Both Voyager 1 and 2 use RTG's (Radioisotope Thermoelectric Generators)[1] for power. Every year the RTG puts out less and less power, so instruments are gradually powered down to preserve the functionality of the critical ones for as long as possible.

According to JPL[2] at least one scientific instrument will continue functioning until about 2025, and the crafts will remain in range of the Deep Space Network until 2036.

[1] https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

[2] https://voyager.jpl.nasa.gov/frequently-asked-questions/