Bad cell .. Any advice for the bad cell warning on a P4P battery or is there nothing that can be done?

What it means: One of four cells in your battery went severely below minimum lipo voltage, which is usually 3.6 volts. Ideally, you should not discharge your battery below 3.65 volts / cell under load (i.e. drone in the air). Now if you fully charge your battery, you will most likely notice that the "bad" cell will show lower voltage than the rest of them. It should not be a problem when flying if you land around 3.7 volts under load, to be on the safe side. If, however, you decide to fly longer, bad cell will most likely experience a significant voltage drop at low voltage (i.e. 3.6 volts or so) and will cause the overall voltage of the pack to drop significantly, leading to a crash. Can it be fixed? No. It is a physical cell damage. I suggest you either fly carefully and land around 3.7 volts / cell or replace a battery. In either case, lipos do not like to be discharged below 3.65 volts I would say, under load. That is because when you land your voltage will most likely bounce back up to 3.72 volts / cell which is considered an empty lipo.

delfin2003 Posted at 2-21 10:27
What it means: One of four cells in your battery went severely below minimum lipo voltage, which is usually 3.6 volts. Ideally, you should not discharge your battery below 3.65 volts / cell under load (i.e. drone in the air). Now if you fully charge your battery, you will most likely notice that the "bad" cell will show lower voltage than the rest of them. It should not be a problem when flying if you land around 3.7 volts under load, to be on the safe side. If, however, you decide to fly longer, bad cell will most likely experience a significant voltage drop at low voltage (i.e. 3.6 volts or so) and will cause the overall voltage of the pack to drop significantly, leading to a crash. Can it be fixed? No. It is a physical cell damage. I suggest you either fly carefully and land around 3.7 volts / cell or replace a battery. In either case, lipos do not like to be discharged below 3.65 volts I would say, under load. That is because when you land your voltage will most likely bounce back up to 3.72 volts / cell which is considered an empty lipo.

Thanks. That's usefull information.

Are the P4 batteries significantly different than the P3 batteries?  I've been running my P3 and P2 batteries below 3.6 volts in a cell for years - about every flight - no problems.  

Mark The Droner Posted at 2-21 12:33
Are the P4 batteries significantly different than the P3 batteries?  I've been running my P3 and P2 batteries below 3.6 volts in a cell for years - about every flight - no problems.  

https://www.youtube.com/watch?v=WP59zpUhP4E

The problem is that go4 won’t let the P4P motors start with the warning I’m getting. There was a similar issue with the Inspire battery’s where the app would stop the motors firing up if there was even a slight drop in any of the cells. I’m wondering if this is a similar fault in the app

fansdb6ff1f7 Posted at 2-21 12:41
The problem is that go4 won’t let the P4P motors start with the warning I’m getting. There was a similar issue with the Inspire battery’s where the app would stop the motors firing up if there was even a slight drop in any of the cells. I’m wondering if this is a similar fault in the app

Is your battery warm enough?  Can you post your flight log via phantomhelp.com?  Maybe we'd see something.  

https://www.phantomhelp.com/LogViewer/Upload/#

Mark The Droner Posted at 2-21 15:34
Is your battery warm enough?  Can you post your flight log via phantomhelp.com?  Maybe we'd see something.  

https://www.phantomhelp.com/LogViewer/Upload/#

When it says bad cell it means bad battery, don't use it. The Intelligent Battery controller won't allow the battery to go below a certain voltage if the battery is good. A battery is a lot cheaper than a new drone.     Battery Care

Hi, fansdb6ff1f7. Thanks for reaching out! If an Intelligent Battery has been stored for an extended period but has not been charged and discharged, its protection board cannot balance the battery's cells sufficiently to avoid a significant voltage difference. When the voltage difference exceeds the safe threshold, the battery's cells will become damaged. As already been mentioned, prolonged usage of a battery that has a bad cell could cause a potential flight risk.

To avoid this and maintain the battery properly, we recommend following the Phantom 4 Series Intelligent Flight Battery Safety Guidelines: https://dl.djicdn.com/downloads/ ... y+Guidelines+En.pdf

In your case, we strongly advise returning the battery back to our Service Center if it is within the warranty period to have it check and replaced.

Mark The Droner Posted at 2-21 12:33
Are the P4 batteries significantly different than the P3 batteries?  I've been running my P3 and P2 batteries below 3.6 volts in a cell for years - about every flight - no problems.  

https://www.youtube.com/watch?v=WP59zpUhP4E

Just because you can, doesn't mean that you should. 3.7 volts at no load, for lipo, is considered empty. At low voltage, under load, cell voltage drops very quickly. This is how you get a bad cell. It will never balance right.

Look at the discharge curve above. Keep in mind, the curve is showing 2C discharge, drone is pulling more.

