These days USB powerbanks are very common – ranging in capacity from about 1Ah, to about 20Ah. Internally, they’ve all got much the same format:
- Lithium Ion cylindrical or Lithium polymer pouch cells for energy storage
- DC-DC boost converter
- Microcontroller & LED Battery Gauge Display
- Lithium cell protection & charge control
As the maximum voltage of a lithium cell for common chemistries is 4.2v, there needs to be a DC-DC converter to boost the voltage up to 5v for the USB ports – There are dedicated chipsets designed for powerbank use available everywhere for this part, and this section is going to be the most energy-wasteful part of the system.
To get a handle on the discharge efficiency of these units, I ran some tests with a constant current load, on different powerbanks from different manufacturers. All were in the range from 5Ah-20Ah, and all had ports rated for 2.1A Max output current.
The load was set for a nominal 2A current, and the powerbanks were fully charged before a discharge cycle. All powerbanks were in new condition to ensure that age-related degradation of the cells wasn’t going to be much of a factor.
Without further ado, here’s some test results:
| Nameplate Capacity (Wh @3.7v Cell Nominal) | Nameplate Capacity (Ah) | Measured Capacity (Wh, Calculated @5V Output) | Measured Capacity (Ah) | Ah Efficiency % |
|---|---|---|---|---|
| 44.4Wh | 12Ah | 31.65 | 6.33Ah | 52.75 |
| 37Wh | 10Ah | 33.8 | 6.76Ah | 67.6 |
| 74Wh | 20Ah | 56.5 | 11.3Ah | 56.5 |
| 22.2Wh | 6Ah | 17.9 | 3.58Ah | 59.67 |
| 18.5Wh | 5Ah | 15.04 | 3.01Ah | 60.2 |
| 18.5Wh | 5Ah | 12.45 | 2.49Ah | 49.8 |
| 22.2Wh | 6Ah | 19 | 3.80Ah | 63.33 |
| 37Wh | 10Ah | 30.85 | 6.17Ah | 61.7 |
| 55.5Wh | 15Ah | 48.6 | 9.72Ah | 64.8 |
| 18.5Wh | 5Ah | 15.25 | 3.05Ah | 61 |
Overall, these efficiency numbers are pretty poor with an average of 59.735% across these 9 samples. I expected at least high 80’s for efficiency on powerbank DC-DC converters, which must be pretty well specialised for the input voltage range by now. I suspect this is mostly to do with keeping costs down in mass production.
What is the watt hour ratings?
The table has been updated with calculated Wh capacity numbers, fill ye boots 🙂
Thanks, I’ve been playing around with some chinese powerbank barebones and 18650 cells from old laptops. This is pretty much what I saw as results as well. I’ve been thinking about more efficient options, I wonder if there’s something to buy. On YouTube there are some impressive examples: https://www.youtube.com/watch?v=0jRsltIW8qM
Hi Markward,
I was having a think about this myself, and Synchronous converters may be the way forward for better efficiency. I suspect most of the issue is actually because there is only a single cell (in effect) in these powerbanks, and the DC-DCs have to operate down to about 2.9v input voltage.
Keeping the efficiency up in any DC-DC converter with varying input or output voltage is difficult, and a problem near impossible to get away from.
Going to 2 cells in series for a nominal 7.4v input, and a buck converter down to 5v output instead of boosting it up as with one cell should get a higher efficiency. This does present some issues with charging the device from a USB source, and a boost converter would be required there along with a 2-cell charge controller, but in theory there’s less of an issue with losses there not only because of the constant input voltage & narrower output voltage range for charging the cells, but there’s often not as much a restriction on input energy for charging powerbanks.
Cheers