When I bought this Powerbank, I didn't found a detailed review regarding this but now as I have used and tested this Powerbank (20000 mAh variant) for 7 days, so I think I'm eligible to write a detailed review for this. Mainly I will write this review based on 4 points, build quality, charging time of powerbank, charging speed and battery backup. So let's start.1. Build Quality : The build quality of this Powerbank feels quite solid & nice. Its sandstone design texture and round edges makes its grip solid in hands & quite comfortable to hold. Before this I used to have Redmi Powerbank (20000 mAh) which in comparison to this Mi Powerbank feels little ugly in terms of design and uncomfortable to hold. Mi Powerbank feels less wider, thinner & more premium as compared to the previous one.2. Charging Time of Powerbank : The day I received this Powerbank, its LED indicators showed 2 LEDs blinking which means it had battery level between 25% to 50%. So before using it for the first time, I charged it fully using my standard 10W charger.One should also note that charging time of Powerbank along with the adapter also depends on what type of cable you are using. Using an old type C cable will take more time to charge it, as current carrying capacity of the cable decreases with time due to frequent usage. Although my 10W adapter is quite old but I have a new type C cable which supports fast charging, so I charged my powerbank with this setup and it took around 8 hours to charge it fully. So you can expect to charge it from 0 to 100% in around 11-12 hours using a 10W charger. Whereas if you use a fast charger like 33 Watt, 65 Watt etc. then ofcourse the powerbank will charge at a faster rate but it will also affect the efficiency and thermals of the powerbank in a negative way in the long run, so keep that in mind.3. Charging Speed Test : I tested the charging speed provided by this powerbank using 3 different type C cables to charge my 4000 mAh mobile device. Each time I charged my device to 80% using different cables (in second case I forgot to check & it got charged up to 90%) and noted the overall time it took to charge the device. All the results are attached in order as screenshots for reference.In first case, I charged it with my 2 yr old type C cable which although supports fast charging but the results were not impressive. (Time 1h 43 min Pic)In second case, I charged it with a new type C cable which came with my tws earbuds but it does not supports fast charging and the results were relatively better as compared to the first case. (Time 1h 58min Pic)In third case, I charged it with the type C cable which comes inside the box of this powerbank and supports fast charging, and in this case I got the most impressive results. The device got charged to 50% in just 30 min, and 80% in 1h 8min. (Time 1h 8min Pic)So, it shows that the cable also plays a very important role in fast charging your device which many people ignore.4. Battery Backup : Other than the above mentioned test results I charged my mobile 3 more times but didn't noted the charging time. So all in all, till now I have charged my device to a total of 406% in 6 different rounds (80% + 94% + 80% + 44% + 82% + 31%) and still 3 LED lights of powerbank are blinking which literally amazed me considering the negative reviews I read about its battery backup before buying it.So, overall I would say you can buy this powerbank if it suits your requirements and budget. Also if you buy this powerbank then I would suggest you to charge it fully before using it for the first time and test other things during that 7 days replacement period, so if you find any issues with it, you would be able to replace it. I used AccuBattery app to record the battery percentage & time it took to charge my phone.P.S : Will update the review in future if felt the need to do it.