by John Salt - Updated March 2023
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.
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 an important data set to monitor in this hobby, yet it's rarely discussed.
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 2 to 6 milliOhm's (0.002 to 0.006 Ohm's) of internal resistance when brand new 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.
As a general rule, 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 almost 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.
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.
As seen here, I will then write that number (or the IR of all the cells in the pack) somewhere on the pack with a permanent marker 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 therefor, the internal resistance decreases. I normally take readings while packs are at room temperature as in that example above. If I took an IR reading right after a hard flight when that pack is "toasty", most of those cells would be reading 1 milliohm or less.
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.
This again is where good computerized chargers come into play. The good ones that support this feature will check the "IR" of each cell.
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.
Pictured here, I am taking the IR reading of each cell in this new 6S Turnigy LiPo using my iCharger RC battery charger.
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!
As I just mentioned, the specific method used to measure internal resistance will yield wildly variable results. Temperature of the pack can also vary the reading a fair amount.
In other words, it's always best to take internal resistance measurements with the same type charger, at the same temperature, and if possible, at the same state of charge (ie. 50% storage capacity) when you are making comparisons between values you took when the pack was new, and as it ages over time.
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 doesn't 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 opposite can also be true where the internal resistance can stay fairly static to new values, but the capacity drops off substantially for various reasons.
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 fairly high, I've re-purposed to run my little TS100 soldering iron which draws very little current in comparison.
Those older and tired 6S packs don't even get the slightest bit warm running the TS100.
For that low power demand application, they likely still have lots of life and charge cycles left in them.
Their capacity is still very 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).