Neat idea, but I don't think there were any onboard batteries -- AFAIK power was just fed straight from the RTG thermocouples, more or less. Batteries would have just added weight, would have had a crappy recharge capacity after a while, and wouldn't have made much sense for a deep space probe which relied on an RTG instead of solar panels.

Also, the RTG might be powering instrument heaters, keeping them at warm enough temperatures to prevent failure. If that's the case, once those are shut off, there's no turning those instruments back on.

EDIT: It appears that Voyager 1's ultraviolet spectrometer instrument, at least, is pretty tough: as of January 2012, it has survived having its heater shut off for power conservation: http://www.nasa.gov/mission_pages/voyager/voyager20120117.ht...

That's one hardy spacecraft! =)