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Word: Potential Problems with Utilizing a USB Powerbank as a UPS

Word: Potential Problems with Utilizing a USB Powerbank as a UPS

2022-10-04 02:15:21

On my latest evaluation of the Blitzwolf BW-P12 10000mAh Power Bank, a reader commented asking whether or not that specific energy financial institution could possibly be used as a makeshift uninterruptible energy provide (UPS) for USB-powered units. That is an wonderful query and one I’ve at all times needed to convey a few of my ideas about, so I made a decision that it deserved its very personal put up.

Does it? Does it not?

If you need a easy reply, sadly, you’re normally out of luck. When shopping for a power-bank, hardly ever do they are saying very a lot about whether or not they work as a UPS or not. More and more, merchandise would possibly often state that they help pass-through charging, however that’s not a common follow and what meaning in follow can have an effect on sure purposes.

For instance, the Blitzwolf BW-P12 is a good instance – the manufacturer’s website doesn’t point out pass-through charging, however the iSmartware SW6208 datasheet states the next:

The SW6208 helps charging the battery and supplying extern machine concurrently, and this perform might be disabled if not needing. When just one port activates, it helps quick cost enter and output. When two or three ports activate, it solely helps 5V enter and output. On this scenario, charger maintain voltage is about to 4.8V. if enter voltage drops because of the weak energy provide of adapter, cost present will mechanically fall down so the enter voltage holds to 4.8V and provides extern machine in precedence.

It might appear that from the datasheet, until explicitly disabled by the producer, the controller would help pass-through charging functionality.

However what does this imply in follow? The best technique to know is to strive it out. I enlisted my “on-loan” Rohde & Schwarz NGU401 SMU to behave as a load on the output of the BW-P12, sinking a relentless 500mA to simulate a reasonably mild load. I used my ZMI zPower Trio USB charger to supply energy to the BW-P12 whereas monitoring the output voltage utilizing a Rohde & Schwarz RTM3004 oscilloscope (though FastLog would have been enough for this goal as nicely).

The outcomes are attention-grabbing to say the least. The above is the hint from a USB-C load connected to the ability financial institution whereas a USB Micro-B cost cable is connected. The output voltage collapses to about 4.2V for 80ms, then down to completely nothing for 960ms. Which means no matter load is related will see its energy out of spec for a few second throughout a charger plug-in occasion. The output energy does return after this, however sits at round 4.8V because the system tries to “steadiness” the ability going into the battery and out to the load with out overloading the ability adapter. The output voltage does have a loss due to the resistance of the MOSFETs within the energy path as nicely.

Doing the identical however as a substitute having the load on the USB-A port and the charger be on the USB-C port leads to a barely extra beneficial results of no output for about 125ms. It is a a lot shorter time, however when mixed with the 80ms of 4.2V, signifies that the connected machine will probably be uncovered to out of specification energy for a few fifth of a second.

Herein lies the drawback – simply because you could have pass-through charging does not imply that your output is uninterrupted. In case you’re simply powering some system that must be operating and will deal with quick energy interruptions, this is perhaps acceptable (topic to different caveats which I’ll clarify in a second). But when your machine completely requires uninterrupted energy, comparable to a Raspberry Pi that will incorrectly shut-down with potential data-corruption outcomes, then this isn’t going to work by itself. You could possibly design one thing to work round this to some extent – maybe a pleasant Schottky diode feeding a big supercapacitor financial institution with some cost present limiting to keep away from over-current throughout charging, however now you’ve actually designed your self a small UPS!

Design Issues

Maybe it’s a good query to ask ourselves why this behaves the way in which that it does. The everyday software schematic within the datasheet is of nice assist.

On this specific design, all of the inputs and outputs are basically commoned collectively into one web, however every of the inputs and outputs has a MOSFET swap permitting for some management and is immediately related to the IC so it may possibly sense if the port is connected to a charger or load. The IC can be the one switching controller in the entire present, so it’s going to be busy fascinated about what to do.

From what I can collect, whereas supplying a load, the unit is boosting from the Li-Poly cell to generate a gentle output on the requested voltage. As soon as a charger is connected, the IC can detect the presence of voltage and signalling strains – it has to cease boosting the voltage (therefore the drop to ~4.2V), flip off the output (nicely, not essentially but it surely does), begin the charging into the cell (maybe performing like a buck converter) after which flip the output again on. This energy path administration takes time, partly to make sure the suitable currents/voltages are monitored and that inrush is managed, thus the output drops-out for a bit.

However it doesn’t should be like this – a few of my favorite Xiaomi energy banks will not be designed this manner. Take for instance, the Xiaomi Power Bank Pro – this one doesn’t use an “all-in-one” chip and is definitely constructed on a extra “discrete” precept.

By that, I imply that the unit has a separate charger and a separate enhance converter, all orchestrated by a microcontroller. Which means they’ll function independently – the 2 inductors additionally signifies that it’s able to completely different voltage fast-charge out and in concurrently. The worth for that is further complexity.

