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Lipo IR reading


Russ P
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When should I take the IR reading of a lipo. Before or after recharging.

The reason I ask is that I have a 6 cell 5000mah that has had 45 cycles of balanced charging, and after the last flight it was hot and slightly puffy. I was going to retire the battery as the IR of 4 cells were 001 but 2 were 031. After a couple of days I thought I'd check it again and all the cells were 001. I will not use the battery again but wondered when the IR reading should be taken.

 

Russ

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The internal resistance of cell changes under different conditions which include, in no particular order...

  • The current(s) it was measured at
  • Temperature it was measured at
  • Chronological age
  • Number and depth of cycles achieved
  • State of charge when measured
  • State of charge during periods of storage
  • Rate of charge under use
  • Rate of discharge under use
  • Temperature during use
  • Historical temperature exposure
  • Time after charge or discharge when measured

IR is a very difficult thing to state without all the conditions it was measured under so no wonder some fanciful figures are advertised because it's very difficult to prove one way or the other.

 

 

 

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1 hour ago, Russ P said:

When should I take the IR reading of a lipo. Before or after recharging.

The reason I ask is that I have a 6 cell 5000mah that has had 45 cycles of balanced charging, and after the last flight it was hot and slightly puffy. I was going to retire the battery as the IR of 4 cells were 001 but 2 were 031. After a couple of days I thought I'd check it again and all the cells were 001. I will not use the battery again but wondered when the IR reading should be taken.

 

Russ

It might be worth running the battery under load with a voltmeter attached and monitoring each cells response to the load in terms of voltage drop. If 4 cells are 0.001Ohms and two are 0.031 ohms that's quite a big imbalance. May also be worth a couple of static flight simulation runs on the ground with balance charges between to see how the battery responds. If the IR increases more and the battery gets hotter/ more puffy it's probably time for the bin.

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Whilst I intend to bin this lipo, its just that there are so many variables and wondered if there was a definitive answer.

Previous lipos have shown their failing when charging and 1 of the cells takes significantly less voltage. Its only recently that I have checked their IR.

 

Thanks for the replies.

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On 04/06/2022 at 13:20, Russ P said:

When should I take the IR reading of a lipo. Before or after recharging.

The reason I ask is that I have a 6 cell 5000mah that has had 45 cycles of balanced charging, and after the last flight it was hot and slightly puffy. I was going to retire the battery as the IR of 4 cells were 001 but 2 were 031. After a couple of days I thought I'd check it again and all the cells were 001. I will not use the battery again but wondered when the IR reading should be taken.

 

Russ


As per @Andy Stephenson’s post, there are a lot of factors that influence IR. For that reason if you want to use it as a measure of battery health you need to measure it at consistent cell state of charge (SOC) and temp, and ideally long enough after charging or discarding that any difference in internal temp vs ambient has dissipated. If you are keeping track over time then you need to note the ambient temp and SOC alongside the IR readings.
 

Purely for reasons of practicality I measure at storage charge level (for me that is 3.7-3.8V/cell off load) just before charging for use - this means if there is an issue I will pick it up before charging, and because IR will be slightly higher at lower voltage which means it’s easier to see if a cell is starting to increase in IR versus the others (discerning between cells that all have very low IR is beyond most hobby grade kit).

 

Edited by MattyB
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The other trouble is that the battery IR is so low that a poor connection on the balance lead can skew the results, and if it is one of the mid pins then it will give a high IR reading on two adjacent cells. It will also mean that the error is the same on both cells.

 

In your case were the 2 cells at 31milmohm adjacent to each other, if so could this have just been a poor connection given how different they were and it has now reverted to normal.

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If you use a 4 wire connection to the battery where two are carrying the current and the other two are measuring the voltage you can eliminate the effects of contact resistance, it's called a Kelvin connection.

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

So you can use the main power connection to carry the current and the balance lead to measure the voltage QED.

Edited by Andy Stephenson
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