That is a bad battery. Sounds like sulphated plates where the battery was deep-discharged for a long time, such as sitting on a shelf somewhere. It will build up a decent surface charge (the 13.1 you see) that that is temporary and it wont be producing decent amperage. had that same issue when I bought a new bass boat in 1992. Thing had a pair of stinger batteries for the 24v TM and they were essentially dead. Got two more (under warranty) and they were bad within 2 months. Went to Delcos and only had a problem every 2-3 years as they would age away and get weak. The best batteries are sold “dry” and you add the Sulphuric Acid when they are placed into service. Otherwise a wet battery will self-discharge and if it is a “slow mover” in that retail outlet, it can go bad just sitting on the shelf.2008 Pantera Classic2014 Mercury Pro XS 200
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