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  • Author Author: jw0752
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On Using a USB Battery for a Portable Project Power Supply

jw0752

Everyone is familiar with the USB Batteries that can be used to charge a cell phone and which can themselves be charged off a any standard USB port or charger. A while back I began using one type of these batteries to power some of my small projects and also I began to advocate their use to my friends for this purpose. Here are two of the many brands and types currently available.

 

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For the rest of this Blog I will refer to them as "Black" and "Silver". My first experience with using these batteries for project power supplies was with Silver. Silver is typical in that it has a 3.7 volt Li-Ion cell at its heart which can be charged with a 5 volt charger and when off the charger and plugged into another device uses an internal boost DC to DC converter to deliver 5 volts output. When Silver is turned on and no load is present it will turn itself off after 60 seconds. When Silver is turned on under load it will maintain an on state as long as the load doesn't drop below 10 mA for more than 60 seconds. Its internal sensor sees loads less than 10 mA as no load and starts a shut down sequence. My experience with Silver as a power source for small projects has been excellent as I seldom build anything that operates at less than 10 mA.

 

It is our nature to generalize and true to my human nature I made the mistake of believing that all other types and brands of these devices would behave just like Silver.

 

Along came Black whose purpose was to power a small frequency counter kit that I built for a friend. Black it turns out is not like Silver. Black will automatically shut down after 18 seconds if it doesn't see a load. Further it turns out that the lower limit for the load that it can see is 75 mA. The small frequency counter did not draw anything near 75 mA and so every 18 seconds the battery needed to be turned back on which was not practical.

 

The other day in a Blog by    -  Working Prototype of a Kelvin (4-Wire) Milliohm Meter  the discussion morphed into some comments about using this type of battery pack for his portable milliohm meter. It was noted that there is a  tendency for these batteries to shut off. In the discussion Frank mentioned that somewhere he had heard about a procedure to pulse the USB battery with a short moderate current load to keep it active.

 

I wanted to have the flexibility to use Black in future projects so I drew up a schematic and did some bench testing finally arrived at a bread board prototype:

 

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Here is a schematic of this circuit:

 

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This circuit has a quiescent current of 2.5 mA and produces a 3 millisecond load of 300 mA every ten seconds. Black which would normally turn off after 18 seconds with a load of less than 75 mA was perfectly happy to stay engaged and run for hours when attached to this circuit. The 10 second load pulse was within Black's 18 second window to shut down and apparently the 300 mA load was sufficient to restart the 18 second time limit.

 

While I hadn't had any problems with Silver I wanted to see if Silver would also respond to the pulsed load to reset its 60 second time out on loads of less than 10 mA. Unfortunately this was not to be. The (6) 300 mA loads that were put on Silver did not change its mind and after 60 seconds it promptly turned off.

 

I do not want to make the mistake of generalizing this situation again but I know there are at least 2 different situations when using USB batteries (Silver and Black) and I might extend this idea to say that there are likely many others. Here is my comparison of Silver and Black:

 

Battery Name       Model & Brand               Time to Shut Down (No Load)        Minimun Stay On Load             Does the Pulse Load Fix Work

 

Silver                XPB15031S  Unknown                      60 Seconds                                10 mA                                                 No

 

Black                 16WMS137  Shen Zhen                   18 Seconds                                 75 mA                                                 Yes

 

I am posting this blog in Test & Tools as I do not want it to belong to me but serve as a place were additional information on using USB batteries to power projects can be posted. If you have a USB battery, and are inclined to test it, any information you can supply would be appreciated. Short of loading the battery down with power wasting loads to keep them on what other schemes can we come up with? Is there a special magic circuit that will work to keep any USB battery engaged powering our small portable projects or will we have to design a specific remedy for each unique USB battery type?

 

Since the use of the USB battery technology can be a benefit to anyone wanting a convenient power source for small portable projects I welcome as many ideas and tested remedies as can be found.

