No, it's not normal. If your alternator is getting ready to go kapoot on you, which sounds like the case, the cap won't really be effective. In fact, it may add a little more stress to the electrical system at this point. The purpose of a cap is like a small secondary battery kind of...it provides a little extra juice to your amps when they need it for heavy bass notes, etc. By no means do they act as a fix all to an electrical system, unless your amp is needing a LITTLE more power than what the electrical system can dish out. Make sense?