I half agree with the person who said something is wrong with the converter.
The reason it's only half is there is a "Suspect #2" (or perhaps #1) and that is the battery.
Either the converter is putting out high voltage (your lights will be super bright and likely not last long story follows) or the battery MIGHT have one or more cells shorted (your volt meter is your friend.. Resting should be 12.6 Put a small load on it (I like automotive test lights.. Look like an ice pick with a bulb in the handle and a wire with clip lead.> Add 25-50 feet of say 18 or 20 ga wire (with a clip on one end and a ring terminal on the other) and you have a very useful tool.... Test fuses swiftly.. Find broken wires (12 volt side of life only) and more very fast and very easy)
If the light goes dim after a few seconds.. Shorted cell (you don't need the extension ground lead for battery testing)
Oh the extension lead.. Like I said put a ring terminal on one end or strip back an inch or two twist loop and solder This is so you can clamp the test lights clip on and a clip on the other.. Then Fold in half and re-roll on the spool it came on (or put one end through the center holes and move it to the middle and re-roll. That way you only need to unroll enough when you need it.
But use a volt meter even if it's a 10 buck horrible frieight special to measure battery and House voltages with the battery disconnected.... Post results.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times