[Motorola Atrix 2] fire_fist_ace:
Sorry for the long delay. Ah I kind-of already disassembled it :D I dont flash often so it wasn't really anything permanent, I'll build a proper one when it happens again, but i can give a simple explanation of 2 methods i tried:

USB CHARGER

I tried the usb charger method, but for some reason, while in fastboot it would show "Battery OK, OK to Program", but then when the FXZ flashing process gets to around 7/20, it would revert back to "Battery LOW, Cannot Program".

If you still wanna try it, just get anything with a usb cable. I used an old mouse that was broken. Cut the cable, & you should see 4 wires: blue, green, white & yellow.

I then used a multimeter to find out polarities of the wire & which one carried 5V, ( turned out it was the blue+white cables. From memory, blue was +ve & white was -ve, but this can vary depending on what you use as the power source e.g computer, mains usb charger etc) .

Once you know this, then just attach the +ve wire to the +ve side of the battery, & vice versa for the -ve wire. Make sure you strip enough of the wire insulation so that only the metal part is in contact with the battery, and also make sure that the other wires don't touch, it could short the power supply.

(see the attached picture USB_Charger, its too big to put here)

ANY OTHER CHARGER

This is the method that worked for me, however, its abit more risky, but i guess if you are into flashing you have already accepted the risks that come with it :p

First off, you will need another charger. Any charger will work, but to make things much easier for you, try to pick a charger that looks like this:

(see the attached picture Other_Charger, its too big to put here)

It may not be clear in the photo, but there is a hole in the middle of that charger. The reason I recommend this type is because most, if not all, of these chargers follow the convention that the hole in the middle is the +ve terminal, & the outside is the -ve terminal. This way you know exactly the polarity of the charger even if you cant measure the polarity of the charger.

Ok, once you know the polarity, just attach a wire to the inside & outside of the charger, or if you picked a different type of charger, attach wires to the correct terminals, then attach to the corresponding battery terminals.

Now, with both of these methods you can do this in 2 ways. One is to return the battery with the wires into the phone, & try flashing. That didnt work for me, so i just attached the wires, & charged the battery outside the phone, then after some time unplug the wires & return the battery into the phone. With the USB Charger, I left the battery charging overnight, but when i came back it was still not fully charged. Thats when i switched to the other charger. Coz this charger is slightly more powerful that the usb charger, i charged the battery for about 40 min, then returned it to the phone to check, & it was charged :victory: . I would suggest if you charge outside the phone, charge the battery for 30-40min at first, then check if its charged. If not, charge in 15 min increments until it's charged, this way you avoid damaging the battery.

Sorry if this post is too long, didnt know how else to explain this in simpler terms :p

Hope this was helpful.