LiPo Internal Resistance
Is your battery still healthy?

by John Salt - Updated October 2023

LiPo Internal Resistance Values In Milliohms

LiPo internal resistance (IR) is a useful measurement that all electric RC'ers should at least be aware of. 

It's not the end all - be all, but it can certainly be used to gauge cell performance, efficiency, and what's important to many of us - Battery Health & Discharge Performance.

Internal resistance of both the LiPo battery and the individual cells within the battery is one of the very best ways to monitor your RC LiPo battery's condition.

By comparing brand new IR readings of the cells to ones you take thereafter as your LiPo pack/s age, you will have useful new vs. old condition data.

As far as I'm concerned, internal resistance of your LiPo's is such an important data set to monitor, yet it's rarely discussed. 

So what exactly is the internal resistance of any battery?

All batteries no matter what the chemistry have an inherent resistance within them, thus internal. It's why batteries get warm when in use; that resistance to current flow is using up power in the way of heat generation. 

A very simplified way to think of this internal resistance is how efficient the battery pack (and cells within the pack) are at flowing power from the battery to your model. The higher the internal resistance, the less efficient the pack is and the less current it can safely provide.

Just think of it in the same context as putting a big stinking resistor between your battery and your ESC/motor. The higher the resistive load is, the larger the voltage drop will be, and the more the resistor (the battery in other words) will heat up, and the slower your motor will run.  

Most decent higher capacity and higher discharge rated RC LiPo cells will have very low amounts of internal resistance, numbers of 1 to 4 milliohm's (0.001 to 0.004 Ohm's) of internal resistance when brand new at room temperature (72F / 22C) are typical.

Smaller micro sized LiPo cells will have higher resistances. It's not unusual for example to measure internal resistance numbers in the region of 150 milliohms on smaller 100 to 200 mAh micro park flyer LiPo cells when they are brand new.

The smaller the capacity, the higher the internal resistance.

To calculate the total internal resistance of a series wired pack, you would then add these cell resistance numbers together. As an example, a 4S pack with each cell having 3 milliohms of resistance will show a total internal resistance of about 12 milliohms (0.012 Ohms).

As LiPo packs age, their internal resistance always increases and that voltage drop will get higher and higher, causing the motor to run slower and slower. The pack will likewise run warmer and warmer while the maximum safe constant discharge current gets lower & lower. 

So, the best way to use internal resistance (if your charger supports this very useful function) is to take an IR reading of your LiPo/s when it/they are brand new.

Recording & Marking New LiPo Internal Resistance Of Each Cell

As seen here, I will then record that number (the IR of all the cells in the pack) somewhere on the pack with a permanent marker or label maker so I will always have a brand new IR base reference for that particular battery. I then put some clear tape over the numbers so they don't slowly rub off over time.

As this pack ages, or if I ever over-discharge it, I can simply reference how the resistance is increasing, or if one or more cells is/are getting ready to take a dump!

Another IR tip is to measure IR at the same temperature when you are comparing IR readings. As LiPo cells warm up, ion exchange efficiency increases, and therefore, the internal resistance decreases.

I try to take readings while packs are around room temperature (72 F / 22C ) as in that example above. If I took an IR reading right after a flight when that pack is "toasty", most of those cells would be reading 1 milliohm or less. If however I took the readings in a colder garage, they would be double or more; so resting temperature does have a big impact on your actual IR readings.

State of charge when taking IR readings. I used to think state of charge had an impact in the IR readings because after fully charging a battery, the IR always seemed to be a little bit lower than when I started. However, what was causing that is the pack was slightly warmer after charging. I have since come to the conclusion state of charge doesn't cause much LiPo internal resistance variation.

I suggest taking your IR readings when the packs are around a 50% storage state (3.85V per cell) and have been sitting for at least an hour after charging or discharging to fully stabilize for the best overall consistency in and between readings.

Use the same charger. Different chargers often give slightly different IR readings so I strive to use the same charger when I'm initially recording, and then later revisiting IR values of my packs. 

Balance Plug & Wiring Condition Can Impact Accuracy of LiPo IR Measurements

Good wiring & connectors.

Balance wiring and connector condition is critical to obtain accurate IR readings.

We are talking milliohm (thousandths of an Ohm) measurements after all, and even a little corrosion or dirt on a connector pin can throw off a reading.

It never hurts to wiggle the balance wires or cycle the balance plug/board to see if the IR values fluctuate much between several readings; especially if you are getting weird numbers that you don't trust.  

One last important point. You can only measure the internal resistance of a single battery at a time. You can't for example have several hooked up to a parallel charging board because the measured resistance values will be much lower due to Ohms law as it applies to resistance in parallel circuits. 

How do you measure LiPo internal resistance?

This again is where good computerized chargers come into play. The good ones that support this feature will check the IR of each cell. They will have a special IR testing mode or will show the LiPo internal resistance while charging. 

All RC chargers I've used that support internal resistance measuring use the "frequency method" (AC conductance); which is fast, but always gives lower IR values than when using the DC load method. For more information on the various ways internal resistance can be measured, here's a good article.

Measuring the internal resistance of each cell in this new 6S LiPo.

