Battery boxes are power cells that can be used as power storage. This is useful when the power supply input is unreliable such as solar which only works during the day. By setting up a bank of these, any power generated that is excess to daytime usage needs can be stored to use at night, meaning an uninterrupted power supply.
Battery boxes can take input, via cables, from any side but the output *must* be from the side with the grey square as shown below. The difference between the sides is subtle.
Battery box showing output side