I have one of the NST2036 36V Battery string trimmer. It lasted about 4 seconds. But I am exchanging it.
I am curious about the possibility of converting the battery pack to a better technology, but they have a seal on the seam so I can't open it up if I hope to return it. If anyone has a dead battery pack they would send me I can take it apart and see what can be done. If I find a solution I will share. The battery pack does not need to have the batteries in it. I just need the case.