I do things the way I do because people want it that way, I don't charge for anything, things seem to be working well, copied from a mate in Tucson who is electronics engineer using the exact same packs
It strikes me as peculiar logic that you rationalize doing something ill advised because "
people want it that way" and you "
don't charge for anything" as if that makes it all OK. As for your "electronics engineer" mate, it sounds like he doesn't really understand how li-ion batteries work despite his credentials. Just because "things seem to be working well" doesn't make it so either. Yes, you can charge a power bank while it is discharging and get away with it for a period of time but it will shorten the life of the cells and will eventually lead to a significant safety issue.
It is been fascinating to observe how many people here on this form rationalize the whole concept of "pass-through charging" despite the all the evidence against it as if wishful thinking is all that is required. It happens so often that we really should have a sticky on the subject.
What "is" required is the proper battery management circuitry of the type that exists in many if not most laptop computers, smartphones and tablets so they can be operated when plugged in but that very rarely exists in lithium-ion power banks because that is not what they are designed to do. They also cost nowhere near what a laptop or smartphone costs and this is another reason power banks generally don't have such circuitry. One reason Zendure power banks are so expensive is that they do have more sophisticated circuitry than other more typical banks.
The reason lithium-ion battery banks should not be charged while power is simultaneously being drawn from them is because it violates the electro-chemisty of how lithium-ion batteries function.
In a lithium-ion rechargeable battery as used in these types of power banks (typically 18650 cells) both the positive electrode (cathode) and the negative electrode (anode) bind lithium ions from lithium oxide in the electrolyte that migrate back and forth between the cathode and anode during the charging and discharging process. When the battery is
discharging, the lithium ions
moving back towards the cathode release energy (electrons) in the process and this is how the battery powers your device. When the battery is
charging, the lithium ions
migrate back through the electrolyte in the other direction towards the anode.
So,
if you charge the battery while it is simultaneously discharging, then you are forcing the lithium ions to move in opposite directions through the electrolyte at the same time which will
cause a lot of stress to the cells and this will eventually lead to a build-up of lithium metal plating which is the dangerous process that can ultimately cause a thermal runaway (chemical combustion), but for the most part will simply shorten the lifespan of the bank's cells and prematurely reduce its charging capacity. Lithium metal plating happens when lithium-ions in the electrolyte precipitate out into actual lithium metal which is highly reactive and flammable and burns fast and hot. This metal plating can cause a short circuit that will initiate a fire within the cell electrolyte if damage is caused to the extremely thin separators that keep the elements of the battery apart. A damaged battery cell will slowly start to swell and when it finally bursts hot chemical gases very suddenly come into contact with air (oxygen) and you have a vigorous fire or explosion on your hands, especially if this happens inside an enclosed space like a battery bank housing.
It is bad enough that we subject high powered lithium-ion power banks to temperature extremes, vibration and shocks when using them in our vehicles to run dash cams but stressing them further by charging during discharge is really asking for trouble.
Some manufacturers falsely claim that their products (especially generics) accommodate pass-through-charging knowing that you are unlikely to notice any problems or loss of capacity until long after purchase but this practice should simply be avoided.