1. The four batteries used in series (such as electric vehicles) on the equipment have the same discharge environment. If there is a big difference in the voltage of the four batteries, it means that the battery with low voltage is seriously aging, and the four batteries should be replaced together.
2. If it is a new battery, you should choose a battery with the same production date (the date is as close as possible) and 4 batteries with the same voltage. This kind of battery is easy to use.