I appreciate your thoughtful reply but I have to disagree with various aspects of your post.
While the issue of operating temperatures for lithium cells is indeed very important, this has nothing to do with the subject of "pass-through charging" so I'm not sure why you bring it into the discussion.
In any event, I have also previously referred and linked to the same article from Battery University you link to here and in fact, it is specifically one of the several sources that informed my understanding of why "pass-through charging" is not a viable method for lithium-ion batteries, as I will explain. So, I am wondering how carefully you read the article you are referring to. Perhaps you didn't read the whole thing?
I think when having a discussion like this we need to compare apples to apples. For this reason I do not believe that your example of charging a smart phone overnight is the same as constantly charging a phone while you are actively using it. Phones are not designed to be charged during use even if this will work for shorter periods. While the phone might reach a voltage cut-off at some point while using it, it will stress the battery. Similarly, and perhaps more closely related to external battery banks, I've often seen people develop problems with laptop computers that they leave plugged in all the time. The battery packs in these laptops overheat, suffer from shorter run-times, swell or fail prematurely. This laptop issue is specifically addressed in the Battery University article in regard to so called "pass-through charging".
"Some portable devices sit in a charge cradle in the on position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic load while charging because it induces mini-cycles, but this cannot always be avoided; a laptop connected to the AC main is such a case. The battery is being charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the 4.20V/cell threshold."
I'm also not able to agree with your remark that, "battery chemistry should have nothing to do with it". I believe that battery chemistry has everything to do with it. Lithium-ion batteries have a positive electrode that is made of Lithium cobalt oxide (LiCoO2). The negative electrode is made of carbon. When the battery charges, ions of lithium move through the electrolyte from the positive electrode to the negative electrode and attach to the carbon. During discharge, the lithium ions move back to the LiCoO2 from the carbon. You simply can't effectively do both at the same time! This is likely the primary reason this battery chemistry doesn't tolerate "pass-through charging". Trying to force the cell to do both opposing cycles simultaneously just places the cells under extreme stress and will damage them. In addition, as explained in the linked article, unlike lead acid batteries (that will happily run with pass-through charging) lithium-ion cells do not accommodate trickle or float charging. Either it reaches the required voltage cut-off or it doesn't. If you are constantly hovering near that voltage point (above or below) you put a lot of stress on the cells as well as on the electronics attempting to provide the specific voltage.
As for the Zendure battery pass-through charging scheme, whether you raise or lower the amperage you are using to charge the battery bank you still end up charging the cells at the same time you are discharging them.
As I've mentioned, this specific BU article was one of the several sources I consulted before I posted my earlier remarks about why pass-through charging is not a good idea. In fact, in those remarks I paraphrased some of what is said in that article in reference to this subject.
Here is the exact quote from the article you linked from Battery University:
"A portable device must be turned off during charge. This allows the battery to reach the set threshold voltage unhindered, and enables terminating charge on low current. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low. A battery may be fully charged, but the prevailing conditions prompt a continued charge. This causes undue battery stress and compromises safety."
People seem to want pass-through charging so they can run dash cams in parking mode using battery packs and not have to worry about keeping them charged. Personally, I believe the best practice would really be to hard-wire one's cameras into their car's 12V electrical system with an appropriate low voltage protection device. One of these days I plan to get off my butt and do just that, although it's beginning to look like that's not going to happen until winter is over. In the meantime, I often run 4 separate dash cams in my vehicle in parking mode using two low priced
generic Chinese battery banks but I go to the trouble of bringing them inside and charging them properly overnight. So far, proper charging and discharging has given me excellent performance and longevity from these units.
I don't think that's entirely correct, the battery chemistry should have nothing to do with it. A properly designed charging circuit, which it sounds like the Zendure could have, would do exactly what they described. The input power is passed through to the load and if the demand does not exceed the supply, then the remaining power is used by the charging IC. The load should not affect the charging circuit's ability to monitor the battery's state of charge because the two are not directly coupled. This is how the charging circuits in most cell phones have worked for years and they seem fine. My last smart phone was almost 3 years old and still had probably 75% of the original capacity, and it was topped off and left connected every night.
What lithium ions don't like are high operating temperatures and being over discharged or charged by a constant dumb circuit, like those cheap e-cigs, which just connect a 5V USB source to the battery and allow it to exceed the 4.2V design limit or have excessive charging current during the saturation charge stage. You might be able to get extended service life by keeping the maximum charge below 85%, but the loss of runtime is generally less desirable than just dealing with earlier replacement. At the typical prices of these packs, I think many consumers would be like me and opt for longer runtime and reserve capacity over replacing the unit a few months sooner.
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
KuoH
