What’s in a Battery Anyway? – Battery Electric Buses in London

As ever there is a lot of interesting news in and around batteries for energy storage in Electric Vehicles and other applications at the moment. Batteries are hugely important for the future not just for vehicles in both fully electric and hybrid form, but also for applications like grid energy buffering and consumer goods, as was highlighted by the recent news of Dyson’s $1Billion investment in solid-state battery technology and manufacturing aiming to improve the performance and desirability of its handheld and robotic consumer products.

The launch of a battery-electric double-decker for London caught my eye last week. This project was originally announced with an in-service date of October 2015 (see the article from July 2015 in the Guardian here) they are slightly later than planned but are now entering service (see the recent article in the Independent here) better late than never! The adoption of a greater number of EV and HEV vehicles in our growing cities is one of the main ways for tackling the huge problem with urban air quality.

What is interesting about these buses is the range and the resulting size of the battery packs. The buses have been specified to operate for a full day on a single charge, I have been asked several times now how they can do it?

The answer is simple they have a huge 345kWh battery pack. BYD specialise in a type of Lithium battery called Lithium Iron Phosphate, (sometimes called LiPo or LiFePO4) which refers to the cell chemistry. This battery chemistry is one of the oldest and lowest energy density of all the Lithium Chemistry’s (read tried and trusted?). With an energy density of around 100Wh/kg this battery pack will weigh in at a whopping 3.5 tons before any packaging, interconnects, cabling or BMS are considered so a weight of 4 tons for the battery is probably more likely. The other limitations of this particular chemistry are a relatively low ability to take and release charge, known as “C-Rate” and relatively total cycle capacity what this means in practise is a relatively long time to completely recharge the battery as it cannot be pushed too hard with too much current causing damage to the cells. Basically, this is a “brute force” approach to achieving range, and the payload of the vehicle has been compromised significantly.

How do these batteries compare to the Lithium Nickel Cobalt Aluminium or NCA batteries currently used by Tesla, the P90D Model S has a 90kWh battery pack, the cells used by Tesla are achieving an energy density of 233Wh/kg. More than double the energy density.

There are also the Blended Lithium NMC batteries used in cars such as the Leaf, BMW “i” cars and the Volt are also breaking the 200Wh/kg mark at a lower price point than the NCA in the Tesla.

BYD can do this of course, because they are first and foremost a battery cell manufacturer, so think of this as essentially a huge battery with a bus assembled around it. A price per bus of £350K is mentioned in the article, with such a large battery it is likely that around half that cost is the battery and its associated hardware. BYD are uniquely positioned to do this as it is their own manufacture so they are not adding a margin to an already expensive battery bought in the form a 3rd party. I am very pleased that such a project is getting off the ground, but I am also disappointed that it is with what is essentially quite out of date technology. It would be great to see a bus with an NMC or NCA battery….

It does not end here, of course, there is a huge investment going into battery systems development both into new chemistries and also construction techniques the Dyson story mentioned earlier is an indicator of one of the most exciting areas of development, Solid State technology. Moving beyond the basic cell chemistry i.e. the reactive ingredients used in the battery, solid-state refers to the construction method. In a normal battery, there is an anode and a cathode and a liquid electrolyte. Within that construction, there is a lot of non-active material which adds significantly to the weight of the cell. In a solid-state battery, very thin layers of solid material are deposited on top of each other with very little or no inactive material in the structure, meaning much, much higher energy densities, lower costs and improved longevity.

About the author; Ryan Maughan is the Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of technology for electric, hybrid and conventional heavy duty and high-performance vehicles.