I'm also in the camp of "oh I'll just eBay a battery" then saw the price and reconsidered.
My unit actually came with a battery but it died a while ago (probably over discharge, possibly a bad cell), so I took it apart to try and revive it (a difficult job). Even with some charged cells connected it refuses to enable the output, so there might be some flag set when it initially died. Thankfully it's all off the shelf BMS parts inside with datasheets available so I'll try and reset it via I2C when I get chance.
Most of my stuff is currently in storage though so that'll be a while in the future - if I do dig up some photographs of the PCB (I probably took some) I'll post them though.