AirData battery deviation- what causes the changes of it mid-flight?

[tmtwomey]

My Spark 3 cell batteries always show minor/major deviations only on cell 3. My Mavic 2 4 cell batteries only show on cell 4. Someone mentioned the last cell being connected to the positive connection to the battery. Could that be why the deviations show up in the last cell ? I now have a battery that has recorded deviations over .11 on cell 4. DJI Go shows the battery as fine.

 

[jzilla80]

My mavic pro batteries are 3 cell , manu . date july 2017 and aug 2018 , I fly for about 18 mins. on average with anywhere from 40% to 20 % battery left --- so max flight time ??????

Hmm, so different dates of manuf, so likely different production lots, so there goes that idea.

It is interesting tho that both our packs show deviation in the last cell!

POLL:
[emoji830]︎ Are other pilots of DJI machines experiencing this with their batt packs: 'Most cell deviations occur in the last cell of a pack (3 for several drones/4 for Mavic 2)'

Seems like if there were something to that, there would be a forum post or something specificly about that, but Ive done a fair amountbof searching and havent seen anything.

 

[jzilla80]

My Spark 3 cell batteries always show minor/major deviations only on cell 3. My Mavic 2 4 cell batteries only show on cell 4. Someone mentioned the last cell being connected to the positive connection to the battery. Could that be why the deviations show up in the last cell ? I now have a battery that has recorded deviations over .11 on cell 4. DJI Go shows the battery as fine.

I wondered about this too... As I understand it, DC electricity flows from Neg to Pos, but Im not sure if that would make the first or the last cell the cell under heavier current draw?

Or it might not make any difference, just dont know.

Hopefully someone can tell us, one way or the other, if were in the ballpark or not?

Thx

 

[WithTheBirds]

I wondered about this too... As I understand it, DC electricity flows from Neg to Pos, but Im not sure if that would make the first or the last cell the cell under heavier current draw?

Or it might not make any difference, just dont know.

Hopefully someone can tell us, one way or the other, if were in the ballpark or not?

Thx

The cell with the highest internal resistance will have the highest voltage drop when a load is applied. Doesn’t make any difference where in the series connection chain it is.

 

[DanMan32]

The cell with the highest internal resistance will have the highest voltage drop when a load is applied. Doesn’t make any difference where in the series connection chain it is.

And current is equal at all points in a series circuit.

 

[WithTheBirds]

And current is equal at all points in a series circuit.

And? Would be interesting to learn how your general observation might prove relevant here. We are interested in the performance of the series connected cells as individual voltage sources. It is the fact they aren’t matched in current delivery capacity with respect to the load that is the issue.

 

[DanMan32]

"Im not sure if that would make the first or the last cell the cell under heavier current draw? "

Just pointing out current is the same for all the cells connected in series.

 

[WithTheBirds]

"Im not sure if that would make the first or the last cell the cell under heavier current draw? "

Just pointing out current is the same for all the cells connected in series.

And I provided the relevance and explanation (including the fact that position in the series chain of a cell was irrelevant).

Matched performance characteristics are critical for cells connected in series. The whole pack is only as good as the weakest cell. Measuring equivalent series resistance (IR) of the cells provides a convenient and arguably the most reliable means of assessing to what extent cell condition might be matched and performance predicted.

Are you able to suggest why it might often be the same cell in the pack that exhibits the highest reported voltage deviation? That is the real question.