A battery pack including a plastic protective layer bonded to the surface of a cell by spreading over the surface of the cell a polyurethane emulsion having a reaction product obtained by emulsifying and dispersing an intermediate product produced from a compound A made of an organic diisocyanate, a compound B1 made of a polyol mixture having not smaller than at least 2.05 average functional groups and a compound B2 having one hydrophilic center and at least two active hydrogen groups in water.