Cell Voltage Deviations

[RadioFlyerMan]

In various posts regarding battery health, etc., I have seen pictures of cell voltage deviation tables from AirData. It appears that AirData calculates a deviation as the difference between the voltage of the two cells that are the lowest of all cells. So for a MP1 battery, they ignore the cell with the highest voltage of the three, and then subtract the voltage values of the remaining two cells. Likewise for an MP2 battery, they ignore the top two cells, and then use the difference in the remaining two cells with the lower voltage.

This algorithm seems illogical to ignore the difference between the cell with the highest voltage and the cell with the lowest voltage as the an expression of voltage deviation.

Either I’ve got AirData wrong, or I failed to understand the meaning of a voltage deviation.

Can any of you more knowledgeable people out there please give me some guidance about this topic?

Best wishes, and have a nice flight!

 

[old man mavic]

i think it has to do with the length of the time, that the lower cell reading would have to do with the flight duration overall, as the voltage would drop off quicker

 

[RadioFlyerMan]

OK, So focusing on the weakest cell is a way to judge the efficacy of the entire battery as to its efficiency, duration and/or potential failure. Makes sense. I guess my problem or misunderstanding is what if there is one strong cell and two very weak cells that just happen to have near equal voltages? Then the deviation would be numerically small and mask a potential problem. This example perhaps is over analyzing and maybe rare, but that’s what prompted my concern.

FYI... I’ve only been flying since last August, and I have never even heard of a LIPO. So I’ve got a lot to learn.

 

[Former Member]

Im not sure about Airdata, but to me, cell deviation is a comparison of the difference between the 3 cells in the MP. It's a good thing that you are seeking help in learning this stuff, instead of not checking your batteries at all.

With DJI batteries, I have found that as long as you treat them well, cell deviations will rarely be a problem. However, after a crash, cell deviations is a telltale sign of potential failure.

 

[old man mavic]

OK, So focusing on the weakest cell is a way to judge the efficacy of the entire battery as to its efficiency, duration and/or potential failure. Makes sense. I guess my problem or misunderstanding is what if there is one strong cell and two very weak cells that just happen to have near equal voltages? Then the deviation would be numerically small and mask a potential problem. This example perhaps is over analyzing and maybe rare, but that’s what prompted my concern.

FYI... I’ve only been flying since last August, and I have never even heard of a LIPO. So I’ve got a lot to learn.

the cells should all be pretty much the same voltage when fully charged ,but they will not be exactly the same, a small difference is not really a problem, but if one of the cells was significantly different to the others then that would not be good

 

[old man mavic]

when the cells are being charged the, balance part of the intelligent charge control in the battery case monitors each cell, and adds or subtracts voltage to get them as near as possible to be the same and in the DJI go 4 app you can view each cells charge state

 

[RadioFlyerMan]

Thanks for the info. Didn't realize how the charger worked to balance things out.

I've read a lot of posts about batteries, how to treat them to sustain their life. The most notable are issues about cell voltage and swelling. And I've also learned that batteries can last a long time if taken care of. However, I was wondering, except for swelling (which is obvious), is it possible to analyze various parameters of a lipo battery such that one can observe its decline, provide a decision point to stop using the battery, or even "predict" its failure. Predicting failure might not be possible.

I am not a subscriber of Airdata because I am too cheap, and mostly because my other hobby is computer programming. So to extend my enjoyment of the drone hobby, I am writing my own computer analysis software of battery data as derived from the flight records. That's where cell voltage deviation comes into play.

My analysis on battery data goes well beyond deviation as a function of time. I'm looking at power and total energy usage (watt-hours) of the battery over many flights as an independent variable to assess voltage deviations and capacity (mAh) decline. And I have noticed a dependency of voltage deviations as a function of power and flying style.

Gotta tell ya, this is interesting stuff and I'm having fun with it!

 

[charliesRig]

Let me see if I can Mud up this topic....
Lipo do not like to be charge or Depealted rapidly They tend to go into a"Runaway" state
and could catch on fire
.....
I recently had a MP1 Battery fail (in Flight)
Born on date 02/17
It charged..
Looked good...
but towards the end of my flight (23 minutes) two of the three cell
discharge abnormaly fast.

The point I'm trying to make...... You wouldn't know how healthy you battery is Unless
you put a load on it and watch all three cell and there values as they discharge
.
here a snapshot of that flight

in hinsight
I lost communication between controller and craft several time before the end ..
That Should have been a big red flag to Bring it home.!
But mavic flight sometimes make it seem that your a better Pilot then you truly are
I"ll admit it i got into that zone


.
.
As a Rule
...... never fly with less then 20% battery
.
.
.....cR
.
.

 

[Former Member]

I always plan to land with 40% reserve. That's usually plenty of flight time for me to get what I went for. If I need to fly more I just pop in a new battery and charge that 1 back up.

 

[DanMan32]

My experience has been, don't let them stay in hot environments too long. Granted heavy loads will cause batteries to get hot, and as they deteriorate, their internal resistance increases which accelerates the heat under heavy load.
We all know power through resistance equals heat.
P=IV where I = current, V = voltage
V= IR
Substitute V in first equation with 2nd equation.
P=IIR or I^2R.
As resistance increases, or the current through the resistance, power dissipated by the resistance increases. If the resistance in question is in the battery itself, the heat dissipated is also in the battery. Battery heats up which can potentially deteriorate it more, increasing its resistance.
A higher resistance reduces the power available to the intended load.

 

[brett8883]

In the DJI Go app under the battery menu the is an additional “sub menu” to check the battery health. It’s something like “additional details.” It will tell you the manufacture date, the #of times charged (cycles), and also a % indicator of the battery life.

Every time a lipo is discharged it loses some amount of its capacity to hold energy. We measure “capacity” in mAh (milliamp hours). The battery life % indicator is a measure of how the battery’s capacity compares to when it was new.

Lastly the menu will in plain English let you know if, according to the algorithm installed on the “smart battery” by the manufacturer, if the battery is operating normally for its age and # of charge cycles.

To give you some sort of benchmark I drive my batteries hard, I do everything you aren’t suppose to do like charging immediately after flight with an 80 watt car charger(normally 50 watts) and each of my 4 batteries have about 300 cycles each and they are indicating 75% battery life, so I’ve lost 25% capacity. In the real world though I’ve maybe lost one minute of average flight time and that could just as well be do too the cold weather.

DJI batteries are beasts

 

[Elude]

I use air data and own two batteries. Air data always says there is something wrong with the cells number four in both batteries

 

[RadioFlyerMan]

So far this has been an interesting discussion. I’m going to continue with my analysis of battery parameters because I don’t have a full understanding of their interdependence, and because I enjoy messing around with it.
In addition, here is my highly sophisticated testing method to determine swelling...