Noname powerbank stopped charging. How to detect and fix the problem with a minimum of devices?

Question

Batteries

Relatively new but unnamed bank, inside 4 green 18650 cells from Samsung, soldered in parallel. The charge indicator of the bank itself blinks, but the situation does not change. On parallel soldered elements, the voltage is 3.2 V now. I tried the "babkin" method with a freezer, it did not help.
Tell me, please, what could be the matter, how to find the problem and fix it? At the disposal is: a multimeter, a soldering iron, a new high-quality 18650 element, another working bank.
APD. After a while, he returned to the repair. The new parts are:
- 4 Samsung ICR18650–26F elements in parallel;
- the total voltage on them was 2.7;
- after charging for 12 hours, the voltage increased to 3.2;
In theory, if not all the elements have deteriorated, then it should have been quickly charged up to 4.2, or not?
In this case, very slow charging, it turns out that either there is not enough current (the controller has deteriorated), or all the elements have deteriorated at once?

Answer 1

Now I will describe the horror method.
1. take 3 AA batteries, solder them in series, attach them to the controller outputs (so that they don’t go into defense, although, if they are literate, they left a long time ago).
2. cut off the battery pack
3. Separate the batteries
4. Measure the voltage on each (after 10 hours of shutdown)
5. Throw away those less than 2.5 V or recharge (!!! explosion is possible)
6. Recharge the rest, either from finger (salt), or from a PSU (no more than 500 mA), up to a voltage of 3.1 V, be sure to look at the heating.
8. Collect and use.
It's easier to buy 18650 elements.
Yes, if you die, don't blame my instructions (many services use them)