Pourquoi vous ne devriez jamais laisser la batterie de votre vélo électrique au soleil

Why sunlight is a risk: what happens inside a lithium-ion cell above 40 °C

Lithium-ion cells in e-bike batteries operate safely between −10 °C and 45 °C. Above 45 °C, internal resistance drops and the electrochemical reactions that store energy begin to accelerate beyond design limits. When cell temperature exceeds 60 °C, the separator membrane that keeps anode and cathode apart can degrade, allowing direct contact between reactive materials. This is thermal runaway: an exothermic chain reaction that produces heat faster than the cell can dissipate it, leading to venting, fire, or explosion.

A battery left in direct summer sun reaches dangerous temperatures quickly. Even on a moderate 25 °C day, a black plastic battery case absorbs solar radiation and can climb to 60–70 °C within 30 minutes. The cells inside lag behind the case temperature by only a few degrees. At 60 °C internal temperature, the electrolyte begins to break down, releasing flammable gases. If one cell vents, the heat can cascade to adjacent cells, igniting the entire pack.

Dutch fire services—grouped under regional safety boards called Veiligheidsregio—have documented this pattern in multiple incidents during the summers of 2023 and 2024. Batteries left on bikes parked outside cafés, stored in car boots during trips, or placed in garden sheds with glass roofs have ignited without any charging activity or manufacturing defect. Post-incident analysis showed that thermal stress alone was sufficient to trigger cell failure.

European cell manufacturers specify storage below 30 °C for a reason. Our 48V 20Ah battery uses Samsung 21700 cells, which carry the same thermal limits: continuous operation up to 45 °C, storage recommended below 30 °C, and thermal runaway threshold around 60 °C. These are not conservative margins. A battery that feels hot to the touch—above body temperature, roughly 37 °C—is already in the warning zone. By the time the case reaches 50 °C, the cells inside are approaching the point where damage becomes irreversible.

Real incidents: Netherlands fire service reports and what they found

Veiligheidsregio Noorderland, covering the northern provinces of the Netherlands, recorded eight e-bike battery fires linked to sun exposure between June 2023 and August 2024. The incidents shared a common profile: no charging was taking place, the battery was structurally intact before exposure, and the ignition occurred after 20 to 90 minutes in direct sunlight or in an enclosed space with poor ventilation and solar gain.

One case involved a commuter who parked outside a café in Groningen on a 28 °C afternoon. The bike was in full sun, battery still mounted. After 40 minutes, passers-by noticed smoke rising from the downtube. The fire brigade arrived to find the battery case split open, with flames visible inside. The rider had left the battery on the bike because the stop was short and the bike was visible through the café window. The battery was two years old, had passed its last service check, and showed no prior faults.

Another incident occurred in a private car during a family trip to the Belgian coast. The battery was removed from the bike and placed in the boot for transport. The car was parked in a sunny car park for two hours while the family was at the beach. When they returned, the boot interior was filled with smoke and the battery was too hot to touch. The fire service used thermal imaging and found the battery core temperature had exceeded 80 °C. No fire started, but the cells had vented and the pack was destroyed.

A third case involved a garden shed in Friesland with a polycarbonate roof. The owner stored his e-bike inside during summer, assuming the shed provided adequate protection. On a sunny July afternoon, the shed interior reached an estimated 55 °C. The battery, which had been removed from the bike and placed on a shelf, ignited without warning. The fire spread to the wooden shed structure before the owner noticed. Post-incident inspection found no electrical fault, no charger connected, and no impact damage to the battery case.

Dutch fire services now distribute parking guidance during the summer months, advising riders to remove batteries during any stop longer than 15 minutes if the ambient temperature exceeds 20 °C. Similar advisories have been issued by fire brigades in Flanders, Belgium, and in Baden-Württemberg, Germany, where greenhouse and car-interior incidents have also been recorded.

Where the danger hides: parked bikes, cars, sheds, and greenhouses

A bike parked in full sun is the most common scenario. The battery case, typically black ABS plastic, absorbs infrared radiation. Even if air temperature is only 24 °C, the case surface can reach 50 °C in 20 minutes. The cells inside, insulated by the case and their own thermal mass, follow the case temperature with a 10- to 15-minute lag. By the time the case feels too hot to hold, the cells are already above safe operating temperature.

The position of the sun matters. A bike parked at 11:00 in the shade of a building may be in full sun by 13:00 as the shadow moves. Tree shade is unreliable for the same reason. If you cannot guarantee that shade will remain throughout your stop, treat the parking spot as if it were in full sun.

