A parallel device for a battery module including a plurality of battery groups, comprising: a plurality of switching units, each comprising a first switch (k1, k2...kn) and a load (R1, R2...Rn) and each connected with a battery group in series to form a branch which is connected to other branches in parallel; a plurality of second switches (kr1, kr2...krn), each connected with a switching unit in parallel; and a controlling module, connected with the first switch (k1, k2...kn) and the battery group and configured to collect voltages of the battery groups, calculate the voltage differences between the voltages of the battery groups, compare the voltage differences with a voltage reference and control the first switch (k1, k2...kn) and the second switch (kr1, kr2...krn) to open or close according to the voltage comparison result, wherein the voltage reference comprises a first voltage reference and a second voltage reference and the first voltage reference is no less than zero and the second voltage reference is greater than the first voltage reference, and the controlling module is further configured to: control the first switch (k1, k2...kn) and the second switch (kr1, kr2...krn) to open, when one voltage difference is no less than the second voltage reference; control the first switch (k1, k2...kn) to close and the second switch (kr1, kr2...krn) to open, when all the voltage differences are less than the second voltage reference and one voltage difference is no less than the first voltage reference; and control the first switch (k1, k2...kn) to open and the second switch (kr1, kr2...krn) to close, when all the voltage differences are less than the first voltage reference. a plurality of switching units, each comprising a first switch (k1, k2...kn) and a load (R1, R2...Rn) and each connected with a battery group in series to form a branch which is connected to other branches in parallel; a plurality of second switches (kr1, kr2...krn), each connected with a switching unit in parallel; and a controlling module, connected with the first switch (k1, k2...kn) and the battery group and configured to collect voltages of the battery groups, calculate the voltage differences between the voltages of the battery groups, compare the voltage differences with a voltage reference and control the first switch (k1, k2...kn) and the second switch (kr1, kr2...krn) to open or close according to the voltage comparison result, wherein the voltage reference comprises a first voltage reference and a second voltage reference and the first voltage reference is no less than zero and the second voltage reference is greater than the first voltage reference, and the controlling module is further configured to: control the first switch (k1, k2...kn) and the second switch (kr1, kr2...krn) to open, when one voltage difference is no less than the second voltage reference; control the first switch (k1, k2...kn) to close and the second switch (kr1, kr2...krn) to open, when all the voltage differences are less than the second voltage reference and one voltage difference is no less than the first voltage reference; and control the first switch (k1, k2...kn) to open and the second switch (kr1, kr2...krn) to close, when all the voltage differences are less than the first voltage reference. control the first switch (k1, k2...kn) and the second switch (kr1, kr2...krn) to open, when one voltage difference is no less than the second voltage reference; control the first switch (k1, k2...kn) to close and the second switch (kr1, kr2...krn) to open, when all the voltage differences are less than the second voltage reference and one voltage difference is no less than the first voltage reference; and control the first switch (k1, k2...kn) to open and the second switch (kr1, kr2...krn) to close, when all the voltage differences are less than the first voltage reference.