Pictured here, I am taking the IR reading of each cell in this new 6S Turnigy LiPo using an iCharger RC battery charger that supports individual cell IR measurements.

It is hard to make out in the photo, but the IR of cells 1-6 are 2,2,1,1,1,2 milliohms each, giving a total IR for the entire pack of 9 milliohms - pretty respectable!

Using LiPo Internal Resistance To Calculate A Pack's Realistic Discharge C Rating

I briefly mentioned this on the LiPo Battery Ratings page, but this is perhaps one of the most useful applications of LiPo internal resistance I know of; using it to determine the safe maximum sustained discharge rate of any LiPo pack. 

As we know from that LiPo Battery Ratings page, the most overstated and misleading number given by most LiPo battery manufacturers is the discharge C rating. It's almost always higher than it really is; sometimes by a lot!

I therefore use this simple formula: 

You multiply the capacity of the battery by the highest cell IR reading in that pack, take the square root of that number and divide it into 2500 which will give a realistic continuous maximum C discharge rate for the pack. 

Below is an example on a brand new Gens Ace 6S 5100 mAh pack. The pack was about room temperature (23C), and at storage state while taking the internal resistance readings of the cells.  

Using The Highest LiPo Cell Internal Resistance To Calculate The Maximum Safe Discharge Rate Of The LiPo Battery.

Overstated 80C Discharge Rate

As we can see from the above photo, that 30C / 153 Amp number is a far cry from the 80C rate on the label. Think about it if true. That would be 80 x 5.1A = 408 Amps! The 10 AWG wire would vaporize as would the XT90 connector within seconds if 400+ Amps were passing through them.

Perhaps that 80C is a one time battery life ending current draw peak event if you shorted the wires together? The most extreme burst rating imaginable in other words; but not at all a safe or realistic sustained draw rating. It's nonsensical. It's also why any battery brand that boasts of 100C or higher C ratings, I just laugh at. Complete and utter marketing BS! Honestly, anything over 50C is dreaming. 

Regardless of most LiPo brands' overstated and often outrageously dishonest C ratings, what I also like about this formula is you can use it as your packs age to know what kind of current you can still safely draw.

Just plug the higher IR values into the formula as your packs get older. Even if/when these packs are getting upwards of 5 milliohms per cell, that works out to about 15C, so I should still be able to get a usable 75 Amps out of them (fine for scale flying). 

There are of course limitations with this C rate IR calculation method since it's only as accurate as the IR values you get, and as we know, IR readings can vary quite a bit depending on temperature, what charger you use etc.  

However, it certainly gets you in the ball park with a reasonable maximum discharge rate that is more realistic than what is printed on most battery labels!

Most folks get pretty upset when they use this formula for the first time on their so called 130C rated packs that they paid a premium for to find out the real number is closer to 30C. 

LiPo Internal Resistance Limitations

As mentioned, the specific method used to measure internal resistance will yield wildly variable results. 

In other words, it's always best to take internal resistance measurements with the same charger, at the same stabilized temperature, and if possible, at the same state of charge (ie. 50% storage capacity) when you are taking reading to calculate the packs C rating or making comparisons between values you took when the pack was new as it ages over time. All that will ensure consistency in your LiPo internal resistance readings if nothing else.  

Don't confuse capacity of the pack with internal resistance either.

Internal resistance can climb as a pack ages, while the capacity of the pack may not degrade too much for example. In this instance, the efficiency of the pack will be lower than it was when new, but it still may safely provide as long a drive or flight assuming you are not pulling more current out of the pack than it can safely discharge with the efficiency drop. The reverse can also  be true (drop in capacity while internal resistance remains low); but in my experience that is rare. 

In short, LiPo battery internal resistance is just one more tool you have to monitor your LiPo battery's condition. Don't rely 100% on it thinking that as long as the IR remains low, the pack is guaranteed to be healthy; also monitor how much capacity the pack takes.

And once again, just like I mentioned on the LiPo Battery Ratings page, feeling the pack temperature after a flight or drive is still my main go-to method of determining how my LiPo's are aging. If one is getting fairly warm for the same type of flying than it used to get, that's reason to think about retiring it or use it in lower demand applications.

Several of my 6S LiPo packs for example that I can no longer use in my helicopters because they were simply getting too warm and the IR values were getting too high, I've re-purposed.  

Pictured below for example, I'm using this tired 6S LiPo pack to power my little TS100 soldering iron which draws very little current (about 3 Amps max). 

Safely using a "tired", high internal resistance LiPo pack in a low power application.

7 year old 6S LiPo IR values have pretty much tripled after many flights.

This older and tired 6S pack doesn't get the slightest bit warm running the TS100.

For that low power demand application, it likely still has lots of life and charge cycles left. 

The capacity is still good, and the IR values of the cells while about triple of what they were when new, are still all very close to each other telling me they are all aging equally and there isn't one or two that are ready to fail outright. 

That's another important point; the IR values of the cells in your battery packs, both when new and as your packs age should all be fairly equal to each other. This is required for safe discharging and charging; your charger won't have to work it's guts out balancing the battery.  :)

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