If the problem is a dead battery, there is a motorola engineering USB cable available that will allow you to power on your phone and and you can then reflash the file. You can find the cable here for $35. I've also heard that there are guides available online somewhere with instruction on how to make your own. Good luck!
turn your phone completely off...then plug it into the wall charger. it will boot to fastboot and it will say battery ok. then unplug it and plug it into your computer. you cant use rsdlite though because it reboots the phone a few times. you will need to manually enter the fastboot commands to flash the phone. i recommend just flashing a good system file. this will probably not be enough to make your phone boot up if youre downgrading from 6.12.173 or 4, but it will allow you to plug a turned-off phone into the wall charger and it will charge.