Regular readers know of my side interest in RC helicopters and quadcopters. All of these models use LiPo batteries. Prior to my interest in RC, I used LiPo batteries during many QRP excursions.
Early on, I bought a West Mountain Radio Computerized Battery Analyzer- a great little device that allows battery capacity to be determined. And over time, trends become apparent with various batteries.
I’ve always been meticulous in my care of LiPo’s – they’re not cheap and I want them to last – and I have them in sizes ranging from 1S-250 mAh to 3S-4000 mAh.
Now about the battery analyzer:
Software is used to set up the analyzer’s parameters: chemistry, initial voltage, desired rate of discharge, etc. Each battery chemistry has a default value to which it will be discharged and at that point, the battery analyzer is finished and the graph (see below) can be saved.
For 3-cell LiPo batteries, the battery analyzer ends the discharge (at default values) when the battery under test’s voltage drops to 8.4 volts (2.8V per cell). I believe this is too low and has shortened the life of my three most recently purchased batteries.
Wikipedia claims that LiPo’s should not be discharged any lower than 2.7 volts per cell, other sites mention 3 volts as the minimum.
Looking at the chart below, it can be seen that each of these minimums are reached within minutes (or seconds at higher discharge rates) of each other due to the steep nature LiPo chemistry’s end of life “knee”.
In other words, 2.7 volts pretty much equals 3.0 volts in terms of how quickly the voltage drops when in this range.
The default LiPo cut-off of West Mountain Radio’s battery analyzer is, I believe, too low to allow testing of a LiPo battery without destroying it.
I noticed all this when all three of my 3S-2200 mAh batteries started giving drastically reduced flight times. Putting them on the tester, I found that their capacity had dropped to 1/3 of what it had been only a few weeks earlier.
Once I realized what may have been the culprit, I recalled past examples of other batteries whose “brothers” purchased with them enjoyed longer lifes than those that were tested half a dozen times or so.
I now define end of life for a LiPo battery to be 3.4 volts per cell. This is the point at which the curve first starts to go south and I now use this value for the cut-off point when testing them.