If it is a high voltage issue, the problem is not so much about damage to your panel, necessarily, but to things like your fridge's control board. You were smart to disconnect things immediately. Hopefully nothing was damaged.
I agree, it could be a bad cell, but I would be more inclined to suspect the used converter. They ARE known to go haywire. BTW... converters have been around for decades... they are not new.
As mentioned, you will need a few simple tools for diagnosing and regular maintenance... a digital multi-meter, a clamp meter, a kill-a-watt meter, and definitely a decent (turkey baster style) hydrometer.