13.8 is probably within specifications and should keep the battery charged. I'd check for a battery drain such as a light that stays on when it shouldn't. Check the car at night in total darkness. I found a door light on my SD that was slightly glowing that would discharge my battery over a week. The quick way is to check amperage flow from the battery when everything is off. There will always be some flow to the clock, etc. but not much. If you don't have a amp meter, pull the lead and check for the magnitude of the spark when you touch it to the battery - little spark = normal, fat spark = a problem. Of course, you could simply have had a bad battery. They do short out occasionally.

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