John Deere LX178 battery light comes on when I turn on lights or mower deck

SOURCE: my 1979 111 john deere lawn tractor deck blades shut off

If it has a clutch, that would be the problem. An electric clutch will kick on and off when it is hot if it is bad.

SOURCE: Why isn't the center blade on a john deere LX178

Of course with everything off, reach under the deck and turn the center blade by hand. If:
1. the blade turns by itself and nothing else moves, you need to tighten the retaining bolt more.
2. the blade, retaining bolt and drive shaft turn, check to see if the pulley above it is turning.
a. If the pulley turns freely check that the drive belt is going around it to power the blade.
b. If the blade turns with the drive shaft and the pulley doesn't, check that the pulley is still properly connected to the blade shaft. Some have a key in them that can split, others just have a bolt on top holding them tightly on the shaft.