I have a pretty new Tattu 4S 1800mAh 45C.  It was involved in a crash that sent the battery flying out of the quad.  
One of the cells now shows 0.00v and two of the other cells shows 7.71v.  Is this fixable or do I need to scrap it?
I have never taken apart a LiPo before.

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I should also note that there does not appear to be physical damage to the battery.  (ie no swelling or breaks)

There must be some physical damage as the only way you could get two cells showing 7V is if the wiring to the balance leads has somehow become short circuited internally so that there are actually two cells connected to the same balance wire.

You can get new shrink wrap that is made for creating your own battery packs from individual cells, so get some of that and carefully remove the shrink wrap for your battery until you can get to the wiring. It should be fairly obvious what is wrong once you've got the wrapping off.

Once you've fixed what has happened to the balance wiring, you can re-wrap your battery and it should be good as new!

(05-Apr-2017, 07:30 AM)unseen Wrote: There must be some physical damage as the only way you could get two cells showing 7V is if the wiring to the balance leads has somehow become short circuited internally so that there are actually two cells connected to the same balance wire.

You can get new shrink wrap that is made for creating your own battery packs from individual cells, so get some of that and carefully remove the shrink wrap for your battery until you can get to the wiring. It should be fairly obvious what is wrong once you've got the wrapping off.

Once you've fixed what has happened to the balance wiring, you can re-wrap your battery and it should be good as new!

Right, I know there is physical damage internally, I was just saying that upon visual inspection, it appears fine. 

So, one of the balance leads has come off and two of the cells are shorted together. (If I understand you correctly)
Ill have to take this apart when I get home from work tonight; and get some shrink wrap to put it back together.

Just found this as well: https://oscarliang.com/fix-lipos-bad-cell/

(05-Apr-2017, 01:34 PM)PaulMek Wrote: Right, I know there is physical damage internally, I was just saying that upon visual inspection, it appears fine. 

So, one of the balance leads has come off and two of the cells are shorted together. (If I understand you correctly)
Ill have to take this apart when I get home from work tonight; and get some shrink wrap to put it back together.

It does sound like it. You have one lead with zero volts and two leads with 7 volts. I'm not quite sure what's happened physically, but you'll soon see once you get the wrapping off!

(05-Apr-2017, 01:45 PM)PaulMek Wrote: Just found this as well: https://oscarliang.com/fix-lipos-bad-cell/

That's a great resource for what it should look like when you unwrap (and good diagrams for how it is wired).  I just want to stress to be careful with the loose wires and your soldering iron if you try the repair.  It's easy to accidentally short circuit things, with pretty bad results. (voice of experience...)

EDIT: Wear protection! (Personally, I would ask myself "Is it worth the risk" - I've built dozens of packs from A123 batteries, but I'm still a little leery of trying to repair lipos.)

So, update:
Pulled the wrap off and everything looked ok. Grrr... what the heck. Grabbed my multi-meter and tested every point along the way with the previous point and everything came out to 3.95v. Now I was frustrated. Tested each at the balance connector again and the first cell still read 7.71v... Then it dawned on me. I must have switched the first two cell leads on the balance connector when I replaced it after the crash. Duh. The charger readout subtracts the previous voltages from the balanced leads to get the single cell voltage.
Plugged it back into the charger and now it looks normal. Now I just have to find the battery wraps to put it back together.
Thanks for the help guys.

Ah... that explains things!

Sorry for misleading you in my first reply, but you didn't say that the leads had come out of the balance connector in your first post and that you had put them back into the connector shell.

I'll remember this next time I see someone post a similar problem!

(06-Apr-2017, 05:43 PM)unseen Wrote: Ah... that explains things!

Sorry for misleading you in my first reply, but you didn't say that the leads had come out of the balance connector in your first post and that you had put them back into the connector shell.

I'll remember this next time I see someone post a similar problem!

It didnt cross my mind either.  I had set that battery aside for about 2 weeks while I waited for the new balanced connectors to arrive. 
The red and the first two blacks had been separated when the connector ripped apart. I just used electrical tape to make sure the ends didnt touch.