PRESSURE MAINTENANCE DEVICE AND BATTERY SYSTEM INCLUDING THE SAME

A pressure maintenance device includes: a hydraulic device inside a housing of a battery module together with a plurality of battery cells, the hydraulic device being configured to change its volume to offset a pressure change inside the housing due to deformation of the plurality of battery cells; an accumulator connected to the hydraulic device through a pipe, the accumulator being configured to store fluid and to discharge the fluid to the pipe or to store the fluid introduced from the pipe according to a change in a volume of the hydraulic device; a heater configured to heat the accumulator; and a controller configured to control the heater according to an outdoor air temperature.