I have a 7.4V battery pack consisting of 2 x 18650 cells.
One of them has 3.4 V, so I tried to charge it with a good old TP4056 board, and it worked:
The second 18650 cell seems nearly dead with ~ 0.1V. When I connect the TP4056 charging board, it doesn't seem to charge (blue light: full).
Is there a way to give an initial "boost" to a dying 18650 cell, to bring it to 2.5 V, so that eventually a TP4056 charging board will recover it?