I got a discounted Li-Ion car jump start / power bank the other day, it is labelled as 12000mAh.
I’m discharging it using the USB port at 1.5A (the port I’m using is rated 5V/2.1A). I’m currently showing 30% remaining (70% discharged) on the battery pack’s LCD, and the tested capacity so far is 3900mAh.
I know there is some inefficiency and loss when converting the original voltage of the pack (I assume it is 3S, so 11.1V nominal, 12.6V max) down to 5V, but I’m not sure how much.
70% of the advertised 12000mAh would be 8400mAh. My capacity tested so far is 3900mAh. So at the moment I’m getting less than half of the advertised capacity, unless my understanding of how this should work is wrong.
How much inefficiency/loss should I expect, and how much of the discrepancy is inaccurate (lies) about the advertised capacity?