The T1 battery was a much larger version of the T30 battery used to power the microphones in field telephones. Like the T30, the T1 was rated at 1.5 volts.
Since it was too big to fit into the battery compartment of a field telephone, and it was only rated at 1.5 volts, it must have been used to power a microphone outside of a field telephone. These microphones are found in the handsets of the Großer Feldklappenschrank.
A wiring diagram of the Großer Feldklappenschrank shows the connection of a battery (MB+/-) for powering the switchboard’s handset microphone via a terminal block in the Untersatz A cabinet.
Since the T1 is too big to fit in the Untersatz A cabinet, it was probably kept externally behind the switchboard.
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WW2 German Field Telephone Equipment 