we all face problems from the fake capacity of power banks ,
mainly the power banks have pretty good circuits but the cells they give are very bad .
Here i am showing how to retrofit a cheapy power bank 1$ with good cells 18650 .
i n many countries the higher capacity power banks cost more
so we can use some good reliable cells and a damn cheap PB to make a reliable
usable device .
Step 1: The Refit
Grab a 2 cell power bank at low price ,
new cells sometimes come seperate ,
so need to scratch with sharp object and solder cells in parallel
install in to the case .//
Step 2: The Safety Cut Off
The litiums must cut at 4.2V ...
DANGER : some of the power bank charger circuits DONT cut so how to know ?
Well charge the cells for 1-2hours , monitor voltage
if it is crossing 4.2 , the cut is not there
So , instead of using OPAMPs etc , we can have a simple circuit like this to cut off
select zener as zevervolt + 0.7+0.7=4.2V zener can be 2.7V