When you charge your electric vehicle (EV), you might notice that you don’t always get the maximum charging speed advertised by either your vehicle manufacturer or the charger. You might even get different charging speeds on different days. Why is that? It's because charging an EV is a lot more like charging your phone than filling at a gas station. Every EV is different, but here are the five most common factors that affect charging speed:
1. How full your battery is when you start charging, which is called your State of Charge (SoC)
Your State of Charge describes how full your battery is, in terms of percentage. Think of it like a fuel gauge. Batteries charge fastest when they are nearly empty—when they have a low SoC.
A helpful analogy is finding a seat in a movie theater. When the theater is empty, it’s much easier to find an open seat. But as the theater fills up, it takes more time to find an open seat (and climb over peoples’ legs, without spilling their popcorn). EV batteries are similar. When your SoC is low, it’s much easier for electrons to find empty “seats” to fill; when your SoC is high, it takes more time, and your charging speeds will be slower.
2. The temperature of your battery
As a general rule, batteries follow the Goldilocks principle: they like the temperature to be just right.
Why is this? The reason has to do with protecting your battery’s health. Your EV has something called a Battery Management System (BMS) to keep an eye over your battery’s safety. It’s sort of like your battery’s brain. Your EV's BMS doesn’t want the battery to get too hot or start charging too fast when it is too cold because extreme temperatures can impact a battery's lifespan.
Most EVs also have what is known as a thermal management system, which can heat or cool the battery to keep it at optimum temperature. Still, EV batteries are influenced by the outside weather. If it’s a really hot day outside (or if you’ve been charging for a while, and your battery is getting hotter), your charging speeds will be slower. If it’s freezing cold outside, your charging speeds will also be slower. These speeds are decided by your BMS, which controls the thermal management system for a fast but safe charge.
3. Other loads in use while charging
If you stay in your car during fast charging, be aware that some of the energy destined for your battery is diverted for loads such as the cabin air conditioning or heating, lights, radio, and other accessories. The thermal management system also uses some of the charging power to heat or cool the battery. This is why sometimes the kW display on the charger may be a few percent more than that of what your in-dash displays indicate.
4. Battery deterioration
Although it takes time, batteries can deteriorate and lose their charging capacity over their lifespan. Because every EV is different, the normal loss of your battery’s capacity should be defined in your vehicle’s warranty. As a general rule of thumb, when fast charging, it’s a good idea to end the charge around an 80-85% SoC. This will keep your battery from getting too hot—and give you more free time (since charging speeds will be much slower as your battery is close to full).
5. Your vehicle’s current and voltage limits
This one can be tricky to understand, but it’s important. The amount of power your EV battery receives in a charge is defined by a unit of power called a kilowatt (kW). And power (kW) is a product of voltage (V) and current (A). Both your vehicle and the charger have voltage and current limits.
Think of it like water flowing through a pipe. The amount of power you receive (water) is a product of the voltage (the force of the water flowing through the pipe) and current (the amount of water flowing through the pipe).
Let’s say you charge your EV with a 100kW charger; you might expect to charge at 100kW of power. However, your charging speeds will depend on the voltage and current limits of both your vehicle and the charger.
When you first plug in, the charger matches your battery’s voltage and delivers current. This current is limited by either your vehicle or the charger, whichever limit is lower. If your EV has a voltage limit below 500V, the charger may deliver less than the maximum power available because the charger reached the maximum current limit. The image below shows an example of how a car with a voltage limit of 380V is limited by the maximum current of the station to 76kW.
Because every EV battery and charger have different voltage and current limits, every charge begins with a negotiation about your EV’s voltage and current limits and the charger’s voltage and current limits.
While it's possible to get the maximum charging speed advertised by your vehicle, you’ll typically get charging speeds on a range lower than that maximum. EVs are smart, and when you plug into a DC fast charger, both your EV and the charger want to give you the fastest speeds possible—in the safest way possible—to keep your EV’s battery happy and healthy.
If you're hungry for more EV knowledge, check out our EV101 learning center. We’re continually creating more content to help you explore this exciting new technology. Stay tuned!