.

Despite the claims from certain producers of power banks that their products can handle it (pass-through charging), they should not be charged while they are under a load.

The reason lithium-ion battery banks should not be charged while power is simultaneously being drawn from them is because it violates the electrochemisty of how batteries function.

In a lithium-ion rechargeable battery 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 to the cathode release energy (electrons) in the process and this is how the battery powers your device. When the battery is being charged, 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 trying to force 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 will eventually lead to lithium metal plating which is the dangerous process than can ultimately cause a thermal runaway (chemical combustion), but for the most part will simply shorten the life of the battery dramatically and reduce its capacity.

@Dashmellow, I am trying to reconcile your statements above with my personal experiences. Since a Lithium battery is a Lithium battery is a Lithium battery..., I tried to apply your statements to my personal experience with my Lithium battery powered laptop. The laptop's original Lithium battery lasted 5 years (2 years ago I replaced it) even as I repeatedly and used the laptop while the AC cable was simultaneously connected to the laptop and visibly charging the laptop's battery. In addition, for 7 years now, when I leave for work, the laptop stays home connected to the AC outlet on the wall and with its Lithium battery re-recharging. I haven't experienced any negative consequences from these practices. Regards.
 
@Dashmellow, I am trying to reconcile your statements above with my personal experiences. Since a Lithium battery is a Lithium battery is a Lithium battery..., I tried to apply your statements to my personal experience with my Lithium battery powered laptop. The laptop's original Lithium battery lasted 5 years (2 years ago I replaced it) even as I repeatedly and used the laptop while the AC cable was simultaneously connected to the laptop and visibly charging the laptop's battery. In addition, for 7 years now, when I leave for work, the laptop stays home connected to the AC outlet on the wall and with its Lithium battery re-recharging. I haven't experienced any negative consequences from these practices. Regards.

Laptops, smartphones and tablets generally have built in battery monitoring electronics/management schemes and so they often can tolerate constant charging more than the subject of this thread which is external power banks, which have charging circuitry but don't benefit from being connected to a computer with a powerful CPU and an operating system. Often when you leave your laptop or smartphone plugged in it may not be charging at all times like you may think it is. Aside from my description of how lithium-ion battery chemistry functions there is also the issue of heat which is another important concern that can degrade batteries, especially in laptops. People here on DCT keep confusing the issue of what they might do with their laptop computers and how they would like to rationalize charging a battery bank while it is simultaneously discharging in their car but it is just not a good practice, especially because unlike your laptop left charging at home battery banks in a car are subject to shock, vibrations and external heat. Well known companies who produce external power banks like Anker previously used to promote their products as being capable of "pass-through charging" but they no longer recommend or advertise that feature. I imagine there is a good reason for that change in their policy and product promotion.

In any event, many people do what you do but battery professionals recommend not leaving your laptop plugged in all the time. Some people have better luck leaving their laptops charging all the time than others. A number of years ago my laptop's battery slowly began to swell up until it ruined the keyboard. When I took it in for repair they told me to remove the battery if I was going to leave it plugged in for long periods of time.

Keeping Your Laptop Plugged in All the Time Will Kill Its Battery Faster
http://www.wired.com/2013/09/laptop-battery/

Reminder: Don't Leave Laptops Plugged in All the Time

http://www.tested.com/tech/pcs/458365-reminder-dont-leave-laptops-plugged-all-time/
 
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