For example, a 5th Ni-MH Battery has a capacitance of 1200mAH, while the other section is 1600mAH. The capacitance of a battery is called 1C. When charging, the charging current is less than 0.1C, called trickle charge.
Trickle charging can charge the battery very well without damaging the battery life, but the time spent in trickle charging is too long, so it's rarely used alone, but in combination with other charging methods. When the charge current is between 0.1c-0.2c, it is called a slow charge. Charging current is greater than 0.2C, and less than 0.8C is fast charging.
When the charging current is greater than 0.8C, it is called ultra-high-speed charging. However, 1 C is a logical concept rather than an absolute value, so the fast charge slow charge according to 1C is also a relative, just as 200mAH charge current is slow charge for 1200mAH battery, and for 700mAH battery is fast charge.