The invention provides a method for preparing a
lithium ion battery pack, and the method comprises the following steps: carrying out
vacuum drying on a plurality of assembled
lithium ion batteries, injecting part of
electrolyte, carrying out constant-current charging to a first preset
voltage, and carrying out constant-
voltage charging at the first preset
voltage until the charging current is lower than a
cut-off current, performing constant-current charging on the battery to a charging
cut-off voltage, measuring the surface temperature of the battery, and recording the charging electric quantity of the
battery voltage rising from the first preset voltage to the charging
cut-off voltage for the first time; determining to inject a first supplementary
electrolyte or a second supplementary
electrolyte according to the temperature value; recording the charging electric quantity of the
battery voltage rising from the first preset voltage to the charging cut-off voltage for the second time, and recording the capacity of each battery; and assembling the batteries of which the difference percentage of the charging constants and the capacity difference percentage are within preset values into one group to form a
battery pack. The
battery pack prepared by the method disclosed by the invention is high in service life performance and good in consistency between single batteries.