Make sure your battery cables are all solid/intact, especially at the battery clamps. If you have broken wire strands at the clamps then that could be shortening the life of both the alternator & the battery. This happened on our '02 Focus wagon (went thru 2 batteries & 2 alts) & was only fixed after rebuilding the positive cable's battery clamp.
The alt-to-starter cable could also be a candidate for problems, but that cable doesn't usually see the amount of disconnecting that the battery cables do, so it's less likely to be a problem.
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