Inside a car, conditions are worse. Even with windows cracked, the interior of a parked car reaches 50–60 °C on a 28 °C day within 30 minutes. The boot, often assumed to be cooler because it is enclosed, can be hotter still if the car is dark-coloured or parked on asphalt. A battery placed in the boot or on the back seat is at high risk. If you must transport a battery in a car during warm weather, place it in the passenger footwell—the coolest location because it is shaded and benefits from residual air conditioning—and drive with the air conditioning on.

Garden sheds and greenhouses create oven conditions. A shed with a glass or polycarbonate roof can reach 60 °C inside on a sunny day, even if the outdoor temperature is only 25 °C. Poor ventilation prevents heat from escaping. Batteries stored on shelves or floors in these spaces have ignited even when not in use and not charging. The same risk applies to conservatories, garden rooms, and any structure with significant glazing and limited airflow.

Balconies and terraces are deceptive. A battery left on a table or bike rack in afternoon sun can overheat even if the surrounding air feels comfortable. Reflected heat from walls, paving, or glass increases the effective temperature. Shade from a building helps, but only if the battery is not in an enclosed or reflective space. A battery sitting on a metal rack in partial shade can still absorb enough radiant heat to become unsafe.

Safe parking habits: shade, removal, and the 30 °C rule

Remove the battery if you park for more than 15 minutes in warm weather. This is the single most effective measure. When ambient temperature exceeds 20 °C, the battery should come with you. It weighs 3 to 4 kg, has a carry handle, and fits under a café table or in a shop corner. The inconvenience is minor compared to the risk of returning to a smoking bike or a call from the fire brigade.

Take the battery indoors—into the café, office, or shop. Do not leave it outside in the shade, on the bike, or locked in a pannier. If you are concerned about theft, the battery is more secure with you than on an unattended bike. Most establishments will allow you to bring it inside if you explain that it is a fire risk to leave it in the heat.

If removal is not possible, park in full shade and verify that the shade will not move during your stop. A building's shadow is predictable; a tree's is not. Check the sun's position and estimate where the shadow will be in 30 or 60 minutes. If there is any doubt, remove the battery.

Never leave a battery in a car, even in the boot. If you must transport it, place it in the passenger footwell and drive with air conditioning on. Do not leave the battery in the car when you reach your destination; take it with you or leave it at home. A car parked in the sun becomes a kiln, and the boot offers no protection.

At home, store the battery indoors at room temperature, between 15 °C and 25 °C. Do not leave it in a shed, garage, greenhouse, or any outbuilding during summer. Even a well-ventilated garage can exceed 35 °C on a hot afternoon. The coolest room in your home—a basement or north-facing room—is the safest location. Avoid attics, which can reach 40 °C or higher under the roof.

What to do on hot days: ride-time cooling and emergency signs

Before a summer ride, check the battery temperature by touch. If the case feels warm to your hand—above body temperature—let it cool indoors before riding. A battery that starts a ride already warm will generate additional heat during discharge, and the combination can push it into the danger zone during a long climb or high-power segment.

During a ride, the battery generates heat from internal resistance as current flows. On a 30 °C day, this is usually manageable because airflow over the case provides cooling. However, continuous high-power climbs in direct sun can overwhelm the cooling capacity. If you are climbing a long hill on a hot day and the motor is drawing peak power, the battery temperature can rise faster than the airflow can dissipate it.

If the battery case feels too hot to hold comfortably—hotter than a cup of tea you would drink—stop, remove it, and place it in shade on the ground. Do not place it on hot asphalt or in direct sun. Let it cool for 20 minutes. The case temperature will drop faster than the internal cell temperature, so wait until the case feels only slightly warm before continuing.

Emergency signs of thermal distress are unmistakable: swelling of the case, a hissing sound, a sharp chemical smell, or visible smoke. If any of these appear, move the battery away from people, buildings, vehicles, and flammable materials immediately. Place it on bare ground or pavement in an open area. Call emergency services—112 in all EU countries—and describe the situation. Do not attempt to open the case, do not touch the terminals, and do not move the battery again once it is in a safe location.

Do not attempt to cool a distressed battery with water. Thermal runaway is an internal chemical reaction that cannot be stopped by external cooling. Water can cause short circuits if it enters the case through cracks or vents, and it can spread burning electrolyte if the cells rupture. The only safe response is to isolate the battery and wait for emergency services, who have equipment to contain and extinguish lithium-ion fires.

