If your converter has a smart charger it will stop charging for a millisecond, sample the battery
voltage, and then either start charging again or change to a float charge level however it will continue to do the sampling. When you disconnected the battery
it stopped checking.
OR: Could you have something wrong with the battery
. It is rare but it happens. My son had a battery explode in the car one day when he started it. We suspect it had some kind of a short inside and the spark ignited the gas. The battery was located inside the trunk thus not vented like it would have been if it was under the hood.
If you can check the water levels inside the battery (One with the caps on top) use a Hydrometer (Around $5 at an auto store) to check that you do not have a bad cell.