Hi
I remember when I did my first soldering repair ,it was on the same generation of iPod you have and I had this issue too!
Here’s how I fixed it
Check the 3 battery soldering pads on both the battery it self and the iPods logic board
Look for any deformity’s on it ,my issue was a couple things
1)in my case the Battery’s contacts (on the battery it self) was loose
2) make sure the battery’s cable is as flat to the board as possible
3)make sure you don’t have a cold joint so use plenty of flux
If you need further help with it give me a shout as I know the iPod touch 6 very very well
Thanks:-)