USB Powerbank Efficiency Testing

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 %
44.4Wh12Ah31.656.33Ah52.75
37Wh10Ah33.86.76Ah67.6
74Wh20Ah56.511.3Ah56.5
22.2Wh6Ah17.93.58Ah59.67
18.5Wh5Ah15.043.01Ah60.2
18.5Wh5Ah12.452.49Ah49.8
22.2Wh6Ah193.80Ah63.33
37Wh10Ah30.856.17Ah61.7
55.5Wh15Ah48.69.72Ah64.8
18.5Wh5Ah15.253.05Ah61

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.

4 thoughts on “USB Powerbank Efficiency Testing

  1. Markward says:

    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

    Reply
    1. The Engineer says:

      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

      Reply

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