I recently picked up a refurbed Kobo Forma, and I absolutely love the device -- with the caveat that, like you mentioned, the battery has been completely unreliable.
Multiple times I have picked up the device to find it completely dead, while it was at full battery less than a day ago. I haven't quite narrowed down the cause yet -- since I did install KOReader and Nickel right after getting the device, it's not running stock software, so I'm not certain if the issue is hardware or software related.
It definitely seems to be doing something in sleep mode that's draining the battery, even with wifi turned off. This really shouldn't be the case -- I'd expect close to 0 power being used when not actively refreshing a page. I've recently turned to mitigating the issue by setting the device to turn off completely after an hour... which is not ideal, but having to wait for the thing to boot up is definitely preferable to waiting for it to charge.
It's annoying because otherwise this thing is pretty close to perfect for me -- the form factor is excellent, extremely lightweight, and I can connect to my Calibre-web server and download any ebook I have on demand. I'd seriously consider buying an extra one to crack open and install my own battery if I knew that would fix the issue.
Edit: Lastly, I have a sneaking suspicion that "refurbished" does not mean "replaced with a new battery", which, honestly, should probably be illegal to advertise a device that way vs "used".