[greggold]

John Hopke (in my opinion the best of the online RC bloggers) has a new video that shows how to assess the condition of your lipos.

The idea is you measure the number of mah during a discharge that goes from 4.2v to 3.6v, using a 2C current. This number should be above 85% of the nominal capacity, if not, he recommends discard the lipo.

The video shows a dedicated RC discharger being used to measure the percentage of nominal capacity, however I think most lipo chargers (that can discharge) can give you the measurement (of mah required to discharge).
The video is doubly worth a look as it explains several important points, such as why the discharge stops at 3.6v.

https://www.youtube.com/watch?v=PwvI5I2x900

The testing has a focus on 4S or 6S, 4000mah capacity lipos, and I think the smaller lipos I use (3S, 1000 or 1300 mah) may have higher internal resistance, and possibly should have a discard figure of 80 and not 85%.

[caffeine]

I have a variety of batteries in different conditions:

4x 3200mAh 3S Zeee - New or as new
7x 850mAh 3S CNHL - New or as new
5x 2200mAh 3S Turnigy - 6-8 years old, not well looked after (stored for years not at storage charge), slightly puffy
1x 2800mAh 3S 'Giant Power' - unknown age, given to me, quite puffy
1x 3600mAh 3S Turnigy - unknown age, given to me, very puffy (to the point that I've never used it)
2x 5000mAh 4S Turnigy hard case - > 10 years old, not well looked after (1 crashed, both hard cases cracked open due to puffiness)

I'm not actively using the last 4 groups of batteries, the ones I've highlighted in red are due to be sent to the battery recycling bin. I bought the Zeee batteries because I didn't want to trust the others.

I find that the older batteries seem to have reasonable capacity, but I find there is significant voltage sag under load.

I have my telemetry voltage alarm set to 10.8V (3.6V per cell).

With the 850mAh batteries I see that they take around 700-750mAh charge after flying, so 10.8 is a good cut off.

With the Zeee 3200mAh I haven't pushed them very low yet, so I only use 1500-1800mAh of the usable capacity. I can fly significantly longer with them than I do.

[greggold]

Some good points here... you are right that there are several existing methods of assessing lipo condition and Hopke's method should be used in conjunction with the existing methods. Hopke's method does not yet have a name, afaik, I will call it "lipo available capacity" or LAC which is the percentage of the total nominal capacity available for use before hitting the 3.6v drop off, under a constant 2C load. I am going to test LAC on my 3S lipos (I have 900 mah, 1000, 1300 and 1800) and 2S (I have 550, 650, 900 and 950mah) to see if the LAC value of 85% works in smaller capacity batteries. Hopke's video above clearly shows what happens at 3.6v in the discharge curve, in lipos of varying capacity.

I see you use your vbat measurement for the total lipo voltage; now I use my vbat for the rx voltage. For me the rx voltage is a key requirement to investigate a crash with the flight log and possibly rule out pilot error. But the lipo voltage is also really important, its something I gave up when I moved from Frsky to elrs. I think/hope soon we will have an elrs system that can provide both lipo voltage and rx voltage, drones with their flight controllers already have this.

At various times in the past I have attached a number to each lipo, and recorded in a log any problems such as sagging power, motor cutoffs, a cell >4.2v or <4.2 when the lipo is fully charged etc. Its a pain but now looks increasingly worthwhile, including info on LAC, on IR, any puffiness etc. I agree puffiness is a clear signal to discard the lipo. A rising value of IR is another signal, but I dont know how fast the rise is, or what the limit (to discard) should be.

[greggold]

Today I measured the lipo available capacity (LAC) on 4 of my lipos:

1. Dualsky 3S 900mah 120C: LAC 87.0
2. Tattu 3S 1050 mah 75C: LAC 79.1
3. Dualsky Ultra 2S 550 mah 50C: LAC 89.6
4. Turnigy Graphene 2S 950 mah 75C: LAC 88.1

Measurements were done on my ToolkitRC M6D charger.

I tried to follow Hopke as closely as possible, including discharging at a constant 2C rate. For the Dualsky 2S, the 2C rate was -1.1A, but for the other 3 lipos the discharge rate was a bit less: For the Turnigy 2S, 84% of the 2C rate, and for both the 3S lipos it was 61.1% of the 2C rate. The reason was the Toolkit charger accepted my request for the 2C rate, but then carried out the discharge at the reduced rate for reasons not explained. My view is this reduction in the discharge rate would have had a minimal impact on the LAC.

If you have seen Hopke's video you know he considers a LAC < 85 as an indication to discard a lipo. However for the Tattu lipo (LAC 79%) I have detected no other sign of anything wrong, it performs well in the plane, the IR is 18 (which is normal/usual ballpark for my 3S lipos, albeit higher than Hopkes 3S), so it may be the case that LAC 85% is too high a standard for smaller 3S lipos.

The Dualsky lipo with the best LAC was Dualsky Ultra 2S 550 mah 50C. Remarkably this has a IR measured during charging of 37, and during discharging of 26. The Dualsky lipos have long been noted by me to have a higher IR than other brands, however I am very satisfied with the performance of the 2S and 3S lipos (but not the Dualsky 3S ECO lipos which I have discarded some time ago).

Measuring LAC is a bit painful, you have to fully charge the lipo, then discharge it, and then storage charge it, all the while keeping an eye on the voltages and IR, and checking for lipo heating. However in my view it makes sufficient sense as an indicator of lipo condition that it may be worth making a schedule to do each lipo at a periodic interval. In future I will do 6 lipos simultaneously.