A containment system for batteries of a data transmission equipment enclosure is disclosed. The system includes a number of identical side plates and integral top flanges for positioning around the batteries along with a front retention bar that has a series of slots for mating with slots formed in the side plates. A top channel frame member is connected to each of the side plates and to the enclosure. The system when installed in an enclosure prevents sideward, frontward and damaging upward movements of the batteries in response to seismic events. The system is careful, however, not to block the region under the batteries so that a heating pad can easily be installed if desired.