What is the acceptable lipo internal resistance number range per cell or in total, to decide whether a lipo is past its good performance window and/or life span?

I think it depends on the device you use to measure the IR. I personally use an i-charger 406 and I found that when batteries get over 2.2 ohm, they seem to start to lack punch. That's the threshold I use, but it's also based on the data I get from my charger. Yours might yield different results.

Martin Paradis

Strongly depends on the charger, cables, battery form factor, temperature etc. Ignoring those caveats, typical 2 cell standard sized packs, ideally between 1-3 per cell. Shorties, ultra thin packs, or the extra thick packs, tend to have a higher baseline, so 2-5 per cell.

Fresh packs from reputable manufacturers should show just under to just over 1 mOhm per cell.

That is really dependent on the device you use to make the measurement and it's calibration. I have an i-Charger and I've never seen IR under 1.5 mOhm. It's possible that my device needs recalibration but then as long as I use this as to compare the performance of my batteries over time, it doesn't really matter.

It all depends what motor you are using. If you run modified with low turns motors then yes, you want a low IR for the max punch and speed. With high turns motors the current draw is much lower and so the voltage loss over the internal resistance is also much lower. The main electric class I drive is 1/10 scale F1, my packs in a good condition have 3 mOhm per cell and my bad packs have cells of about 8 mOhm and it is hard to notice the difference on the used 21.5T motor.

It is interesting that those are your findings. It has been my experience, and of course one persons experience in not a universal truth, that batteries make more of a difference than motors for spec racing.

I use the Wayne Giles ESR (not a true ESR meter, it is his acronym) meter to check my batteries. I have one of original units, the newer one have more features. I check my batteries at room temp for comparisons. When you check them on a charger you are checking them when they are heating up a bit and this lowers the IR. You'll find generally with a charger that the discharge cycle will have a lower IR than charging. Well, that is if you are charging at 10-15 amps and discharging at 30-40 ams (model dependent). Some will say their batteries are 1mΩ/cell this is usually during a 40A discharge cycle. My good batteries will generally read around 2.0-2.4mΩ/cell (i've had a couple that get down to 1.8mΩ/cell) when charging at 12A and at room temp on my ESR meter they will read between 4-5mΩ/cell.

I'd also argue that mod, while certainly drawing more power than stock from the battery, doesn't rely on an optimal battery as much, since even a "normal" quality battery can provide enough power. Stock however, is much more limited by blinky ESCs and low power stock motors, so people chase after every bit of extra power / punch they can find, including the batteries.

You don't need a punchy battery for mod. They have an abundance of punch.

IR is a variable based on quality of battery and capacity. For a typical race grade pack in the 5Ah range a brand new pack might measure all cells below 1 mΩ, and then as the pack ages/degrades you will slowly see the IR increase over time... anything below 3 mΩ is still considered "low IR" but I start to notice pack swelling and degradation of performance (fade, etc) as soon as any individual cell goes past 7 mΩ... it's a very subtle difference and is hard to tell if you use the same pack and that's where having an IR Meter helps you decide when to retire a pack. I might still keep an aged pack for practice up until any cell hits 15mΩ but at that point I typically toss them out or give them away. I have also found that when any cell is farther than 1-2 mΩ apart, then the pack tends to take longer to balance and that's when I may consider selling the pack provided the cells are still below 7mΩ... I make sure to post IR readings of any packs that I sell and I would refuse to buy any used pack without IR readings so you understand the current condition of the pack!

Basher grade packs tend to start out around 5mΩ so it's all relative to your starting point.

I always check my IR readings immediately after a full charge, I also get lower IR readings when pumping/cycling the batteries as well.