Keeping Ahead of the Curve

Neelesh Vasistha Jun 18, 2021 · 3 min read

Range anxiety remains one of the heaviest logs on the road to EV adoption. Its existence proves a living contrast between EVs and traditional cars: e-mobility may have come a long way, but many drivers still aren’t confident of somewhere to recharge. Despite this, most manufacturers recommended charging only to 80% capacity. What gives? Wouldn’t filling a car to 100% help range anxiety? Turns out it’s not as simple as that. In this article, we learn about the charging curve, and why drivers should be mindful of it.

What is a charging curve?

Ever pulled a pint of beer? If so, you’ll know how deceptively simple it seems. Filling the bottom is quick, but the game is won or lost in the top half. The perfect pour requires visual calculation and a steady hand; slowing down as the beer fills to the top, and taking care to avoid spillage.

A similar principle applies to EV charging—charging power must decrease as the battery fills, to avoid overheating and protect battery health. This forms the charging curve. Beware; there is no ‘standard’ curve. The graph above is an ideal type, but in reality, this trajectory varies greatly between vehicles.

What’s constant, however, is that charging power drops once the EV reaches a certain State of Charge (SoC). By around 70%, most EVs will receive dramatically less charge, and by 90%, the charge speed is glacial. This is why the industry often measures chargers by how fast they charge to 80%.

Why the drop-off?

Because of chemistry, basically. EVs are super safe, but never forget: fast charging means you’re zapping high-power direct current (DC) into a battery—essentially a giant cake of hazardous metals— and then sitting directly above it.

Overcharging these batteries puts them under stress, which can lead to all kinds of nasty things; like toxic gases, or what scientists charmingly call ‘thermal events’ — explosions and fires. So caution is smart, and cutting the current as the battery fills is a simple way to avoid overheating and stress.

For charge point operators, it also makes business sense — it incentivizes you to leave a public charging point after an 80% charge. This solves a few problems; not only does it create faster turnaround, more availability and thus less charge anxiety, but also lets the remaining cars at the station charge faster.

Environment

The charging curve is mainly a result of cautious design choices. But it's also influenced by environmental factors, like weather. When the outside temperature drops to below freezing, reactions within the battery slow down, and excess charging current causes overheating. Here, the onboard battery management system (BMS) limits the rate of charge; effectively modifying the charging curve.

EV batteries like the same kind of weather we do, working best between 20 and 30 degrees Celsius (68–86 Fahrenheit). Extreme temperatures in either direction slow charging speed. And in hot weather, this is compounded by the fact that fast charging itself warms the battery. Some models are equipped with thermal management, which can cool or heat the battery as needed. But just like heating up a car cabin, this can take a while.

What does this all mean for drivers?

Beyond safety concerns, a recent paper in Energy Reports studied battery behavior under real-world conditions. It concluded several strong reasons why heeding the charging curve lies in the driver’s financial interest.

Firstly, charging over 80% almost doubles energy loss—where energy is ‘wasted’ traveling between the charge point and car battery. Between a 20–80% SoC, this figure sits at a lowly 16% (for comparison, ICE cars have a typical energy loss of 75%). Charging over 80% also accelerates battery degradation, shortening its lifespan and reducing potential for second life applications.

Our Chargetrip Routing Engine is powered by an obsessive level of analysis. We will soon ingest lakes of data that allow us to predict the pattern of charging curves, slowdowns and drop-offs for practically every EV model. In the future, this will become one of the sharpest swords in our armory, giving EV drivers accurate charging and destination times. It will also help us play our part in beating the industry devil of range anxiety—routing drivers to a station that may not be available now, but will be when they arrive.