Is it safe to use a power bank to power a USB LED lamp?

[John Jones]

I've just bought a power bank to power my USB LED lamp, but I found that the LED flickers every 2 seconds.

The LED works fine when plugged into computers so it never occurred to me that a power bank would be any different.

The power bank is 2A. I've no idea what current the LED is but this is the model: https://www.amazon.co.uk/Charging-10-Level-Brightness-Function-Reading/dp/B07GSCX6RW

Is this risky? Would using a 1A power bank be any better? Is there an easy fix?

 

[alec_t]

Welcome to ETO!
It is the LED lamp, not the power bank, which determines how much current the lamp draws. If the LED flickers, it is likely that the lamp current exceeds that which the power bank can safely supply continuously, so the bank is switching off periodically to protect itself. Hence the 2A bank rating (if indeed that is a genuine 2A) may not be enough.

 

[John Jones]

Thanks! I'd imagined the LED current must be too low, but you're right - I just found someone who tested this LED model and it uses between 0.5W and 5W.

So does anyone know if using this could be dangerous and what could happen? Could anything explode? The power bank is from a very reputable company so it is definitely 2A and functioning properly.

 

[John Jones]

Update - I've just noticed that the flickering ONLY happens when I have the LED turned down low. At full brightness, it doesn't flicker.

Could this mean that the power bank IS handling the higher current fine, but struggling with a lower current?

And is this safe?

 

[alec_t]

Could this mean that the power bank IS handling the higher current fine

Looks like it. Can't explain the flicker though.

 

[Diver300]

A power bank has a Li-Ion battery that has a voltage of between around 3.4 and 4.2 V. It needs a boost converter to boost that to 5 V. The boost converter takes some power to run, so the boost converter will shut down when not needed, which is usually when the current drawn by the load is less than a certain amount. It needs to do that so that the power bank doesn't need charging every week even when ont used.

It seems your USB LED lamp, when on low, is not taking enough power to keep the power bank turned on.

 

[Visitor]

....The boost converter takes some power to run, so the boost converter will shut down when not needed, which is usually when the current drawn by the load is less than a certain amount....

Some power banks behave like this but not all. Many of the cheap single-cell ones that don't have a power switch will output 5 volts regardless of load, down to zero current (well, down to whatever a DVM draws at least). There's a post around from last fall where I documented this after Nigel made the same claim.

 

[Nigel Goodwin]

Some power banks behave like this but not all. Many of the cheap single-cell ones that don't have a power switch will output 5 volts regardless of load, down to zero current (well, down to whatever a DVM draws at least). There's a post around from last fall where I documented this after Nigel made the same claim.

And most of them still seem to, as most seem to use the same circuit. I would imagine it's probably what's causing this strange problem - perhaps doing some kind of test?.

 

[John Jones]

Thanks for the answers!

I can live with the flickering, probably won't have the lamp on low much anyway.

But please, does anyone know if this could possibly be dangerous/blow up?

 

[Nigel Goodwin]

Thanks for the answers!

I can live with the flickering, probably won't have the lamp on low much anyway.

But please, does anyone know if this could possibly be dangerous/blow up?

No, I wouldn't have thought the flicking was of any concern, most likely just a natural part of it's operation - which is really intended to charge a phone.

 

[Diver300]

Some power banks behave like this but not all. Many of the cheap single-cell ones that don't have a power switch will output 5 volts regardless of load, down to zero current (well, down to whatever a DVM draws at least). There's a post around from last fall where I documented this after Nigel made the same claim.

Does that mean that the cheaper ones will self-discharge due to the boost converter running all the time?

 

[Nigel Goodwin]

Does that mean that the cheaper ones will self-discharge due to the boost converter running all the time?

If it's running all the time, then obviously so - but even the cheap ones (and mine are all cheap) mostly switch off at a minimum load. That's why I gave up on the idea of using them - but even the ones that do switch off, still self-discharge, just at a slower rate with the converter turned off.

 

[Visitor]

Obviously, I don't know what I'm talking about because my proof doesn't match Nigel's experience.

 

[Nigel Goodwin]

Obviously, I don't know what I'm talking about because my proof doesn't match Nigel's experience.

Your experiences on a small range of power banks were different to mine, and to many others - I don't deny the possibility that you may be right on a small number of power banks, but leaving the converter flattening the battery permanently is a bad idea (in my view) which is why it's not that common.

 

[Diver300]

I know that the power banks that I have contain timers. If nothing else, when the button is pressed, and the level of charge lights show, they turn off after a few seconds.

So when the load detected by the power bank is low, the output won't switch off immediately, there will usually be a timer which will keep the output on for a bit. That might be seconds, it might be a few minutes. It could allow lots of time to measure the voltage.

Another consideration is that most boost converters will have an output voltage that is the input voltage less a Schottky diode loss. So the output voltage will be over 3 V even with the converter turned off. It means that some power banks can detect the current taken by the load at 3 V as a signal to turn on. It would be quite easy to get one to turn on without realising that you had done anything.

 

[Nigel Goodwin]

I know that the power banks that I have contain timers. If nothing else, when the button is pressed, and the level of charge lights show, they turn off after a few seconds.

So when the load detected by the power bank is low, the output won't switch off immediately, there will usually be a timer which will keep the output on for a bit. That might be seconds, it might be a few minutes. It could allow lots of time to measure the voltage.

Another consideration is that most boost converters will have an output voltage that is the input voltage less a Schottky diode loss. So the output voltage will be over 3 V even with the converter turned off. It means that some power banks can detect the current taken by the load at 3 V as a signal to turn on. It would be quite easy to get one to turn on without realising that you had done anything.

None of the ones I've tested output any usable amount of power once they have shut down, I suspect there's probably an output switch in the chip used? - as you say a boost convertor would normally feed the input directly to the output via a Schottky diode.

One of the chips used is an SY20755CQDC, and that chips internal 'schematic' clearly shows FET switches on input and output.

If you google for "usb power bank without auto off" there's a LOT of information out there - which appears to basically be "try and find one of the very few that don't" - a common suggestion is the Voltaic Systems V15 or V44.

 

[AWG]

driver3000 is correct, it looks like the LED lamp on low does not draw enough power to keep the power-bank on. But LED's have a small inrush current which might turn it on again for a few seconds. Play with the dimming level and you might find at a certain spot the light is stable. If you need the low dimming level you can add an resistor load to fool the power bank