Thanks for your post.  So you're saying the P4 batteries are no different than the others.  Okay.  That's what I thought.  

Your comments don't jive with my experience.  For example, as you can see from my video above, I hit a low cell of 3.62 volts within the first 60 secs of flight.  And yet I flew 15 minutes with no problems.  And this flight was last spring.  And the battery is still fine.  I actually flew with it yesterday.  Same battery.  Serial #804.  I've got thirteen DJI Phantom batteries, 11 of them are authentic DJI, two are aftermarket.  One or two I've had since 2015.  They are all in great condition which is important since they are extremely difficult to find these days.  I try to take care of my batteries.  I treat them like pet hampsters.  I keep a close eye on them.  I make sure they stay warm and dry and clean and well fed, but not too well-fed.  I make sure they get plenty of exercise, but not to the point of exhaustion.  I keep them at around 50% charge.  I check them all the time to be sure.  I pre-warm them before flying in cold weather.  I keep the posts clean.  I've never had a battery problem with a DJI battery in 6 years of flying.  Hundreds of flights.  I've never gone to auto-land during flight except a few rare times when I was already landing - and it was because of the percentage, not the voltage.  I've never had one shut off.  I don't like to fly them below 3.5 volts / cell.  Obviously that's under load.  That's where I might get a warning - when I drop below 3.5.  I'm always on my way to homepoint anyway by then.  I get the warning, but nothing ever happens with the flight.  Typically I'll land at or above 3.5 volts at the lowest cell.  It's very rare I would get into the 3.4 volt range.  Maybe one flight in one hundred.  After I land and turn the motors off, the voltage jumps up.  In my video, you can see the running low cell pops up to 3.69 after the motors are shut off after being down as low as 3.49 during flight.  

BTW, that P3 battery in the vid is technically my worst one.  The balance tends to routinely misbehave early in the flight.  Then it calms down.  It's been like this since I bought it.  And ironically it's also one of my newest ones.  Point is, my old batteries are in fantastic shape.  So based on my experience, I would not be concerned about 3.7 or 3.6 volts.  I would be concerned about 3.5 volts.  And I am.  I know routinely flying below 3.5 volts per cell will dramatically shorten the battery life.  My understanding is, the DJI Phantom will go into auto-land when any cell hits below 3.3 volts.  

Here is an in depth video discussing battery behavior under different conditions.  He doesn't mention anything regarding 3.7 volts or 3.6 volts per cell, however.  



EDIT:  I think the problem is the graph in post #9 does not apply to DJI Intelligent Lipos because the C ratings in the graph are too low.  I believe the C rating for the DJI Intelligent Lipos is something like 10C.  I actually have an aftermarket P2 battery that says "10C" on it in big bold letters.  Here's an example:

Rf7da4c29c1196fd154b8eb6a759df637 (600×600) (bing.com)

So adjusting for the graph, the line for a DJI intelligent Lipo would be below the green 2C line.  It makes sense that the voltage would drop quicker, from 4.2 v to below the green line, then smooth out in the 3.7 - 3.5 range, then begin dropping quickly again below 3.5.  Hence, the warning in the Go app, the warning in the log, the auto-land at below 3.3v, etc.  And that's exactly how my batteries have been behaving over the years.  Thanks again.  

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Interesting video, but, don't treat it as a rule to go by. What I can say is this, it depends on the battery chemistry. In the given video, at around 2 min mark, you can see that the voltage under load is 3.4, but once the drone lands, it quickly recovers to around 3.8 volts. I would say that it is fine, it's a good indication that that the battery still has enough juice to feed the drone. However, it may not be the case with all the batteries. I have been flying RC for over 10 years, used all kinds of batteries, and settled with Pulse Lipo. The reason is, they hold voltage under load extremely well. You won't ever see a huge "bounce back" as you see in the video above. They will recover, but maybe to around 3.55 volts a cell (I would imagine), assuming I drive them low. I can't tell you how much P4 battery will bounce back, but I doubt it will be 0.4 volts, that's huge, but again, depends on the battery. Thunder Power RC lipos have a good bounce back, but they also experience huge voltage sag under load, which in my case, wasn't acceptable.

From a personal experience with drivng a lipo too low. I've driven two of my 6s Pulse Lipos to around 3.3 volts under load, never intended for it to happen, but it happened because I wasn't paying attention. After a couple of weeks I went flying, and once i landed my plane, battery was puffy. Same thing happened to the second battery as well. Could never balance it right after that. So the lesson was learned.

It's good that you keep them at storage voltage around 50%. It is how it is supposed to be. Ideal storage voltage for lipo is 3.8 to 3.85 volts a cell. They can be stored for a very long time.

I would still strongly advise anyone flying their drone to not drive them below 3.7 volts a cell when landed.