I don't know if that's normal. Most of my training has been early morning in the dark with those battery powered lights so they get used almost every day rain or shine, and my training bikes don't have fenders. I think it's the water and vibration that eventually kills them. I have the same issue with every headset I've tried except for CK. The Ixon lights aren't all that expensive in the grand scheme of randonneuring expenses, especially compared to dynamo lighting, so I just bought a few spares.

Another tip is to get a bunch of eneloop rechargeables and a smart charger.