Batteries have enabled us to bring energy everywhere, but they’re far from perfect. We want them to be as small as possible, to carry as much energy as possible, and to charge as fast as possible. However, these desires are often in conflict with each other, and battery designers face a challenge in balancing these tradeoffs. In this article, we’ll focus on just one of these criteria: charging.
The intensive commercial development of electrical vehicles (EVs) has made fast-charging batteries a very desirable goal. Batteries already have significantly lower specific energy densities compared to fossil fuels, so the faster they can be charged, the better. Filling up with fossil fuel is very quick—a few minutes in comparison to the tens of hours required to charge an EV’s batteries.
Why Is EV Charging Slow?
It’s a true challenge for engineers to make a fast charging system. They face two sets of requirements:
- Battery chargers
- Battery design
The main challenge for battery chargers is the output power. For example, typical household chargers are limited to 1 – 3kW of input power, so these outlets do not allow for chargers that require more power. Depending on the battery size, the charging process can take up to 30 hours. This is not practical for long distance drives and for high-power EVs.
Ultra-fast chargers must have a high output power, 120kW for example. This requires electrical infrastructure adapted for high power, and these chargers are quite expensive. The fastest EV charger in the world today is 350kW. However, this charger is not yet useful because most commercial EVs are not capable of absorbing energy at such a high rate.
Batteries Must Be Specifically Designed to Withstand Fast Charging
A battery is a sensitive electrochemical device. Many factors can damage the battery: temperature, sudden discharging/charging, too many charging cycles, etc. The battery is designed to absorb a specific amount of energy in a specific amount of time, and pushing more energy into it than it can handle damages the battery and shortens its lifespan. In the case of the most commonly used lithium-ion (Li-Ion) batteries, doing this causes lithium plating to form on the anode, which creates dendrites and shortens the battery life.
Generally, batteries can be designed to be energy-optimized or power-optimized. Energy-optimized batteries have more capacity and are able to supply more energy, but with limited power. Power-optimized batteries can supply high power and are able to generate high current. These types have a different battery cell design, which is the most sensitive component for fast charging. So-called power cells perform great for fast charging (because of a large cell surface) but have a low specific energy, which is not good for EVs. Energy cells, on the other hand, have a good specific energy rate but require more time to charge. Designers of EV batteries must use a hybrid solution of these cells (with focus on the specific energy-capacity). One technical solution is to use lithium-titanate batteries, which charge faster than lithium-ion. However, these batteries are too expensive for commercial purposes.
Fast chargers also require a high-charging current. However, current also causes the battery to heat up. This effect can be calculated as follows:
- W is the heat energy dissipated across the battery
- J is a constant, Joule’s mechanical equivalent of heat
- R is the internal resistance
- I is the current
- t is time
The equation above shows that, the faster the charging, the higher the heat. Batteries cannot handle high temperatures well, so fast-charging batteries must have low internal resistance. However, it should be mentioned that much lower temperatures have a negative effect on the battery as well.
Fast-charging batteries will eventually make their way to commercial EVs. However, while the batteries may be fast, the process of developing them is not. As we’ve seen, there are many factors that pose a challenge in building fast charging batteries:
- Fast chargers require high-power electrical infrastructure
- Fast-charging batteries must be able to absorb a high amount of energy in a short period of time
- Internal battery resistance needs to be low
- Environment conditions are important for fast charging