I finished a battery life test today with my OKCell USB LiPo batteries. Over four listening sessions I got two hours and ten minutes total.
Shorter than I hoped, but close to what I expected. An Amazon reviewer warned that the 800mAh rating was grossly exaggerated and that the real capacity was around 400mAh. This seems to be the reality.
On the bright side, these batteries have normal power-on behavior and keep their voltage over 9V til the bitter end. They measure 9.18V when freshly charged and I think the lowest I measured was 9.13V, circa 30 minutes before one cell quit. The weaker cell just switches itself off when it gets low, which is quite obvious, because volume drops a bunch and sound gets distorted. Out of curiosity, I charged the dead cell only and listened to the amp some more to let the stronger cell run its full course. It only lasted an additional 15 minutes.
For me, the convenience of charging my batteries in the tin, via micro USB, plus the steady, surprisingly high voltage these put out, makes them keepers. I can also confirm that these batteries have never gone into any sort of protection mode when charging up my amp's 1000uF rail caps (other users have reported having to switch on/off a few times for proper power up. These just work with the first click.
I wish they lasted a little longer, but I never really listen for more than 2 hours at a time, so for $11 shipped from AliExpress, I'm happy with these batteries. I plan to buy two more for my second build, and I hope they will give the same turn-on behavior with the larger rail caps I plan to try.