Fast to Slow: Unpacking the Charging Speed Shift in EVs

If you're an EV enthusiast, you've likely observed a curious trend while powering up your ride. Take my Xpeng P7, for instance. When the battery dips to 20%, it greedily gulps power at a brisk 90KW. Yet, as it climbs to 80%, the pace noticeably tapers off to a more modest 20-30KW. And past 90%? The speed dwindles even further. Ever pondered why?

It's tempting to wish for unrelenting high-speed charging – imagine the time saved! However, this isn't feasible, primarily due to the intricate dance of lithium-ion battery charging and a focus on preventing battery damage. Let's delve into the why and how:

1. The CC/CV Charging Tango: Lithium-ion batteries pirouette to the rhythm of Constant Current/Constant Voltage (CC/CV) charging. Initially, it's all about constant current, powering the battery up to a specific voltage level (circa 80%). Post this milestone, to sidestep the perils of overcharging, the routine shifts to constant voltage. This switch cues a gradual current reduction, aimed at maintaining the voltage, inevitably slowing the charge.

2. A Toast to Battery Health: As the adage goes – slow and steady wins the race. Easing off the charging throttle as full capacity nears is a toast to the battery's health and longevity. Rapid charging can be a heated affair, both literally and figuratively, straining and degrading battery performance over time. Slower charging equals less heat, spelling a longer, healthier battery life.

3. Safety First: Lithium-ion batteries aren't fond of high-stakes risks associated with overcharging. Nearing full capacity, a high charging rate could turn your battery into a hot potato, heightening the chances of thermal runaway – a fiery or explosive scenario. Reducing speed as full charge approaches is a safety net.

4. The Chemical Shuffle: Think of it like finding a parking spot in a lot. When the lot is mostly empty, it's a breeze to pull in and park your car. But imagine the lot is 80% full, it becomes a bit of a challenge to find that open space. The more cars there are, the harder and more time-consuming it is to locate a spot. This is a lot like how charging an electric vehicle works. When the battery's charge is low, it's easy to 'park' more energy in it quickly. But as it fills up (say, around 80%), finding room for more energy takes more time, slowing down the overall charging process."

In essence, the throttle back in charging post-80% is a strategic ballet, choreographed to harmonize charging duration with battery health, safety, and lifespan. It's the dance of modern EV tech, ensuring your trusty steed is both swift and sustainable.