# How efficient should the USB output of a power bank/jump start pack be?

3 posts / 0 new
sac02
Offline
Last seen: 8 hours 15 min ago
Joined: 09/26/2016 - 15:18
Posts: 682
Location: TX
How efficient should the USB output of a power bank/jump start pack be?

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?

sac02
Offline
Last seen: 8 hours 15 min ago
Joined: 09/26/2016 - 15:18
Posts: 682
Location: TX

Test finished in 3hours, 23min.
5,289mAh
26.652Wh

Rated capacity = 12,000mAh

44% of rated capacity (assuming zero loss/inefficiency)

gauss163
Offline
Last seen: 5 hours 54 min ago
Joined: 10/09/2013 - 18:04
Posts: 722
Location: USA

Unless otherwise notated, the mAh rating is at the nominal cell voltage, typically 3.7V. So 3.7V * 12Ah = 44.4Wh, and 26.6Wh/44.4Wh = 60% of rated capacity.That’s in the ball park after accounting for all potential losses (from the buck converter, cable/connector losses, early discharge termination (to preserve life), aging, capacity exaggeration, etc).