet still be able to withstand the high pressures.’ Once the cooling fluid has flowed into the battery, extracted the heat and then exited the battery, it flows to a conventional radiator where that excess heat is rejected to ambient air, similar to a conventional cooling system for an internal combustion (IC) engine.
It is interesting to note that F1 and WEC use water for their cooling systems. ‘In a racing application you might only use dielectric fluids if instructed to do so by the regulations for safety reasons,’ highlights Campling. ‘This was the case in the first four seasons of Formula E – there was no compromise on safety and this has also been carried over to the new Gen2 batteries.’
Keeping a watchful eye on the temperature, voltage and performance of each cell in a battery pack that could contain as many as 5000-6000 cells requires an intricate network of wiring all controlled by a complex brain: the BMS. ‘We had thermocouples and voltage sensing on every single cell so we could monitor the voltage differential and therefore identify which cells were having a harder time than others depending on where they were located in the battery pack,’ explains McNamara. ‘We could therefore ensure the performance of each pack was identical for fair competition.’
In addition to thermocouples, there is also the positive and negative high current connection that goes from one cell to the next, to allow current to flow between the cells as well as a low current sensing connection. Finally, the cooling system, which can require the cell to be connected to a coldplate through a thermal adhesive, at either the top or bottom of the cell. ‘You would also often have cell interconnect busbars that carry the current from all the cells in parallel into the next group of cells or module,’ says Law, ‘The cells connect to the busbars via either a welded connection, wire bonds or bolted terminals.’
The number, type and packaging of these diagnostic connections are one of the key development areas in motorsport today, along with research into new chemistries. All in an effort to extract the maximum energy out of the battery for the lightest possible weight. Since 2007, the F1 ERS system has seen an 81% weight reduction, a 56% efficiency increase whilst achieving 12 times the power density and twice the energy density. It is this development drive to win in motorsport that will continue to change the face of battery technology as we know it.