I have to agree here.
Definite signs of Arching on Battery and
device Negative. May be a weak Soldered wire off the Contact or at the Board input side.
Either way, it either needs to be Soldered properly or returned for the factory to correct/scrap.
Best to Stop and decide which way you wish to approach the correction.
New, I would return with pics of the Damage being caused by and to the
mod.
As stated earlier - Likely a one off occurrence but they do happen. Replacement would be Prudent.
Edit: Adding this for reference:
Reference to Negative path ^^^^^
Part of the Temperature Control - D2
thread