 

 

Parents

  • As an experienced power bank tester - I've come across a wide variation of types, but I haven't really taken the time to test the shut-off behaviour. I have found some that:

    • Have high quiescent current but maintain a 5V output at all times, until the battery is depleted and it abruptly switches off. Unfortunately this type needs regular topping off with the charger or it'll be flat when you come to use it. Don't store it with your keys in your bag either, otherwise nasty things can happen.
    • Have microcontroller-based switching control and can be turned on/off manually with a button - some of these types have an output that is off (0v) when turned off, others are more like the latter.
    • Have integrated boost chips that turn on whenever sensing a load greater than a given current - this type has an output that is "battery level" (3-4.2v) when the boost is not engaged, thus if your circuit can work on "unboosted" Li-Ion voltage, it will continue to work. Other appliances (e.g. cheap USB lights) tend to show reduced performance (e.g. dim/slow/etc). Charging is done by basic "linear" regulator chips (e.g. TP4056) which is slow and inefficient.
    • Have integrated power-bank controllers - these tend to use one coil but can boost for output or buck for charging, just not both at the same time. Issues are similar to the above.
    • Have more intelligent Qualcomm Quick-Charge capable controllers - some are dual-way QC capable resulting in buck-converted charging and boost-converted output. For QC2.0 power banks, some tweaks on the D+ and D- lines can signal for 12V, 9V, 5V output from most power banks, whereas for QC3.0, this can be commanded in 0.2V steps between about 3.6V - 12V. These are much more expensive and more efficient, but some will have shut-off behaviour.

     

    Now that I've mentioned this - I suspect some units can't be fooled by short pulse loads as they only sample the load resistor output infrequently and aren't really integrating the current over time. If your pulses aren't long enough to overlap with the sample time, then there's basically as good as no load at all. I suspect some of the "microcontroller" based units might be fooled by changing the resistor, but at the risk of breaking overload protection/voltage regulation feedback (as some have a "voltage slope compensation" function that boosts the voltage higher under higher loads to compensate for cable drop).

     

    But more than this, sometimes, you just need to find a model you like and settle down. The last one I used and I still use regularly is the Xiaomi Mi Power Bank Pro (QC 3.0) power bank. The QC feature is quite nice for phones, but if you build something like this, you can turn it into a 3.6V-12V variable power source up to 1.5A. Furthermore, if you read the manual, there is a low-current charge mode too - double-press the power button and it locks itself into supplying 5V regardless of current load, but only for two hours. This mode is intended for charging accessories such as Bluetooth Headsets and Smart Band Fitness Trackers which have <40mAh batteries and even lower charge currents. I believe some other power bank manufacturers are beginning to offer this mode.

     

    - Gough

  • in reply to Gough Lui

    Hi Gough,

    Thanks for the information. Until I read where you have a unit that runs all the time and doesn't shut down I thought there was something wrong with this one as it too runs all the time:

     

    image

    With no load other than the scope probe it cycles between 4.88V and 5.68V until a load is placed on it. With no external load it draws 30 uA from the battery.

     

    John

  • in reply to jw0752

    I find the quiescent current can depend on things such as how clean the PCB is (sometimes they leave slightly-conductive flux residue), how leaky the output capacitors are, how clean the USB connector is, etc. But 30uA is hardly a big issue for a massive Li-Poly battery ... but I've found some to be more like 100-300uA. The thing I find more annoying is that some seem to also emit bursts of RF interference every time they wake/sleep causing some odd interference when I'm doing radio stuff, so not my favourite type.

     

    - Gough

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  • in reply to jw0752

    I find the quiescent current can depend on things such as how clean the PCB is (sometimes they leave slightly-conductive flux residue), how leaky the output capacitors are, how clean the USB connector is, etc. But 30uA is hardly a big issue for a massive Li-Poly battery ... but I've found some to be more like 100-300uA. The thing I find more annoying is that some seem to also emit bursts of RF interference every time they wake/sleep causing some odd interference when I'm doing radio stuff, so not my favourite type.

     

    - Gough

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