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 %|
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.