06-07-2018, 09:55 AM
(This post was last modified: 03-26-2019, 10:53 AM by thanar. Edited 7 times in total.)

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?

There's a critical bug with these power bank cases... Because the cells are tightly squeezed, looks like that if using the build-in springs/strips, a couple of cells get disconnected as soon as the case is closed, which could even be dangerous in some cases. That's why I've opted to scrap the strip/springs altogether and solder the cells with wire instead.

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?

There's a critical bug with these power bank cases... Because the cells are tightly squeezed, looks like that if using the build-in springs/strips, a couple of cells get disconnected as soon as the case is closed, which could even be dangerous in some cases. That's why I've opted to scrap the strip/springs altogether and solder the cells with wire instead.