The UK is experiencing record-breaking heat, with eight days at or above 34°C this year, surpassing the previous record of seven days set in 1976 and 2020. As temperatures soar, experts warn that a common habit—charging your phone to 100% in hot weather—can drastically shorten battery life.
Heat is the Battery's Worst Enemy
Galo Paez Fajardo, a research assistant professor at the University of Warwick, told The i: "Basically, the worst enemy a battery can have is heat." He explained that when a phone gets hot, it signals the device to cool down, as heat accelerates battery degradation. Leaving your phone on a sun lounger, car dashboard, or in direct sunlight can reduce its battery lifespan.
Scientists say the chemistry inside lithium-ion batteries speeds up as temperatures rise. Every 10°C increase above around 25°C can roughly double the rate at which batteries degrade. Charging a phone at 35°C to 45°C causes additional wear, and a battery stored at 40°C and fully charged could lose about 35% of its capacity in just one year.
The Simple Fix: Avoid 100% Charge in Heat
Experts recommend making one simple change: avoid keeping your battery at 100% charge in hot weather. David Howey, professor of engineering science at the University of Oxford, said: "You don't want to be sitting at high states of charge at high temperatures." Many newer phones include smart charging features that pause charging at around 80% and top up closer to when you usually unplug. Billy Wu, director of research at Imperial College London's Dyson School of Design Engineering, noted: "Normally we say about 80 per cent charge is a good number to reach and doesn't accelerate the degradation too much."
Modern Batteries and Charging Myths
Paez Fajardo said modern batteries have improved significantly, so it's often best to rely on your phone's built-in charging features rather than manually unplugging it. Experts also dismissed the old belief that batteries should be regularly drained completely before recharging. Adam Speight, senior researcher for product testing at Which?, said: "Having your phone at zero per cent and 100 per cent are both high-stress states for your battery." Paez Fajardo added: "Now, current batteries are fine if you charge them at any percentage. Going all the way down to zero per cent is actually detrimental for some chemistries."
Fast Charging and Heat
Fast charging remains useful but should be used carefully, especially in hot weather, as it generates more heat. When buying a new charger, Speight recommends choosing one that supports USB Power Delivery (USB PD), which manages charging more efficiently and reduces stress on the battery. Vodafone research found that one in four people say the fear of their phone running out of battery is the most stressful part of their day, highlighting the importance of battery health.
