1.1 - Cell Chemistry
1.1 - Cell Chemistry
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Written 14/08/22 by Tom Johnson
There are a variety of battery technologies that can be used in a Formula Student car. Important factors to consider are that of specific energy density, voltage drop, cooling and packaging. Cell chemistry has the biggest influence on specific energy density, which is what we will be looking at today.
One of the most common cell chemistries is lead acid, which is used in most car batteries. They work by suspending lead plates in a mixture of sulphuric acid, and a chemical reaction takes place, allowing energy to be stored. Lead acid batteries have the ability to transmit lots of current, but are the least energy dense chemistry I am looking at today. To reach our desired capacity (~6.5 kWh) would require a whopping 153kg of lead acid batteries, and a considerable amount of space. Additionally, most lead acid batteries are designed to output at 12V, meaning that, even with the 13 batteries in series for our 6.5 kWh, you would still only achieve a nominal voltage of around 144V.
Another cell chemistry to look at is nickel metal-hydride (Ni-MH). This has actually been used by several Formula Student teams in the past. These batteries have a much higher specific energy density than lead acid, but suffer with poor charging and discharging characteristics. Ni-MH batteries have a high self-discharge rate, which can often result in unnecessary wear to the battery, decreasing its longevity. Ni-MH batteries also typically have a much lower nominal voltage than lithium-ion batteries, which is not ideal for our application.
The vast majority of Formula Student EVs now use lithium-ion batteries, thanks to their excellent specific energy density and high nominal voltage. Their technology is also commonly used in the automotive industry, as it it suited to multiple charging cycles and has optimal discharging characteristics.
There are many types of lithium-ion technology. In the past, UH Racing has used Lithium Nickel Manganese Oxide (LiNiMnCoO2) in the Murata US18650VTC6 cells used in the Energus Li1x5pVTC6T modules. This is represented by the green ellipse in the following graph.
While there are other lithium technologies with a higher specific energy, they are compromised by either being unavailable for public use or having too low of a nominal voltage, making them unsuitable for our application. For example, one chemistry which appears to be a significant upgrade is that of Li2MPO4F, which is represented by the light blue ellipse on the graph. However, research is still being done into creating better fluorophosphates in order to increase the capacity and improve the chemical and thermal stability - which makes the current technology a safety risk.
At this moment in time, I do not think a more suitable chemistry than the LiNiMnCoO2 we have been using for the last few years exists. Additionally, this technology is available in many different form factors, which gives us greater flexibility for packaging and cooling. However, depending on when you are reading this, it may be beneficial to see how far the technology has come along and consider implementing it in your system.
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