Why EV batteries suffer from cold
The range of an electric car can drop by as much as 50% in freezing temperatures, a recent AAA study found. What happens – or doesn’t happen – in the battery cells when the mercury drops below zero?
Lithium ion batteries are basically an assembly of cells containing chemicals that react to either produce or absorb energy. As the cells discharge, they release electrons to power to the electric motor. As they charge, they absorb electrons that flow to the battery cells either from the grid or from the regenerative braking system.
The problem is that electrons, i.e. electricity, move a lot slower in cold circumstances. You can keep a chemical reaction going by applying heat – which is what the battery management system does, but that heat comes from the battery itself and thereby compromises the equation.
It is therefore crucial to pre-condition the EV before you leave and take power from the grid to get the electrons in a happy state.
This chemical reality also means that at a standard 50-kW DC fast charger, the vehicle will charge a lot slower when it’s freezing than in moderate temperatures. The battery management system slows down the “absorption” of electricity to protect the cells.
As the power flows in, the cell temperature will rise, allowing an increased charging speed, but you won’t get the same rate as on a summer day. We found out the hard way two weeks ago, when we charged a Jaguar I-Pace for three hours at a 50-kW DC charger, only to find out that it was not enough to get from 21% to 100% SOC (state of charge).
The Nordics know
The report Assessing range and performance of electric vehicles in Nordic driving conditions of Nordisk Energiforskning confirms that the Nordic climate necessitates slower charging and battery warming.
Interestingly, Tesla has designed its battery pack so that it always stays in the best possible temperature zone — not too hot, not too cold — combining heating, cooling and insulation. Better still, if you use the navigation system of your Model S or X towards a Supercharger, the cells are prepared for charging, i.e. preheated when it’s cold, so they are at the ideal charging temperature upon arrival.