Just wanted to share that the batteries for Sony's Ericsson K750 fit the X5 rather good.
My brother's X5 was about to be scrapped (cracked case, intermittent contacts, dead battery) when I remembered that we had a K750 in the household that recently got a new battery whose only use was to reveal that the phone's charging circuit is broken. It turned out this battery fits perfectly in the X5. At 980 mAh measured, it is almost the same capacity as well.
The gold plated contacts of the battery took the solder nicely. It charged fine without incidents. Looks like the player just got a new lease on life... Image attached (hopefully)
regards
andreas