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Random Power Banks for low-quality cells
#1
I have been building Power Banks using the lowest-quality cells I scavenge that are still above 1000mAh at a discharge rate of 0.5A. These cells obviously will not end up being used on a Powerwall project, they did however pass all testing. I will share my thoughts on the projects and my questions, since I was expecting this venture to be more successful that it actually is turning out to be.

First Power Bank uses 6 Samsung 28A cells from a laptop battery pack. They sum up to 6600mAh (3.0V to 4.2V), discharge at 0.5A, which adds up to 24.42Wh (nominal 3.7V). The empty power bank was bought through e-bay for a couple of bucks. These power banks need some grinding on the inside, since the cells sit too tightly and sometimes a couple of them tend to disconnect from the positive bar once you forcefully close the lid. That's why all testing is being down with the cover open, for the moment.


The power bank is supposed to be able to provide both 1A and 2A of current on two different USB ports. I found out that both ports provide up to 1A maximum, which calculates to 10W max power when both ports are being used. That calculates down to 450mAh (nominal) per cell. And here's where things get interesting...

I first completely discharged the power bank until it shut off. It shuts off at around 2.85V, which is not too bad, the cells will shoot up to over 3V as soon as current draw disconnects anyway. I then completely charged the power bank and it took 7766mAh at 5V nominal, which calculates to 38.83Wh (that's 60% MORE than the cells' discharge capacity).

I then completely discharged the power bank and it gave out just 2887mAh, which calculates to 14.44Wh, which is 40% LESS than the cells' discharge capacity.


All of the above, end up at a 63% efficiency while charging, and 59% efficiency while discharging, with an overall efficiency of 37%, at least by my calculations and the hardware at hand. This is kind of awful. Can you see any obvious mistakes?
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#2
A few thoughts during lunch pause :

-According to what I find on Aliexpress the board is obviously the 1A model not the 2A one
- I can't find any real data about the IC that's on the board but from what i can see i would bet the charge circuit is a linear one wich mean (for 5.15V input to 3.6V mean at cells) 3.6/5.15 = about 70% efficiency
- Accounting a 90% efficiency for the cells themselves (i have no precise data about that but it doesn't sound too bad for old cells that usually have a higher internal resistance *) we should have about 3.6*7.766*0.9= 25Wh available when discharging the pack
- You got 5.21*2.887 = 15Wh out of the bank
- 15/25 = 60% efficiency wich is pretty bad for a small boost but nothing to be really surprised of from a cheap gadget

... And we get back your overall 37% = 0.7*0.9*0.6 !

Even with a better boost part at 80% the overall efficiency would be only 0.7*0.9*0.8 = 50%

* If anyone has measured datas about cells efficiency with age please tell us !

Edit : typos, poor english
thanar and DCkiwi like this post
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#3
That powerbank uses a linear regulator.

I don't have it anymore, since it has a major fault in which sometimes, the output of the IC can cause a micro short, which causes the capacity to be drained in about a day.

Also, it has a tiny inductor for boosting 1.5A 3.6V at 5V 1A. The boosting IC is an unmarked one, but it looks decent.

If you want a good one, get one with a TPower IC, which is the company which manufactured the original high quality TP4056, TP5000, TP5100, and others.
The power of lithium ion is in our hands!
We'll show them what we're made of!
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#4
Assuming that you tested that each cell could hold about 4.1V for a few days without losing more than 0.03 V ( ie they did not self discharge )

Then there can be no explanation for this poor efficiency , I think , except that the electronics if not allowing effective use of the cells potential....

Even with a terrible internal resistance of 400mOhm per cell , the losses on one charge discharge @0.5 A is only 11% ...

It seems the only solution is to bypass the electronics and then test cycle efficiency.
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#5
(06-10-2018, 03:16 AM)ozz93666 Wrote: Assuming that you tested that each cell could hold about 4.1V for a few days without losing more than 0.03V

Of course I did.
I actually charge them to 4.2V and let them rest for around two weeks.
Anything that stays above 4.1V gets discharged and then charged to 3.7V and stays for another couple of weeks.
Anything that is still above 3.65V at the end passes.
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#6
Did a couple more of the same power banks, this time soldered the cells with copper wire. Total efficiency jumped to a little over 60%, which I was not expecting... I suspect some cells were disconnected from their contacts when I closed the unit prior to doing the initial test, which is a bummer and also a word of notice for said power bank shells. Will do all my powerbanks by soldering copper wire to the cells from now on.
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#7
(09-04-2018, 04:35 PM)thanar Wrote: Did a couple more of the same power banks, this time soldered the cells with copper wire. Total efficiency jumped to a little over 60%, which I was not expecting... I suspect some cells were disconnected from their contacts when I closed the unit prior to doing the initial test, which is a bummer and also a word of notice for said power bank shells. Will do all my powerbanks by soldering copper wire to the cells from now on.

That's interesting - but not surprising. I shall try soldering mine from now on.

How did you test the cells? If you used the Opus, it often overstates capacity by about 10%. Another factor is what voltage does the bank cut off at?
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#8
I test all my cells on an iMax charger. These banks cut off at around 2.85V (voltage drops to under 3V for a very small amount of time under load on the bank), and charge the cells up to spot on 4.2V. Discharge tests done at 500mA for these cells, since they will never see more than that in these power banks.
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#9
(06-10-2018, 02:51 AM)BlueSwordM Wrote: That powerbank uses a linear regulator.

I don't have it anymore, since it has a major fault in which sometimes, the output of the IC can cause a micro short, which causes the capacity to be drained in about a day.

Also, it has a tiny inductor for boosting 1.5A 3.6V at 5V 1A. The boosting IC is an unmarked one, but it looks decent.

If you want a good one, get one with a TPower IC, which is the company which manufactured the original high quality TP4056, TP5000, TP5100, and others.

I am waiting for a DIY powerbank.. hope it isn't using linear regulator..
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