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From what you told me, that sounds like the battery is shorting out. The converter may be fading as well. A good one should put out about 13.7 VDC at the battery. Have your battery load tested and see if that is good or not and then check the output of the converter again. Then get back to me if we need to dig deeper into this. All follow ups are included in our fee.
Not sure you may have missed my earlier response to your question. Just checking back in to see if you were able to get this resolved or do you still need additional input from me here?