Since summer began, temperatures in many places have exceeded 35°C, and quite a few EV owners have run into the same puzzle: when you go to an outdoor fast-charging station on a scorching day, just a few minutes after plugging in, you hear the roar of fans from the front of the car. Watching hundreds of amps of current being "poured" into the battery, you can't help but wonder — with the temperature already this high, could fast charging cause the battery to go into thermal runaway? Do we really have to stick to slow charging in the summer?
Some owners have also noticed that even though they're plugged into a 120kW supercharger, the actual charging power is only 30 or 40kW — painfully slow, like a "senior mobility scooter." Is the charging station cutting corners, or is something wrong with the car? Today, let's clear up all those summer charging questions.
First, let's understand the difference in heat generation between fast and slow charging:
Why do the fans go crazy during fast charging?
The principle behind this is something we all encountered in middle school physics: Joule's Law tells us that the heat generated by an electric current is proportional to the square of the current. This means the heat intensity of fast charging and slow charging is worlds apart.
With DC fast charging, the current can reach up to 250A — it's like forcing a large volume of water through a narrow pipe. The friction generates an enormous amount of heat. At this point, the battery's "smart manager," the BMS (Battery Management System), immediately kicks in the air conditioning compressor and liquid cooling system, running at full capacity to cool the battery down. That loud fan noise you hear is the cooling system working at full throttle.
In contrast, the current for AC slow charging is only 32A or even lower. The chemical reactions inside the cells are gentle, generating minimal heat. The cooling system barely needs to activate, and the battery remains in a very comfortable charging state, with minimal impact on battery health.
Supercharging turning into "slow charging" in the summer?
It's really not the charging station cheating you.
Many owners complain that supercharging is actually slower in the summer. In reality, this is the vehicle's BMS trying to "save the battery's life."
Summer temperatures are already high, and under direct outdoor sunlight, the radiant heat from asphalt roads can exceed 40°C. The battery is already quite hot just from sitting there. When you add the massive heat generated by fast-charging currents, even with the liquid cooling system running at full capacity, it's hard to dissipate all that heat in time.
Once the cell temperature approaches the safety threshold of 45°C, to avoid the risk of thermal runaway, the BMS decisively triggers high-temperature protection and actively limits the charging power, cutting off that high current. At this point, owners are not only paying the premium electricity rates for fast charging but also enduring charging speeds comparable to slow charging — which is really not cost-effective at all.
How to protect your battery and save money when charging in the summer:
It's actually very simple.
Many people think fast charging in the summer damages the battery, but the real culprit isn't fast charging itself — it's the double whammy of "high ambient temperature + high heat from fast charging." So the key to choosing between fast and slow charging is simply avoiding peak temperature periods.
The most recommended charging method is overnight slow charging at home: generally, from 10:00 PM to 8:00 AM the next day is the off-peak period for residential electricity, with rates as low as 0.3 yuan per kWh. Fully charging an average household EV costs just a little over ten yuan — less than a cup of coffee. Plus, nighttime temperatures are low, there's no harsh sun exposure, and the battery charges at a comfortable temperature with minimal wear and tear. Just plug it in and sleep soundly, no need to make a special trip to a charging station and wait — it's both hassle-free and cost-effective.
If you do have an urgent need to use fast charging, try to choose underground parking lots or supercharging stations with sunshades, or avoid the peak temperature window from noon to 4:00 PM. Don't plug into a fast charger immediately after the car has been baking in the sun. This can effectively reduce battery degradation and also prevent the charging power from being throttled.
These summer EV charging tips may seem small, but sticking with them over the long term not only saves you a fair bit on charging costs but also effectively extends your battery's lifespan. If you have any other questions about using your EV, feel free to leave a comment and join the discussion. And please share this guide with any fellow car owners who might find it useful.
