Why does a car battery get hot during charging?
4 Answers
Car battery overheating during charging can be attributed to the following reasons: 1. Battery malfunction: Increased internal resistance, dried-up electrolyte, or internal short circuits. As the battery undergoes repeated use, water molecules within gradually deplete, leading to gas evolution and water loss during electrochemical reactions, which causes heating. 2. Battery aging: If the battery has been in use for two years or more, thermal runaway may occur due to quality issues or usage conditions, typically manifested by the charger not switching to green light after overnight charging. 3. Charger issues: Conventional chargers lack pulse functionality and negative pulse depolarization capability. Such chargers often fail to maintain constant voltage in late charging stages, causing battery voltage to exceed permissible values, resulting in excessive gas evolution, water loss, and heating.
A couple of days ago, the battery in my car got extremely hot while charging, which really scared me. After consulting a car-savvy friend, I learned that it might be due to the charger's excessive current or high voltage, which forces the battery to charge and causes intense internal reactions generating heat. Additionally, if the battery is old or has an internal short circuit, it can easily overheat during charging—this is quite dangerous and could lead to swelling or even catching fire. Also, if the electrolyte level is low, the increased resistance during charging can make the battery hot to the touch. I later switched to a smart charger that automatically adjusts the current, and now the charging process is much gentler. Remember to regularly check the electrolyte level and avoid charging under direct sunlight to extend the battery's lifespan.
Yesterday, I helped my neighbor check his car battery, and it was extremely hot while charging. It was most likely caused by overcharging—either the charger's output current was too high or it was charged for too long, both of which can make the battery overheat. Additionally, when the battery ages and its internal resistance increases, this can also happen. The last time I disassembled an old battery, I noticed that if the plates were sulfated or short-circuited, the charging efficiency would drop, generating excess heat. Another easily overlooked detail is that if the ambient temperature exceeds 35°C during charging, the battery temperature rise becomes more noticeable. It's advisable to use a fan for cooling during charging and pause every two hours to check the temperature. If the problem persists, it might be time to replace the battery—after all, safety is the top priority.
Last week when charging the car, I found the battery was too hot to touch. Common causes include charger power mismatch, especially high-current fast charging which can lead to overheating of the battery plates. Another reason is insufficient water inside the battery, where lack of electrolyte increases resistance and generates heat. I've read that overheating during charging could also signal an internal short circuit or cell damage in the battery—in such cases, it's best to disconnect the charger and check if the voltage remains stable. Additionally, loose terminal connections can increase contact resistance and cause heating. Nowadays, I use a charger with temperature monitoring, set to automatically cut off power if it exceeds 40°C.
