I would not pass sentence on the OBC yet. You could get the same conditions with a bad cell in a battery.
The brown spot on the OBC could be a sign of failure as well. If it is bad there are kits available to bypass the OBC and chargers that do not require an OBC to operate.
My recommendation is to get it checked out before your final decision as to cause and correction. Guess work could be costly and could not correct the problem.
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