Hi, my dad has an mtd 17 1/2 hp gear drive mower, just after he bought it, it started blowing the fuse. Warrentee took care of that one. That was six years ago, now its blowing fuses again. Replaced the fuse again, but then I installed wire loom on every wire I could get my hands on. The wires coming down from the key switch I noticed rubbed through and into the wire itself, so I taped the wire, loomed that gang of wires also, and yesterday, dad calls and sez it won't start, I mean turn over, he put a new fuse in and it works. I have noticed that the starter goes slower than when it was new. The key has to be turned sometimes a couple of times in order to engage the starter to turn over... I don't know where to look first now...
Oh, 17 1/2 opposed twin, and runs super...

How old is the battery?

The battery is about two years old

Have it checked out, 2 years is about the lifespan for most.