Battery Management System Strategies

Hannes Kuusisto is a senior design engineer in battery control at Volvo Car Corporation in Sweden. He has a background in engineering physics and works within the electrical propulsion system department.

Will Hornick: Tell us a bit about your background leading up to your work with Volvo Cars.

Hannes Kuusisto: I am a physicist. My background is in engineering physics. I studied at Chalmers here in Sweden and focused on condensed matter physics. This is related to how the cells work on the microscopic and nanoscopic level. I did my master’s thesis on battery cells for the Advanced Battery Research Group at Renault on physical cell modeling so I have a good understanding of cell behavior and their characteristics on the electrical side. I‘ve been working at Volvo for two years and before that at Lund University doing electrical modeling. I am also working on efficiency for the control algorithms of the battery, where we spend time on the key technical areas in a BMS system.

I am working on the BMS at a system level – as a system designer. It’s truly interesting to get a holistic view of the battery pack. In a sense, it’s like a mini-car in that it has everything from advanced control algorithms, advanced safety features, but also mechanical safety regarding crashes. The latter is particularly important for Volvo Cars which has a historical background and association with vehicle safety.

Could you discuss more about BMS strategies that the car industry employs and what some of the advantages are for different systems?

It’s very interesting when you look at the cell design and how that impacts packaging. We follow our own test flow very carefully. If you look at the Volvo V60 plug-in hybrid, we use -ion pouch cells. They are very soft. Their only characteristic is to store energy and give power. We have to pack them in. The packaging and enclosure has to be very strong because pouch cells swell, so if you don’t have anything internally to hold them together, you need to package them in a way to fit them in the vehicle without cracking after aging. That comes with some packing challenges because they are flat but quite big.

Li-ion cells can come in different forms. One of them is as pouch cells. These look more like in your A4 notebook. They are hard and rigid, but don’t have a lot of internal strength. They can handle some crash but can grow. So what you do normally is to pack them into modules which have some of this mechanical strength to hold the swelling in place. And they give you a specific size and you can package them that way. Another method is prismatic cells. They can hold against their own pressure, so you don’t need to package them as tightly but they tend to be a bit more expensive and heavy. Some manufacturers use this method.

Yet another method is to buy really inexpensive cells and buy a lot of them. So many that you can drive extremely far with them - 200 or 300 kilometers or so. But the problem is that these cells age quite badly. They age quickly which is why most manufacturers don’t look at these cells. Volvo, like most car manufacturers buy very good cells of very good quality for what is available on the market. These cells are very homogenous in how much energy they contain and how they age and there are very very few defects and they therefore age very well.

On the other hand, as a comparison, if a company buys a lot of lower quality cells cheaply, the problems have to be solved from an engineering perspective.

If you choose the lower quality cells in a large quantity to achieve a very long driving range, the cells can’t handle that amount of cycles if you drive the maximum range every day. Your car will die within a year. However, if 95% of customers only drive 50km per day then that’s a small fraction of the range.

Here there are the two main ageing factors. One is load cycles from fully charged to fully discharged, and the other factor is calendar age. Regarding these lower quality cells, they can’t handle so many cycles but most customers would not use many cycles because there is so much energy available in the car – they only use a fraction of it every day and therefore for most customers, the battery pack would last.

With this lower quality cell, you need to have a battery system. You really need this load-balancing approach, because you need to be able to turn specific cells off if a cell is bad. You need to distribute power unevenly between the cells.

Otherwise the weaker cell is going to be the limiting factor for the whole battery I believe?

Exactly. Our cells are extremely good quality and are extremely homogenous. You see defects in the cells but it is one in a million or one in 10 million. It is very uncommon. Therefore you don’t need to spend a lot of money on electronics to be able to circumvent the cells as they age quite similarly, rather than having an advanced system to be able to distribute energy between cells so that the cells reach the bottom at the same time.

How can you detect how charged the cells are? Is this accomplished via software algorithms or with sensors that are detecting how charged the cells are?

