If I had a dollar every time I helped someone who couldn't flash/repair their device because the battery was flat even though they thought it was charged, I'd have about $10.. that asides, I don't think the phone will charge the battery unless the OS is booted.. I think it is deceptive.. and flashing (or doing anything with flasher-3.5) becomes more and more unreliable the lower the battery.. I'm lucky, gf has a 5800 so I always .have a backup.. but so far, I've not needed to flash