Actually, the record for that was just set. Can't remember the exact time, but sub 20 hours.

From TFA:

"To provide enough power, we need certain methods with high energy density," said Jae Kwon, assistant professor of electrical and computer engineering at MU. "The radioisotope battery can provide power density that is six orders of magnitude higher than chemical batteries."

Power density.

My daughter is still asking for the pink slashdot back.

Exactly.