When you plug a Pacemaker into your computer you'll notice how it enters 'charging mode' and then if you turn it on it then becomes visible to Windows and you see it as a Mass Storage Device eg. E:\. When in this mode I think because the hard-drive is active the battery actually drains, despite USB power. Perhaps it's possible after 12 tries as you said the battery had enough juice to complete the update because left in DFU mode the device is effectively in 'charging mode' again. I have no-idea if the firmware regulates device charging, if so then this might explain even why leaving it on for hours the device hadn't appeared to charge. Obviously if your firmware was damaged then it wouldn't be charging.
Again it's possible you were unlucky the first time when you installed the Beta Firmware initially and the battery died then? Who knows, but either way - glad we could help!.
Have fun mixing!
