One of my P2 batteries with rather few cycles shows four green bars, but when I fire up the drone, I immediately get the audible warning of critical low battery voltage.  The DJI Vision app on my phone shows the battery status bar across the top as over 80% charged, but at the same time I'm getting red text flashing the Critical message  (can't remember the exact wording).

Is this battery toast?

Sounds like the battery cells are badly out of balance, ie. you have a bad cell.  Take a look with Phantom 2 assistant.

Maybe do the deep discharge cycle....   below 8% and then recharge.

Please try the advice in post #2 and #3 at your earliest convenience.

Thanks, I'll take a look at those suggestions today. I'm in the UK, so my timezone is likely different from yours!

OK,   Assistant says all three cells more or less at the same voltage, all around 4050, see pic, so next is how do I do a deep discharge cycle?

Run the battery down until 8% or less left (or the aircraft turns off)...    It's also mentioned at the bottom of your battery status picture....  Here is a thread talking about it

https://forum.dji.com/thread-89494-1-1.html

This is a puzzle.  Battery looks fabulous according to the data.  I'm not sure I'd do a deep discharge quite yet.  Could this be an aftermarket battery?  Have you been storing it correctly?  Is it swollen at all?  Maybe try cleaning the battery contacts - all of them including data contacts on both battery and bird and test it further.  

Otherwise, if you do decide to do the deep discharge, read the manual and do it correctly.  Start with a fully charged battery.  Discharge to BELOW 8%.  Fully charge the battery once again.  See documentation.  

pasaytenpete Posted at 2017-5-18 05:07
Run the battery down until 8% or less left (or the aircraft turns off)...    It's also mentioned at the bottom of your battery status picture....  Here is a thread talking about it

https://forum.dji.com/thread-89494-1-1.html

Thank you.  I hadn't actually read that bit at the bottom of my screenshot!

Mark The Droner Posted at 2017-5-18 05:15
This is a puzzle.  Battery looks fabulous according to the data.  I'm not sure I'd do a deep discharge quite yet.  Could this be an aftermarket battery?  Have you been storing it correctly?  Is it swollen at all?  Maybe try cleaning the battery contacts - all of them including data contacts on both battery and bird and test it further.  

Otherwise, if you do decide to do the deep discharge, read the manual and do it correctly.  Start with a fully charged battery.  Discharge to BELOW 8%.  Fully charge the battery once again.  See documentation.

Cleaned the contacts with surgical spirit (aka rubbing alcohol here in the UK) but it made no difference.  This is a genuine DJI battery, supplied with the drone when new a little over a year ago (I know, already obsolete even then!)  Stored at room temp, but I didn't do anything about discharging it, just left it (with its mate) in the carrying case. I read that they will discharge themselves automatically after 10 days, but mine never have, as far as I can tell.  The other battery behaves perfectly.  This one is not swollen and looks the same as its pair.  I guess I'll read up the deep discharge and take it from there.

The P2 series batteries don't auto-discharge.  So if it's fully charged and you don't plan to fly, you need to discharge it manually.  

Mark The Droner Posted at 2017-5-18 09:03
The P2 series batteries don't auto-discharge.  So if it's fully charged and you don't plan to fly, you need to discharge it manually.

How do you discharge manually? Leave the drone turned on and running until it turns off?

You could, but I have a discharger.  

https://www.ebay.com/p/?iid=262384376907&&&chn=ps

Mark The Droner Posted at 2017-5-18 10:02
You could, but I have a discharger.  

https://www.ebay.com/p/?iid=262384376907&&&chn=ps

Thanks, I will look it up.

Homer Simpson Posted at 2017-5-18 09:36
How do you discharge manually? Leave the drone turned on and running until it turns off?

That's what you would do in this case, it can take a while when just sitting idly. Do you recall whether there are any error messages in the Assistant software?

I found a thread that said just leave the drone switched on but not doing anything, but no indication as to when your know you've reached 8%.  Anyhow, I left it like that and forgot about it, and now the battery showed no lights at all, and when I removed it and pressed the On button, it shows just one flashing green bar, so I assume that's somewhere near discharged, and tomorrow will charge it and see what happens.  I'll leave myself a note (It's bed time! to plug in the Assistant and see what that tells me first)  I may change what I do, depending what that tells me, but will report tomorrow.

Homer Simpson Posted at 2017-5-18 09:36
How do you discharge manually? Leave the drone turned on and running until it turns off?

I would just fly it to the level you want to store it and then not charge. More fun that way.  

I left the drone switched on until the battery lights all went off, left it overnight, then today charged it up.  It took a long time for the lights to come on, but then seemed to charge normally.  First time I fired up the bird, I got a critical battery error, saying (sorry, didn't write it down) there was something wrong with one of the cells.  Shut down, fired up again and it looks perfect.  Vision app says Ready to fly.  Motors fire up (inside with no props on) and run for a minute or more without problem.  If the weather ever improves, I'll take it out and fly cautiously to see what happens.  Also looking at battery discharges now.

Good to see that the deep discharge cycle has hopefully restored your battery!  

Rob Clack Posted at 2017-5-19 05:35
I left the drone switched on until the battery lights all went off, left it overnight, then today charged it up.  It took a long time for the lights to come on, but then seemed to charge normally.  First time I fired up the bird, I got a critical battery error, saying (sorry, didn't write it down) there was something wrong with one of the cells.  Shut down, fired up again and it looks perfect.  Vision app says Ready to fly.  Motors fire up (inside with no props on) and run for a minute or more without problem.  If the weather ever improves, I'll take it out and fly cautiously to see what happens.  Also looking at battery discharges now.

Good one, glad that worked, have to do it to mine now.

The same thing happens to me with two original DJI batteries. They do not have more than 11 charges one of them and the other 12 charges.
Either way, it's the same for me.
Charged at 100% one of them at the first minute of flight flashes red quickly and descends at full speed to the ground. And the other does the same but two minutes of flight. The DJI application of the PC does not give me errors in the batteries.
What can be?

Note: I must say that I did several deep discharges to the two batteries and still they still give me problems.

Was wondering how about doing deep discharge by running motors (props off) at idle until AC shuts down?