Batteries: THE IMPORTANCE OF THE BATTERY MANAGEMENT UNIT
All current EVs use lithium-ion batteries. Lithium is a highly reactive element, and modern batteries use a combination of lithium cobalt oxide as the positive cathode and just simple carbon as an anode. These elements allow electrons and ions to be pushed back and forth between the carbon and the lithium repeatedly, but not forever.
Lithium Dendrites
Lithium-ion batteries eventually die, mainly due to internal corrosion. Small crystalline formations grow inside lithium-ion batteries as they age. These are known as dendrites. Dendrites grow inside the battery and gradually reduce efficiency by consuming the liquid electrolyte. Dendrites are like stalactites in a cave, are metallic, and eventually form little wires that short circuit the battery cell. In modern electric car battery packs, there are thousands of individual lithium-ion cells. The failure of any one cell does not destroy the entire battery, it’s a redundant design. Indeed, even if half the cells failed the battery pack would still function, although you would lose half your charge output and range. The invention of the lithium-ion battery pack fundamentally created the modern electric car today.
While the use and advancement of lithium-ion cells has transformed modern electronics from smartphones and laptops to cars, we are approaching the fundamental limits of the technology. Solving the problem of dendrites is fundamental to advancing further.
Research and prototyping into lithium metal cells, new forms of electrolyte and solid-state batteries is advancing at a rapid pace. Promises of a future battery capable of a 1000-mile range and five-minute charging have been claimed. For now, lithium-ion battery density and output per kilowatt has been the focus for the past decade. Reaching a battery pack cost low enough to make mass affordable electric cars is the EV Holy Grail. Massive achievements in battery technology have occurred in the past 20 years, but this is not just down to the elements used but also to modern battery management systems. I’m going to explain how battery management systems work and why they are fundamental to progress the electric revolution.
Since the development of the lithium-ion battery, most research has been on managing the cells rather than producing a completely new battery technology. This is about to change with massive research, spurred on by the advancement of electric cars, into new battery technologies, but I’ll cover those in a future blog.
Battery management systems are crucial for power management and your safety. Lithium-ion cells can be extremely dangerous. You've heard lots of stories about Teslas catching fire after crashes and I cover in chapter 6 of my book all the facts and fiction about electric car safety.
Eberhard and Tarpenning (the co-founders) in the early Tesla days developed the first electric car battery management unit (BMU) capable of taming thousands of lithium-ion cells. BMUs are perhaps the important part of an EV. While most modern devices powered by lithium-ion batteries have BMUs installed, how an electric car’s BMU operates is completely different to say a mobile phone. The BMU in an electric car isn't just focused on safety and performance, but also longevity, whereas mobile phones are constantly upgraded and discarded after two or three years.
Tesla Battery Module
The actual full capacity of a new EV is more than is made available or visible to the driver. The battery management unit keeps this excess power and range in reserve. This is what allowed Tesla to release more range to Tesla drivers during hurricane Irma in Florida.
As battery packs age, the efficiency and range of the vehicle begins to decrease and so the BMU frees up some of this spare reserve capacity. This means that even though the physical capacity of the battery pack has diminished, the perceived capacity for the driver has little changed. You may think this is false advertising and misleading, but this secret reserve capacity also protects the pack for when power is needed for heating and cooling. Some cells in the pack can be almost fully charged while others can be nearly depleted.
This is one of the functions of the battery management system, to make sure that there is uniformity in the charging so that no one cell is over or under charged. Without this cell balancing, the longevity of the battery would diminish at a much faster rate. Indeed, some cells even die and the BMU affectively removes them from the rest of the collective. The BMU therefore separates cells and groups of cells into a series of protective gates; a gate can be shut to protect the wider battery pack.
If the first priority of the BMU is to protect cells, the second is temperature control. Charging and discharging lithium-ion cells creates heat and controlling thermal runaway is vital for safety and performance. Using coolers, heaters and fans the BMU regulates temperature for the battery pack. Coolant strips snake in between the battery cells to provide terminal protection in the event of under or overheating. Of course, cooling the battery requires energy and this is perhaps one of the least efficient aspects of an electric vehicle.
However, unlike an internal combustion car, the heat loss is relatively small, hence contributing to an electric vehicle’s 80% efficiency rate. Monitoring individual cell health, state of charge, state of discharge and temperature is all part of the BMU’s role. The rate and speed of recharging, particularly supercharging, is controlled by the BMU to optimize charge time safely. Therefore, supercharging slows down as you approach 80% or more charged.
As with lithium-ion cells, battery management systems are largely reaching the limits of their capabilities. With over the air updates Tesla was able to slightly increase the efficiency of supercharging by releasing software that pre-heated or cooled a battery to the optimal charging temperature as the car approached a supercharger.
These enhancements are reaching the peak of the physical hardware, but I'll explain later how Tesla and Volkswagen are taking existing lithium-ion configurations and BMUs to the next level. This all means the cost and efficiency of batteries is changing rapidly, making EVs increasingly more affordable.