Certainly, I attempted measuring this on the oscilloscope when the enter was being related and disconnected – completely no interruption in anyway. A straight line is just not so thrilling, so I didn’t hassle with a screen-grab.

Oh My! Have a look at the Time!

Having simply discrete parts is just not enough to maintain issues operating easily. The easy matter of “steadiness” signifies that you might want to make sure the charging price is at the least as quick because the depletion price, in any other case the pack might deplete itself underneath load. That is particularly related to older generic power-banks which can have used cheaper TP4056 linear regulator cost ICs which regularly had currents of 800-1000mA into the cell.

However it’s also possible to be bitten by one other “characteristic” of charger ICs – the cost security timeout. Most charger ICs on their very own count on to be charging a cell with none further load on the cell. Nonetheless, in case you’re working like a UPS, you can be taking present from the ability financial institution. In a super design, the ability path might be able to juggle calls for assuming a single voltage and have the load served from the enter energy, however this isn’t the way in which the Xiaomi energy financial institution above works.

As an alternative, in that energy financial institution, it behaves extra like an “on-line” UPS – every little thing coming in has to undergo the charger and into the cell, and every little thing that does out has to return from the cell and thru the enhance converter. Whereas this ensures a clear and regular output, it comes with effectivity losses.

Many charger ICs have a security timeout which is normally set to a while between eight hours to underneath 40 hours to make sure they don’t hold placing vitality right into a doubtlessly partially-shorted cell. That is to scale back the chance of catastrophic hazard from lithium-ion cells. I’ve not used my Xiaomi energy financial institution as a UPS for lengthy sufficient to know if there’s a time-out, however I do know many standalone charger ICs do have a timeout, during which case the charger will merely cease operating leaving the cells to deplete for no good cause.

See Also

Treating Li-Ion/Li-Poly Batteries Effectively

One other difficulty with the entire idea is just that it’ll usually abuse the lithium-ion and lithium-polymer cells. It’s because they’re normally designed to be charged in a CC/CV regime, terminating at a set present degree to keep away from overcharging. Whenever you run them in a UPS position in the way in which the Xiaomi energy financial institution does, then it’s uncovered to the warmth from the circuitry and goes to expertise “micro-cycling” at a high-state of cost.

In truth, it’s well-known that retaining Li-Ion/Li-Poly batteries at excessive states of cost for lengthy intervals contributes to speedy lack of capability and issues with swelling. My private expertise appears to indicate that is extra extreme for extended-voltage Li cells comparable to these used within the Xiaomi energy banks and trendy cellphones which attempt to maximise vitality storage density. The result’s usually swollen cells which current a security threat.

To deal with the cells nicely requires sacrificing some capability to scale back the voltage stress on the cells. I managed to dig up this desk from a Samsung SDI datasheet for their INR18650-30Q cell, however the steerage is relevant generally for battery pack improvement:

It might be barely tough to learn, however I would like you to note the charging voltage for 4.20V and 4.35V termination voltage cells is beneficial to be 4.00V or 4.05V for Vitality Storage Programs (ESS) or Uninterruptible Energy Provide (UPS) purposes. The identical threshold applies for prolonged voltage cells suggesting that prime voltage stress from being maintained at excessive state-of-charge situations are certainly seemingly the trigger for a lack of capability and swelling of Li-Poly packs particularly.

Sadly, most energy banks prioritise extracting the utmost capability from the cells and thus principally have charger chips or controllers which can be both mounted to a nominal 4.20V or 4.35V termination, or might be adjusted from 4.20V and up slightly than permitting for decrease capability, better security and longer storage life. It is a huge disgrace as most energy banks are saved absolutely charged in anticipation of a necessity, and thus they’re basically sitting underneath excessive stress that causes them to degrade extra quickly by way of cell capability.


Understanding if a given energy financial institution can carry out the position of an uninterruptible energy provide for USB units is just not a straightforward activity, particularly counting on producer info alone. An marketed “pass-through charging” functionality doesn’t essentially indicate that the output is freed from interruptions.

Even in case you did discover a energy financial institution that was seemingly able to uninterruptible operation, this might not be underneath all situations because the enter charger IC might time-out on over-time safety. If it doesn’t, what it might be doing is mistreating the lithium-ion/polymer cell sufficient that it’ll shorten its lifetime by holding it at a most state of cost, producing voltage stress on the cell which can contribute (together with temperature) to untimely capability loss and swelling of lithium-polymer cells.

Whereas there could also be some energy financial institution merchandise that may seemingly function within the UPS position, until they’ve been fastidiously engineered to finest keep away from these pitfalls, they’re unlikely to be dependable, secure or sort to the lithium-ion/polymer cells within the long-run and I might advise in opposition to long-term unmonitored use as an sad lithium battery is usually a fairly catastrophic occasion.

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