It is extremely interesting when it comes to the control algorithms, because the cell has a few states that you want to know. One of them is the state of charge – how charged it is. This is like how full your fuel tank is. That is very important in determining how much further you can drive. And when you make pure electric vehicles, then it is extremely important particularly if you live up on the east coast in the US or in Kiruna or Abisko in northern Sweden or somewhere else there in the far north. We do a lot of testing there in the winter, where you can get down to minus 30° or 40°. You have to be very careful because people can die if their car dies on the road, and if it is minus 30° in an area without cell phone coverage. You really need a car that can tell you how much further it can go. You don’t want accidents to happen because of a bad algorithm or control. For that reason you really need to know the charge level.

You also need to know how good your cells are. And what your internal resistance is. How much energy will you have to pass through them? Compared with a car that has a fuel tank where the volume of the tank stays the same over time, the volume in batteries decreases, meaning the amount of available energy, or charge, goes down from full. When it will die really depends on the chemistry and the design of the cells. You need algorithms to calculate how much energy you get as a maximum, or the capacity, how much electricity you can get out before empty - the age of the battery, in a sense.

Also very important for the customer’s perception of the vehicle is how much power you are allowed to take. And there are a lot of interesting optimizations and opportunities here, because if you take a lot of power fast, you can take a lot less power after a while. Or you can take an even amount of power for a longer time. And also you need to optimize, because if you take a lot of power, the high peaks tend to age the pack faster.

After saying all that regarding the charge, how much energy, the resistance, etc, none of those things are directly measurable. The only thing you could measure is the temperature of the cell if you could afford a temperature gauge for each cell. You can’t, of course, but you need to know the voltage of each cell, or at least cell power. Ideally both, because you need to know what charge level you are on and also from a safety perspective, because cells can go bad though it is very unlikely. Nevertheless, you have to measure the cell voltage from a diagnostic point of view. You have to make sure that it doesn’t drop, or that it is suddenly different from the other cells and then you measure the current through the pack.

Do you see the future of vehicles as being a plug-in hybrid where the IC engine works in conjunction or do you see that rather as a bridge?

I definitely see it as a bridge, in the sense that I am a technological idealist. When you look at it from a very high system view, you see a system that is very efficient and very good but we can do a lot better. On the other hand, you see where petrol and combustion started and that it has taken a long time to get where we are and we are still optimizing. It is extremely interesting to see what has happened over the past ten years when it comes to the fuel efficiency of the combustion engine. And I think we are in for the long run when it comes to the electric engine and hopefully batteries for storage. You can look at other ways of transporting energy. But storing electrical energy, I would say, would systemically be the most optimal, because you can transport it very easily, you can have it wherever you want, you can store it, or you can give it back to the system. Electric vehicles functioning as part of the grid has great potential.

Theoretically it would even be possible to sell electricity from EVs back to the grid especially if you have a bit larger capacity battery. The battery cost is falling quite fast, which is very good, the energy content is increasing, also very good, but the longevity, the ability to handle a lot of cycles, this is very crucial to look at from a systemic solution point of view.

From a personal perspective, one of the reasons that I chose to work in this field is because I think I can make an impact. In our generation you look at technology that is not only fun to work in but that is also green, in a sense – to have a chance to have an impact on the future and the way we live. And Volvo Cars are moving quite a bit in that direction.

Look at the four cylinder engines that we are making, both diesel and petrol, they are both extremely fuel efficient and good. And we are very proud of that. We can’t deliver as much power as a V8 engine but they are powerful and efficient for what they are.

Our top of the linepremium car at the moment is the V60. It has the biggest engine and it has an electric engine for the extra oomph. That strategy is very interesting and it gives us a bold position heading into the future. It was surprising how well the V60 Hybrid has been received and extremely fun to see how well that turned out along with its low fuel consumption and we feel there is definitely a future in this field. But the future has to be scalable and cheap as well.

Thank you so much for taking the time to speak with me.