How does the recharging curve

of an electric vehicle work?

When we talk about a 100% electric vehicle, recharge time is one of the first things we think about, right after range.

These two characteristics determine a car's ability to cover long distances quickly, i.e. with a minimum number and duration of recharging stops.

During these stops, the most coherent strategy is to connect the vehicle to a fast-charging station, which offer high electrical charging power (at least equal to 100 kW) delivered in direct current. This type of terminal can be found in many service stations on the motorway network and in some retail outlets.

For a clearer picture, let's take a look at the figures for the new E-308 (not currently available in NZ),  powered by 156 hp. It is equipped with a battery with a useful capacity of 51 kWh (enabling it to travel 413 km according to the WLTP cycle), which can be recharged at up to 100 kW DC. If you plug the compact car into a rapid charging station, you'll soon realise that this maximum value is not maintained throughout its charge. Why is this?

Imagine your mission is to fill a glass of water as quickly as possible without overflowing. You'd go fast enough to fill three quarters of the glass, before gradually decreasing the flow rate so as not to exceed the maximum level. The same applies to a lithium-ion battery. This means that the recharge curve, which represents its capacity to store electrical power (kW), is not a straight line that can be maintained from end to end. 

Rest assured, intelligent battery management software manages this famous recharging curve (or charging speed) according to a multitude of parameters. In the vast majority of cases, when a car arrives at a fast-charging station with a battery at around 20%, the curve will peak at the outset, hold steady for a while, then gradually decrease.

Above 80%, the curve falls sharply, so the charging speed slows down. That's why it's best to get back on the road as soon as this threshold is reached, to optimise your journey time.

So it's the battery's ability to maintain its recharging curve at the highest possible level that determines how long you stay at a DC charging station. For example, the 51 kWh battery in the new E-308 can go from 20% to 80% in 27 minutes, recovering many kilometers of range.