Problems charging my NS device 19 Nov 17

I am having problems with recharging my nerve stimulator device.  When I have it charging, it overheats after 3-4 hours.  It beeps and a warning sign comes up.  The charging efficiency is usually 4 squares.  This has happened for 4 nights so far and the past two nights I have not charged past 3/4 as I have stopped when the arial gets too hot.  Last night, Saturday, I fell asleep while I was charging and I was woken by the beep telling me I had overheated.  I had noticed I was very tender over the device when I started, and when it overheated it was painful - I thought I had burned my skin.  Despite that, as it was nearly midnight, I just took it off, noting I had 3/4 charge, and went to bed.  

This morning, Sunday, I asked the nurse to look at my wound.  I have more than 10 staples in the wound.  It is red but not burned.  It is not so tender today.  I charged a 1/4 charge after lunch, for an hour, and I intend to charge again this evening as much as I can.

I am ensuring there is nothing in the way of the charging process.  I lay on the bed with nothing over me so the charging process won’t overheat because it is covered.  I cannot understand why it would overheat - perhaps it is the staples?  Or the dressing over the wound making the charging process harder thus leading to overheating?

I am contacting the Medtronics product manager tomorrow to ask for help.