The main difference is the current size and charging speed. Fast charging voltages usually exceed 5V, while slow charging voltages are below 5V.
Fast charge provides a high current quickly, allowing the battery to be charged quickly and usually fully in a short period of time. But at the same time, fast charging will also cause the possibility of battery heating and capacity loss. Therefore, manufacturers will add some protective measures to avoid damage to the battery from fast charging.
Slow charging is lower than 5V, the charging current is small, and the charging speed is relatively slow. However, compared with fast charging, slow charging can better protect the battery, extend the service life of the battery, and avoid excessive loss. Therefore, for people who use mobile phones for a long time, slow charging voltage is more suitable.
Both fast charge and slow charge essentially charge the battery by providing different voltages and currents. Fast charge usually uses high voltage and low current mode, such as 9V2A or 12V1.5A, while slow charge usually uses low voltage and high current mode, such as 5V2A or 5V3A. The goal is to improve charging efficiency and safety.
Fast charging can put more energy into the battery in a short period of time, but it also generates more heat; Slow charging, on the other hand, injects energy into the battery at a slower rate, but generates less heat.