How do 'cell to body' batteries work, and how will they make cars lighter and cheaper?

Batteries are bulky and heavy and no one wants that. Worse, a lot of that excess stems not from the actual cells that store the electricity, but from structures designed to house and cool them.

EVs use individual cells, wired in series to reach the approx 400V or even 800V that the drive systems use. Standard procedure is to package a number of cells, maybe two dozen, into a structure called a module. This is basically a metal box, incorporating wiring between the cells, and cooling channels. Multiple modules are fixed inside a bigger, stronger box. The battery pack.

This sounds safe. The cells are mounted in three layers of box. If one group of cells fails, only that module needs replacing not the whole pack. But it’s also wasteful. Cooling and wiring channels need to worm their way among all this structural steel or aluminium, and the structure itself is bulky.

So innovators such as BYD are skipping the modules, and stacking the cells straight into the battery pack. Cooling channels and wiring are redesigned to keep temperatures even, and careful design maintains repairability. Cells seldom actually fail.

BYD’s cells, as with increasing numbers of cars, use lithium-iron-phosphate (LFP) chemistry which has cheaper minerals and is less prone to fire than the lithium-nickel-cobalt-manganese (NCM or NMC) type. LFP cells are less energy dense than NCM, which is why saving space and weight in the structure of an LFP battery matters, and so binning the module structure is good.

BYD calls this the ‘blade battery’. Tesla calls it a ‘structural battery pack’. Many Chinese are there now, and Renault has it on the Twingo and VW on its new small EVs.

Next stage then: cell to body. Here the battery isn’t really a separate unit at all. The battery box is part of the car’s monocoque. Again BYD is there with the Seal, and BMW with the Neue Klasse. In other words the body isn’t self-supporting without its battery, and the battery isn’t a finished article until it’s united with the body.

With a cell to body battery, the car uses its space better, allowing for more cells and range, and can be lighter and potentially cheaper. But it demands tricksy design of the cell crash protection and temperature regulation, when each element of the understructure around the battery has to do at least two jobs.