I think this is the cheapest way to check your powerbank capacity and of course with lots of patience as well. In the video I am using the popular 2600mAh powerbank in eBay as an example.
Concept: Basically you charge the powerbank (full discharged first) and get the Amp reading from the start and again at the end when is fully charge. Then note the full charge duration of the powerbank. Once you got the duration and the two Amp readings you can calculate the mAh.
What you need?
2. USB current voltage meter, $5 in eBay, also known as Charger doctor
3. USB power source such as USB AC adapter
4. Timer, clock or a stop watch
5. Continuous recording camera such as car dash camera. This is optional if you are lazy to keep checking full charge time
Here are the steps.
Step 1: Fully discharge your powerbank.
Step 2: Plug the USB voltage current meter to the USB power source.
Step 3: Begin charging the empty powerbank and start timer. Immediately take note of the Amp reading. You can use a camera to record.
Step 4: Note the last Amp reading just before it is fully charge because there will be no current when the powerbank is full. If you use a camera, you can check the last Amp reading.
Step 5: Once you get Amp reading and charge duration time. Here is you can calculate, I will show by example [as described in video]:
Start at 0s powerbank empty ------ 0.7A = 700mA
Last at 3hours 53min 5sec powerbank full ------ 0.34A = 340mA
So, average at 0.52A = 520mA Then you multiply 520mA with the duration in hours, which is 2,019mAh (520mA X 3.88h)
Here you go, my 2600mAh powerbank actually has 2,019mAh only.
This calculation is approximately 10% to 20% accurate, see additional note below. If you want to get more accurate measurement you have to record the amp at short intervals like (every 10 mins) or use device such Power Analyzer Pro.
Additional note: The average calculation is assumed on linear regression (to make it easier to explain and demonstrate), however the actual charging profile is a non-linear regression meaning the current decreases more over time. So if you plot an actual graph of current over time, you will get a non linear graph.