Electronics > Projects, Designs, and Technical Stuff

USB Battery Bank Auto Shutoff

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engrguy42:
Interesting...you learn something new every day.

I have a 10 amp-hour battery bank for USB stuff (cellphone, etc.) lying around, and it occurred to me "hey, it's free 5 volts, why the heck not use it?". Quick and easy for a quick, flat, 5V....and 10 AH is nothing to sneeze at.

So I tried it (modified a USB cable to just use the power wires) and it works great. Until it suddenly shut off. I was powering some stuff, and suddenly it shuts off all by itself. WTF??

So apparently those battery banks have internal circuitry to shut the thing down when it thinks nothing is connected. Go no, right?

So the other day I just happened to finish some software to access my Labjack USB data acquisition device, and it grabs data and plots it. So I figured I'd try to get to the bottom of this. So I hook up the battery to the Labjack, turn on the recorder, and it gives me the attached plot. With no load on the bank it shuts off about 35 seconds after you turn it on.

So I then applied a 20 ohm resistive load (250mA) and it stayed on forever. So I tried a 47 ohm resistor (106 mA) and it stayed on forever. So I applied a 100 ohm load (50 mA) and it shut off in 35 seconds.

So apparently these things shut off if there's less than 100-150 mA of output current. So what people do is put together a circuit to apply a burst of load every 10 or 15 seconds to keep the thing awake. Of course you could just tie a resistor, but that's just a waste of amp hours.

I'm curious, anyone know of a cheapo device I can get to do this load cycling? I saw a TI paper on this phenomenon, and they have some recommended circuits, but I'm too lazy  :D

Thanks.

Peabody:
Power banks are designed  to charge other batteries, such as your phone's battery.  Partly to save their own battery power, and partly out of concerns about lithium battery safety, they are designed to shut down when the other battery has been fully charged.  And they conclude the charging is finished based on the low current draw.  But if "low" means 100mA, that won't work for most microprocessor projects.  And you know, sinking excess current just to keep the powerbank turned on kinda defeats the purpose of battery power.

There is a line of powerbanks by Voltaic Systems which have an "always on" option, but they are about US$30.  And there is this much cheaper design that's only safe to use for 60mA loads or less if you are also charging the powerbank at the same time:



Apparently, sinking extra current periodically, as you described, works to keep most powerbanks on.  I'm not familiar with the circuit you described to do that, but maybe someone will point us to it.

engrguy42:
Okay, well I kludged together a 555 timer with some parts I had lying around and came up with the attached, which I tweaked in LTSpice.

Basically it turns a MOSFET on with 50% duty cycle to apply a 120mA load to the battery bank. It's pretty much 7 seconds on, and 7 seconds off. I suppose I could further tweak it to see how short I can make the ON time, but my knowledge of 555's is lost in a foggy memory from years ago.

Attached is the LTSpice circuit, which I duplicated on a breadboard, and then applied to the actual battery bank. BTW, that's the C# app I just finished last week to access and plot from my Labjack data acquisition unit, showing the input to the gate of the MOSFET. It came to the rescue just in time  :D

And, SUCCESS!!! The battery bank stays on indefinitely.

Any suggestions for shortening the duty cycle would be welcome. Thanks

engrguy42:

--- Quote from: blueskull on May 12, 2020, 01:58:58 pm ---If you need a really well built and full featured power bank, consider my pick, Huawei CP12S.

It is stupidly expensive at $45 for just 12000mAh, but it has a lot of features that are not common.

It offers the always on feature, which is enabled by double clicking its power button.

It can charge PD or FCP devices at up to 40W, including laptops and tablets.

It also can output 12V in QC mode, which is very rare. This made it perfect companion for my TS80 iron.

The only downside is it does not support UPS mode when output is not 5V. Other than that, it is an overall perfect power bank that fits in the most slim pockets.

--- End quote ---

Thanks, but it's something I rarely/never use in the first place, so replacing it isn't really under consideration. In fact, I could probably just tie a 22 ohm resistor to the output without all this stuff and be done with it.

But as it stands I can just look for a tiny plastic box, stuff these parts inside, and I'm good to go.

Thanks.

SilverSolder:

Some battery banks don't have the auto-shutoff feature.

I have an old 13000mAh power bank here where the cells are so worn out that the capacity is only around 1,000mAh now...   but it doesn't auto shut down, so is great for powering an Arduino etc. for several days!

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