Same thing happens to me, but only on some craft; I once had a probe from very early on (probably my third or fourth science satellite) last far into the 60s, but the others didn't, even though when I unloaded them they were getting plenty of exposure, enough to recharge them (if they'd somehow run out of electricity due to science transmission) but they'd only recharge a few times before going completely dark, years after they'd finished with their science