Complex sentence: The voltage during discharge remains practically constant at 1.35 volts, and the capacity is much greater than a similarly sized zinc carbon battery, From the time of World War II mercury batteries became a popular power source for portable electronic devices.
Split into simpler sentences: The voltage during discharge remains practically constant at 1.35 volts, and the capacity is much greater than a similarly sized zinc carbon battery. For a time during and after World War II, batteries made with mercury became a popular power source for portable electronic devices.