Abstract: A handheld, battery-powered telecommunications test instrument with a talk battery circuit capable of supplying power for up to four telephone sets, which may be connected either in series or in parallel. The instrument determines the configuration of the telephone sets as having serial connections or parallel connections by sensing the amount of current flowing through the circuit of the invention. In response to the amount of current, a transistor automatically operates to supply power to the telephone sets as either a voltage source or a current source and maximizes power transfer between the telephone sets. The instrument has a low power mode to reduce current drain on the battery that is automatically engaged when the telephone sets are not in use.