Charging in summer: why heat and current do not mix

Charging generates heat inside the cells. The chemical process of moving lithium ions from cathode to anode is exothermic, and the internal resistance of the cell converts some of the incoming electrical energy into heat. A battery that is already warm—above 30 °C—starts charging with less thermal margin. The heat from charging adds to the existing heat, and the cell temperature can rise above safe limits even if the charger is functioning correctly.

Always charge indoors in a cool room. Do not charge in a garage, shed, balcony, or any space that can exceed 30 °C ambient temperature. During a heatwave, choose the coolest room in your home—a basement or north-facing room with blinds closed. Avoid charging in direct sunlight, even indoors; a battery on a windowsill in sun can overheat during charging.

If the battery is warm after a ride, let it cool to room temperature before plugging in the charger. This takes about 30 minutes if the battery is placed in a cool, ventilated space. Do not charge immediately after a long ride on a hot day. The cells are already warm from discharge, and adding charge current on top of that heat can push them past safe limits.

Our 48V 4A charger has thermal cutoff protection: if the charger itself overheats, it will stop. However, the charger cannot measure the internal temperature of the battery cells. If the battery starts charging above 30 °C, the charger will continue to deliver current because it has no way to know the cells are too warm. The thermal cutoff protects the charger, not the battery.

Never charge overnight in an unattended space during summer. Charge during the day when you are home and can check on the battery every 30 minutes. If the case feels warmer than room temperature during charging, unplug the charger and let the battery cool. A normal charge should produce only a slight warmth; if the case is hot to the touch, stop charging and investigate.

Long-term storage in summer: the 50–60 % rule and cool placement

If you will not ride for more than two weeks in summer, store the battery at 50 to 60 % charge. This is 2 or 3 bars on the display, or approximately 50 to 52 V measured at the terminals of a 48 V nominal pack. A fully charged battery under heat stress degrades faster because the high cell voltage increases the rate of parasitic reactions that consume electrolyte and grow resistive layers on the electrodes. A battery stored at full charge in a warm environment can lose 5 to 10 % of its total capacity over a single summer.

Store the battery indoors in the coolest room of your home. A basement is ideal; if you do not have a basement, choose a north-facing room on the ground floor. Avoid upper floors, which are warmer, and avoid attics, which can exceed 40 °C under the roof on sunny days. The storage location should remain below 25 °C throughout the day.

Check the battery once a week during storage. Touch the case; it should feel cool, the same temperature as the surrounding air. If the case feels warm, the storage location is too hot. Move the battery to a cooler spot immediately. A battery that feels warm while idle is self-heating, which indicates either a fault or that the ambient temperature is too high.

Do not store a fully charged battery in summer. The combination of high cell voltage and elevated temperature accelerates calendar aging—the gradual loss of capacity that occurs even when the battery is not in use. A battery stored at 100 % charge in a 30 °C environment will age twice as fast as the same battery stored at 60 % charge in a 20 °C environment.

When you return to riding, charge the battery to full only on the day you need it, not in advance. If you know you will ride on Saturday, charge on Friday evening or Saturday morning. Do not charge on Monday for a Saturday ride. The fewer days the battery spends at full charge in warm weather, the longer it will last.

Frequently asked questions

Can I leave my e-bike battery on the bike if I park in the shade?

Only if the shade is guaranteed not to move and the ambient temperature is below 25 °C. Even in shade, a black battery case can warm up from reflected heat off nearby walls, pavement, or vehicles. For stops longer than 15 minutes, removal is safer. If you cannot remove the battery, verify that the shade will remain in place for the entire duration of your stop and that there are no heat sources nearby.

What temperature is too hot for an e-bike battery?

Internal cell temperature above 45 °C is the threshold for accelerated degradation; above 60 °C thermal runaway becomes likely. Surface temperature of the case should not exceed 40 °C. If the case feels too hot to hold comfortably—hotter than a warm drink—it is too hot. As a practical test, if you cannot keep your hand on the case for five seconds, the battery is overheating.

Is it safe to charge my e-bike battery during a heatwave?

Yes, but only indoors in a cool room and only after the battery has cooled to room temperature. Do not charge in a garage, shed, or any space above 30 °C ambient. Charge during the day when you can monitor the battery, not overnight. If the battery case feels warm to the touch during charging, unplug the charger immediately and allow the battery to cool.

What should I do if my battery case starts to swell or smell strange?

Stop using it immediately. Move the battery outdoors away from buildings, cars, and flammable materials. Place it on bare ground or pavement in an open area. Do not attempt to cool it with water. Call emergency services—112 in all EU countries—and describe the situation. Do not transport the battery in a vehicle. Stay at a safe distance and keep others away until emergency services arrive.