Engine battery

After 2 and a 1/2 years of new operation, I have to change my engine battery which was a DETA950 AGM, www.deta.info/dk950. ; I do have solar panels which charge the service battery and engine battery well.  And we just did a 2 and a 1/2 day trip to the Bahamas involving some motor sailing with no issues or problems, but the engine battery is not taking a charge and the folks at a local Napa auto parts store said the battery has sulfated, which could result from lack of use. Historically, we have left the boat since new for up to 3 or 4 weeks at a time, without any issues. But again, the solar panels have indicated a successful charge, as indicated by the sentinel monitoring system. Curious, if any 588 owners have any issues with their engine battery? The cost to replace at the Napa auto parts store was about $230, which is not too bad. In my opinion. I'm just surprised at the short time lapse since new.  ...of course the dealer and the factory are pointing figures at the battery manufacturer and at volvo penta, which is standard in my experience that neither the factory nor the dealer like to take ownership of any problem post delivery.