I ordered some days ago a new ZachTek WSPR desktop transmitter for some field tests. This transmitter is powered by 5VDC via a simple USB cable.
As an avid reader of Tom's K4SWL Blog I've seen HAMs using many times a small powerbank from Talentcell - the YB1203000-USB - which is advertised as a 12V 3.000mAh powerbank.
As can be seen on their website, that the powerbank uses three 18650 LiIon cells. The regular voltage of an LiIon battery is 3.6V - three cells will give 10.8V and not 12V?!
Initally with not further searching on the internet, I ordered the powerbank on eBay, as the specs looked fine on first glance :-/
Intro
Searching the internet reveals, that also Martin from Sotabeams has also tested this powerbank some time ago with bad results. So I powered up my lab equipment and did some tests on myself.
Discharge Test
First I did a discharge test using my RIGOL electronic load DL3021. The load was setup to Battery-test mode with a constant discharge current of 1A and a cut-off-voltage of 9.6V (3.2V per LiIon cell).
The measured capacity of the powerbank is 2352mAh 
The powerbank is rated at 3.000 mAh - so 78% of the rated capacity - I would say "failed".
The discharge diagram looks as expected 
It can be clearly seen, that the 12V output of the powerbank is directly connected to the cells (maybe via BMS). So this matches their specs - Output voltage 12.6VDC - 9.6VDC. 12.6VDC only for some seconds, as the voltage drops fast to the cells nominal voltage.When I look at the measurement for my Penntek TR-35 I would expect to have a working power source to generate 5W HF on all 4 bands.
Only the 5V USB output is regulated. The USB port is powered, when the powerbank is switched on via the hardware switch. So there should be no problems, when the USB current is low - other powerbanks may switch of the USB port below a certain level of power drawn from it. So I expect, that this powerbank can be used with my ZachTek WSPR desktop transmitter.
Charge test
After the complete discharge I used the supplied charger rated 0,5A @ 12.6VDC to recharge it.
As can be seen, the maximum output voltage of the charger is 12.5VDC. I assume they reduce the charge voltage to 12.5VDC for safety reasons.
The charge time was roughly 6h to reach a 12.5VDC cell voltage. But even after 8h of charging time, the LED on the charger did not turn from red to green as advertised.
Conclusion
Hm - I bought the device for €47,- - this is in my opinion too expensive - why?
- This is not a 12VDC power source - this are three 18650 cells in a simple case with an additional USB port
- The capacityis too low for the advertised 3,000mAh, But this is a common problem of powerbanks. Even higher priced powerbanks deliver on ~80% of the stated capacity.
Will I return it to the dealer - propably not - I have to make some field tests with the unit.