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Need simple circuit for external power bank level indicator

Sidecar Bob

Dec 19, 2021
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I'm reworking a project that is powered a USB power bank. This works well and the 5300 mAh PBs I bought for it will power it for a reasonable length of time after which it shuts off abruptly (unless I'm mistaken that's what they are supposed to do). Until now I've been able to see the charge level lights on the PB through the translucent front panel and switch to the fresh one before it dies.

It it would be much easier if the new case can be completely opaque; I'll add a small window in front of the LEDs if there's no other option but if I can find a simple circuit for an external charge level indicator that can connect between the power bank and the device (without altering the power banks) and mount its LEDs in the opaque front panel I think that would be more elegant.

I don't know a lot about how the controllers in power banks work but I have a feeling that the output voltage stays pretty constant until it shuts off so I won't be surprised if I find out that this is impossible but it doesn't hurt to ask.
 

Bluejets

Oct 5, 2014
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True capacity meters are not "simple".
If using LiPo cells, you could monitor the output voltage and act on that.
"reasonable time" is not a standard measure.
Batteries in good condition and charged according to specs to a predetermined level will discharge over a given time depending on the discharge current.
If the battery shuts off abruptly, I don't see any indicator being any use as you will have to watch it 24/7 like a hawk.
 

Sidecar Bob

Dec 19, 2021
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If I understand what I think I understand the controller in a power bank converts the actual (higher) voltage of the cells in it to approx. 5V and feeds that to the output port as long as it is above whatever the set full discharge voltage is and when it reaches that set full discharge voltage it turns the power to the output port off.
And the charge level indicator LEDs measure the voltage at the actual cells, not the output.

I'm not surprised to hear that true capacity meters aren't simple and I really don't expect that anything connected between the power bank and the device it is powering can tell me its state of charge but I figured it would be better to ask now and be told it can't be done than to learn later that I could have done something with LEDs & resistors.......

The project is a music player (portable stereo) that I use for listening to recorded radio programs when I'm working outside in the summer (when I work indoors I listen to the programs on the shop computer). Depending on the content of the program and how loud I have it a charge can last from 5 hours to more than 8 (which I consider reasonable for the application).
The reason I'm concerned about monitoring the charge level is that if you turn the power off without first pausing the player it forgets where it was. That can be annoying when the power bank shuts off in the middle of a program that is between 1 and 3 hours long so I periodically look at the level and when only 1 LED is lit I change to the fresh power bank.
That has worked well for me for a couple of years but I was never really happy with the case I put it in and it has cracked so I want to replace it before gardening season gets here and what I've chosen for the case doesn't have a front panel I can see through.
 

Bluejets

Oct 5, 2014
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More like bms board inside the battery.
Capacity indicator is also more than likely not really a capacity meter but a basic voltmeter.
Smart chargers have capacity circuits but not really a requirement on most portable gear batteries.
This is sufficient really on a LiPo or similar as the bms will cut the charge/discharge at set levels.
As an example, 3v7 is nominal for the LiPo so as it starts to drop 3v6 or lower, obvious almost at the end, or at least on it's last legs.
You can see more details on LiPo cell (or any other) charge/discharge curves over at batteryuniversity for example.
I don't really see the necessity for it anyhow as you already know the expected down cycle time.
So change the battery well before then.
 

Sidecar Bob

Dec 19, 2021
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The problem is that I don't know exactly how long the battery will last and I don't work outside for the same number of hours every day so sometimes a charge lasts less than a day and sometimes it can be 2 or 3 non-consecutive days and my short term memory isn't good enough to remember how many hours I've listened to on previous days.

So I guess I'll have to figure out how to make a window in the case and mount the power banks so I can see their LEDs as I figured I probably would.
 

kellys_eye

Jun 25, 2010
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A very simple comparator circuit with a variable set-point would suffice. Power it from a push button that only works when the equipment is powered (so you are measuring the battery state under load) and the LED can be set to light when the voltage has fallen to a level that you determine is the point at which you should recharge it.

You could even switch in a fixed, known, load resistance to pull the battery voltage down as the test is made and miss out the equipment powering up stage.

Such battery voltage test circuits abound on the interweb......
 

Sidecar Bob

Dec 19, 2021
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Ah, that would work if I was using batteries but I'm using power banks (= lithium cells with controllers) so the output voltage should be fairly constant until it shuts off.
 
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