I upgraded to an Xtar VC4S charger.
It can test and rate a cell. It can answer questions like capacity or ability to deliver current.
It's best to have as near matching cells as possible in a multi-cell applications.
If the cells were matching when You placed them all into service, most replacements won't match the (good)cells that are still in your multi-cell configuration.
I would retire the other working cells to single cell applications and upgrade the multi-cell application(s) with new matching cells as needed.
I can say Your attentiveness with the use of Li-lion cells is a good safety practice.
I was going to get the Xtar Dragon, but the SkyRC MC3000 changed my my mind. The ability to update it's firmware is what convinced me.
And yup, i am now using brand new cells plus i upgraded my power bank that now supports QC charging and uses 6 instead of 4 cells.
I like the fact that this new power bank has a locking mechanism to keep the cover from sliding open and how the metal contacts are tucked away from being exposed unlike the old one.
These are the brand new 18650GA, and the seller was liionwholesale. They threw in 3 battery cases plus a 18650 XML-T6 flashlight for free.
I do notice a "B" marking on all 6 cells. So i am guessing it's an internal grouping letter?
As for the old cells. They will be used for light duties such as now my single cell flashlight, and bluetooth radio.
This will be my 2nd set of cells since introduction to cylindrical li-ion world. Before that, it was just eneloops, sure they were safe, but they just don't cut it these days anymore at least for what i use them for. AA power banks are a joke lol.
And I hate devices with sealed-in batteries from x brand company as you are at their mercy should something goes wrong with that device lol.
And lastly, Awesome seller if looking to buy new cells.