I love this R900 that I got a few months ago. The seller indicated that the internal battery wasn't working, but the unit came with the AA sidecar (the black thing connected to the player in the photo above) so I wasn't too concerned. I figured it would just be a matter of cleaning the internal battery contacts and it would be good as new. When I received it, the internal battery contacts were a hot mess (the original Sony gumstick battery was stuck inside the unit), but I got it out and cleaned the contacts with vinegar and IPA. Afterwards, it worked with a new gumstick, but performance (i.e. how long the battery actually lasted) was less than satisfactory. A lot of the gold plating had been lost from the contacts on the battery door (the end where the leakage occurred) from the corrosion, so I tried re-tinning the contacts on the door, hoping to restore a bit of that conduction. No matter what I did however, it seemed the full charge segment on the battery indicator would never stay lit for more than a few minutes (not normal operation). After months of dicking around with this I finally threw in the towel a few nights ago and removed the gumstick battery, attached the sidecar, and put a drop of super glue on the door (because it wouldn't stay closed) and called it finished. The sidecar is obviously not as elegant a solution as just using the gumstick, but it works. The new AA battery I put in it last night has been in use nearly all day today and the full battery indicator stayed lit for nearly 10 hours.
I'm inclined to believe that the majority of the R900s out there suffer from this same internal corrosion because nearly all the working ones you see for sale are shown with the sidecar attached. Ideally I'd like to find a working, non-corroded unit of any color and swap the guts into this one, but then I'd also need to find another red unit with a door that latches properly to harvest, so I think I'll just live with this as it is.