I have a 2016 13 in Macbook pro. I did some research and the design capacity of the battery is 5550 mAh. I purchased a 26800 mAh RAVPower external USB-C power bank to charge my laptop. I specifically got this one because it has a really high capacity and is 30W, which is enough to charge a MBP. I use it whenever I'm not near an outlet. It works fine, except it takes the whole battery to charge my laptop once. I don't know a whole lot about batteries and I assume there would be some power loss charging the internal battery from the esternal, but the battery is 4.8 times the capacity of my internal laptop battery.
The only reason I can think that it would eat through this much battery is it thinks its connected to wall power and it does not use power as efficiently? Is this a software issue, a hardware issue, can you compare batteries just based off of mAh?
The latptop is a 2016 13 in Macbook Pro with touchbar. It has a 2.9 GHz Core i5. Here is the battery I am using.