During the use of the battery, the percentage of the capacity released by the battery to its rated capacity is called the depth of discharge. The depth of discharge has a deep relationship with the battery life. The deeper the depth of discharge, the shorter the charging life. Therefore, deep discharge should be avoided as much as possible during use. The storage battery discharge depth is about 10% to 30% for shallow cycle discharge; the discharge depth is about 40% to 70% for medium cycle discharge; the discharge depth is about 80% to 90% for deep cycle discharge.

    Generally speaking, the deeper the daily discharge depth of the long-term operation of the battery, the shorter the battery life, and the shallower the discharge depth, the longer the battery life. Shallow cycle discharge is beneficial to prolong battery life. The shallow cycle operation of the battery has two obvious advantages: first, the battery generally has a longer cycle life; second, the battery often retains more backup ampere-hour capacity, which makes the power supply guarantee rate of the photovoltaic system higher. According to actual operating experience, a more moderate depth of discharge is 60% to 70%.

    Excessive discharge mainly occurs when the battery supplies power to the load for a long time after the AC power supply is cut off. When a battery is over-discharged, it causes "sulfation" at the cathode of the battery. Because lead sulfate is an insulator, its formation will have a great negative impact on the charging and discharging performance of the battery. The more sulfates formed on the cathode, the greater the internal resistance of the battery, the worse the charging and discharging performance of the battery, and the shorter the service life of the battery. It is very difficult to redox the lead sulfate formed under the condition of low current discharge. If the lead sulfate crystal is not cleaned for a long time, it will inevitably affect the capacity and service life of the battery.