In electric vehicles, one of the biggest sub-systems (and indeed the one that holds the key to the future of the industry) is the energy-storage system: the battery pack.
At Ather, we are developing an all-electric scooter that is powered by a lithium ion battery pack. What really goes into a ‘battery pack’? Fundamentally, any battery pack is a bunch of electrolytic cells that are connected in a certain configuration to give a required voltage and pack capacity. While functionally similar to their lead-acid cousins, most lithium-ion packs differ massively in the efficiency and complexity from them.
But first, a primer on lithium ion batteries:
The lithium ion cell was invented by a scientist working for Exxon in the 1970s. But it was commercialised by Sony and a company called Asahi Kasei in 1991. The technology has come a long way since. In fact lithium ion cells are freakishly common and can be found in most handheld gadgets. In the marketplace it has established its superiority over other chemistries like Nickel Cadmium and the “aging” lead acid batteries (pun intended). Lithium-ion cells can store a high amount of energy in a small structure, leading to a high energy density – useful for power back up applications. Also with minor alterations in the electrode makeup it can be made to deliver exceptionally high power densities – a feature that power tools often need. Combined with a good cycle life those make it the market favourite.
Of course, lithium ion comes with its own set of problems and limitations. Due to the high energy content, the cells can become unstable under certain conditions. There is a safe operating area recommended for any lithium ion cell; this is usually the temperature range, voltage and maximum discharge/ charge current. Taken outside these limits, the cell can become unstable. Indeed, several high profile accidents have dominated the tech discourse over the years (the infamous Dreamliner fire is one). These parameters are also meant to deliver the maximum possible cycle life for the cell.
..an Engineering Optimization:
Every battery’s capacity degrades as it’s being used – that much is (currently) unavoidable. But the rate of degradation can be controlled by various ways. One of them is keeping the cells in the ‘safe operating region’. That often is the crux of a great pack design. Pack design involves:
Designing a smart Battery Management System (BMS)
Designing an efficient thermal management system
Building in layers of safety
Temperature in particular, has a very significant effect on these cells. Operating at high temperatures degrades capacity while low temperatures cause a drop in performance. It is imperative for the design team to find the sweet-spot in between these extremes and keep the cells there.
Another critical factor regarding cycle life is the voltage operating range. For example, adjusting the operating band by a mere 1.5% can yield a 20-30% higher life: more optimization.
Then there is a trade-off between cycle life and operational time. There are variants among the lithium ion family that have the best of certain properties- lithium iron phosphate cells that offer high power density along with a great cycle life but have very poor energy density. Then there is lithium cobalt oxide which offers a high energy density and satisfactory cycle life but falls back in the stability department.
The wide amount of options lends lithium-ion well with loads of applications. And that’s why you find these batteries everywhere: phones, laptops, cameras, power tools, vehicles and so on. Selecting a chemistry that does the job most efficiently for your application is the key.
That is why the yields from great optimization in battery pack design are massive.
Application in EVs:
There are several ways of building a battery pack for automotive applications. The one typically used is to employ large prismatic (cuboid) modules or pouch cells and wiring them up together. It’s quick, reliable and takes minimal design effort on part of the OEM.
An example of pouch cells.
An example of small cells.
The other approach is to design an integrated pack using several standardized cells. The best example is Tesla Motors battery pack which employs north of 7000 cells for a single sedan (the Model S). We will get into a detailed comparison of both approaches in later articles but with the technology available today and the relevant price targets, the Tesla approach has more pros than cons.
One of the biggest issues with the current batteries are the upfront costs and the range they deliver for the same. While the current lithium ion cells are improving at the rate of 8% per annum, there are a host of chemistries in the labs which are promising super-fast charging, an extremely high energy density and (sometimes) a significantly low cost.
In the short term, silicon anodes can potentially improve the energy density by 20-30%. In the medium term, lithium titanate chemistries hold the commercial potential of being charged up in a matter of few minutes. In the long term, there are several interesting technologies like graphene anodes which can radically change the densities and life-cycles of the cells – we mean an order of magnitude improvement on most parameters.
We primarily talked about the optimization required in a pack design and the various approaches that the industry is using in this article. In later articles, we will delve deeper into battery management systems and the state of art of the technology