Is it safe to leave a power bank in a car in hot weather?
On hot sunny days, many people have the habit of leaving spare batteries in their cars without thinking about the potential risks. But is this really safe, or can it turn the car into a “time bomb”?
Many people have the habit of keeping a spare battery in their car, considering it a convenient solution when they need to charge their phone or deal with an emergency. Cars, with their inherent mobility, seem to be the perfect “storage” for all devices. However, this is a mistake that carries great risks.
When parked in the sun, the temperature inside a car can spike to extreme levels, creating a harsh environment far beyond what we can imagine. In such conditions, not only is the performance of the backup battery affected, but the chemical reactions inside the battery can also become abnormal and dangerous.
The consequences go beyond shortening the life of your equipment, and can even lead to serious damage and even safety risks. A seemingly harmless decision can cost you dearly in the long run.
Safety or not depends on the living environment?
If you thought that leaving a spare battery in your car was harmless, the truth is much more complicated. The safety of this habit depends directly on your living environment, especially the outside temperature.
Batteries, whether they’re used in phones, laptops, or cars, have a natural enemy: heat. This problem is exacerbated with lithium-ion batteries, which are common in electronic devices and power banks. They’re known for their high energy density and long lifespan, but they’re also extremely sensitive to extreme temperatures, both hot and cold.
As temperatures rise, chemical reactions inside the battery occur more rapidly, leading to premature degradation. Battery life is shortened, charge capacity is significantly reduced, and more dangerously, there is a risk of damage or even explosion.
The worrying thing is that the temperature inside a car parked outdoors is always much higher than the air outside. The greenhouse effect causes sunlight to pass through the glass, trapping heat and causing the temperature inside to skyrocket.
On a day with an outdoor temperature of only 24°C, after 90 minutes, the car cabin can reach more than 43°C. In extremely hot regions, such as when it is 43°C outside, the temperature inside the car can reach 70 - 80°C, enough to turn this space into an "oven" for all electronic devices.
It’s worth noting that most lithium-ion batteries are only recommended to be stored between 10-40°C for safety and longevity. While they can theoretically handle up to 63°C, that’s the extreme limit.
In fact, even leaving a battery in a car at high temperatures for an hour can cause irreversible damage. Above 35°C, the solid electrolyte interface (SEI) layer on the anode begins to rapidly develop, depleting the amount of reusable lithium and causing the battery to deteriorate over time.
One study found that a fully charged lithium-ion battery stored at 60°C for a year can lose up to 60% of its original capacity.
There are many cautionary tales. In places where summer temperatures often exceed 40°C, leaving electronic devices in the car is almost a “death sentence”. Some users have shared their experiences of forgetting their phones in the car, only to return to a sunburned screen with permanent discoloration. In many cases, they were forced to replace the device because it could not be repaired.
In other words, whether or not it is safe to leave a battery in your car depends entirely on where you live. In cooler climates, the risk may be less, but in hot environments, especially on hot summer days, leaving a battery in your car is like having a “time bomb” right next to you.
The safest solution is still to carry electronic devices that use batteries or spare batteries with you or store them in a cool, airy place, instead of letting them "heat up" in the car compartment.
The best way to store spare batteries
When it comes to storing spare batteries, especially lithium-ion (Li-ion) batteries, which are common in most electronic devices today, it is important not only to keep them ready when needed, but also to ensure their longevity and safety. To do this, users need to pay attention to three main factors: temperature, charge level and storage environment.
First of all, temperature plays a key role. Li-ion batteries are extremely sensitive to temperature changes, especially high temperatures. The ideal storage environment should be maintained between 13°C and 29°C, but keeping it below 24°C will help the battery to be more stable, reduce self-discharge and maintain its capacity for longer.
Conversely, when temperatures exceed 35°C, chemical reactions inside the battery occur more rapidly, leading to irreversible degradation. This means that leaving the battery near a stove, heat-generating device, or in direct sunlight is a “death sentence” for battery life. Meanwhile, excessive cold temperatures are also harmful, as it can cause the electrolytes inside the battery to lose flexibility, affecting its ability to function.
In addition to temperature, humidity also needs to be controlled. The ideal level is usually around 50%, and absolutely should not exceed 70%. High humidity can cause condensation, corrosion of components, and even lead to short circuits. Many people used to think that storing batteries in the refrigerator is the optimal solution to avoid heat, but this is a misconception. When removing batteries from the refrigerator, water vapor can easily condense on the surface and inside, creating the risk of damage and unsafety. Instead, choose dry, cool and stable places, such as air-conditioned rooms.
Another aspect that is often overlooked is the battery level during storage. With Li-ion batteries, it is not recommended to charge them to 100% or to completely drain them. The ideal state is to maintain a charge level between 20% and 80%, as this is the range that puts the least stress on the battery cells. If the battery is left at 100% for a long time, especially in a hot environment, the battery capacity will decrease rapidly and the aging process will be more intense.
On the contrary, if the battery is allowed to drop too low (below 20%), the risk of permanent battery damage is very high because the voltage drops to a level that the battery cannot recover. Some modern chargers have integrated Storage mode, which automatically returns the battery to the optimal level, helping to significantly extend its life. To be safe, you should periodically check the battery every 3-6 months, and recharge it if the battery level drops below the safe threshold.
Last but not least, never leave a spare battery in your car, especially in the summer. The temperature inside a car parked in the sun can double that outside, reaching 70-80°C, which is extremely dangerous for the sensitive chemicals inside the battery. This seemingly harmless habit can cause the battery to swell, deteriorate quickly, reduce performance, and even pose a risk of fire or explosion.
In an age where we increasingly rely on backup batteries to stay connected and productive, understanding and applying proper storage principles not only saves on replacement costs, but more importantly, ensures the safety of you and your devices./.