If you charge a powerbank flat out and simulateously discharge at full power on every output for a long period then it may get a bit stressed, and if it is a poor quality one then it may fail, but if you are just running 1 or 2 low power B1Ws while not actually charging the powerbank they are connected to because it is already full then it will be closer to being underloaded than overloaded, and nothing to worry about! I've got one powerbank that wont run a B1W, it turns off because the B1W uses so little power the powerbank thinks that it has fully charged the "phone" connected to it. I also have one that will turn its outputs off while it is charging, most inconvenient.
The battery cells only get stressed by the heat if you overload the power bank, as long as it is running cool then there is no problem.
This is merely wishful thinking as there are numerous reasons not to charge lithium batteries, especially high power battery banks while discharging.
Although most laptop computers, smartphones and tablets have the required battery management intelligence and circuitry to accommodate pass-through-charging this is reflected in the price of these devices along with the fact that they have sophisticated operating systems. Power banks are relatively inexpensive basic devices that are primarily designed for occasional charging of mobile electronics when away from mains power and therefore for the most part, were never intended to be charged while discharging during full time powering of other devices.
As explained by Isidor Buchmann, the man behind
Battery University, the founder of battery charger company
Cadex and author of
Batteries in a Portable World: A Handbook on Rechargeable Batteries for Non-Engineers,
"A portable device should be turned off during charge. This allows the battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low enough by drawing a leakage current. A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress."
Most authorities on batteries and many manufacturers advise against pass-through-charging of power banks. Anker, for example, originally claimed that their power banks could be charged at the same time they are being discharged but they now strongly advise against it.
As Buchmann explains on Battery University, pass-through-charge causes "mini cycles" (rapidly powering on and off) where a battery charger's circuitry will become confused and eventually damaged as it constantly hovers just above and below the charge threshold.
Anker agrees. For example, here are some of their responses to the oft asked question about pass-through charging they receive on their web site forum. Using a daisy chained power bank rather than a wall charger makes no particular difference here. In fact, it is worse because it will eventually damage both power banks.
Charger Harbor, a review service specializing exclusively in Power Banks suggest you might be able to use pass-through-charging in the short term but they too advise against the practice.
For some peculiar reason a great many people here on DCT engage in a form of "magical thinking" about charging power banks while at the same time discharging using the odd belief and mental image that lithium batteries are somehow like a bucket full of water with a little hole in the bottom that you can simultaneously keep refilling with an equal amount of water from a spigot to keep it topped off, but this is a false metaphor. Charging a lithium battery is a multi-step process depending on varying charge and current factors. It's not a contantly replenished bucket full of electricity with a hole in it.
Aside from the charger related issues with pass-through charging, 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 dendrites and ultimately 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. Dendrites are basically whiskers of lithium that grow inside batteries and can cause the devices they’re powering to lose power more quickly, eventually short out, and in some instances, catch fire. 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 lithium metal and dendrite build-up 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. These power banks are very energy dense devices that should simply not be abused or use in ways they were never designed for.
There are some alternative, safer lithium battery chemistries that are entering the marketplace that are better suited to our purposes using power banks in an automotive environment such as the
TQKA 20,000 mAh lithium iron phosphate (LiFePO4) that some of us here on DCT have recently been experimenting with but it would still be unwise to attempt to power
any lithium type battery during discharge unless it is specifically designed with the proper circuitry. It's simply a BAD idea no matter how one wants to rationalize that it is somehow OK.
BTW, there is no indication whatsoever that the Poweradd X7 power bank uses lithium manganese cells. The
Amazon sales page for the Poweradd X7 states that this banks uses lithium polymer cells. The
Poweradd web page for the X7 itself merely states that the X7 uses "Lithium Ion" cells. I am not currently aware of any currently available standard consumer power bank on the market using lithium manganese cells but these cells are becoming more popular in more and more devices so I guess it is possible. Currently, they seem to be used primarily in medical devices EVs, and a growing number of other professional tools.