I would do this to bypass the battery low thing:
:::HOW TO FLASH A PHONE WITH "BATTERY LOW CANNOT PROGRAM":::

Then you are going to have to sbf your phone back to stock to get it running again.
How to SBF back to stock - Android Forums at AndroidCentral.com

Hope this helps- Phatninja55

 

just beware you can fry battery and phone, using the stripped usb wires to charge
there are safer ways
like a spare battery charger or a black hat programing cable links to both in my list

sd_shadow's list of links for Droid FAQs, SBF, Rooting, ROMs.... https://docs.google.com/document/d/1Goawxdx_UBF4Y8lqzHYWf8Ha3yUcRK4faq0UWIlXLWA/edit

 

bootloader says low battery
that means you can not flash sbf until a charged battery is in phone
maybe battery has failed

edit: generally when phone is stuck in bootloader, fix is to flash sbf recovery image
but you could try wiping cache or data in android recovery
try reading
the 2nd link in 2nd post or try
http://droid.koumakan.jp/wiki/Welcome

but your main problem is low battery
pull battery, replace, connect to wall charger, does charging mode start, or bootloader
if it charges great, let it charge completely
if no charge, find a charged battery
then post back here

 

if phone boots straight to bootloader, from power off then connected to wall charger
wiping data is likely not going to help

 

not that i know of, and if you haven't charged battery,
if phone died in the middle of update it may really be bricked

 

think there is a link in my list to 621, but will only work if phone is on 601

 

yes, that's what you need, and all you'll get
now flash D2 601 ezSBF

or use rsd lite, moto drivers and 601 sbf
links and instructions in my list
sd_shadow's [Collection] of Links: FAQs, Guides, Rooting, SBFs, ROMs... - Google Docs