So how well does it work in practice?
I have run a number of tests on batteries with identical specifications and found that the “Max current” number from the calculator is a pretty good indication of the reasonable maximum current in practice for the sorts of batteries I use.
For example I tested a pair of new identical Turnigy 3S 1300 mAh 2030C and got an IR of between 17 and 18 mOhm. Suggested max C is then approximately 21 Amps and when I tested them at 20A they were clearly at or just beyond the sensible limit. The cell voltage is under 3.5V at midrun, discharge curve is flat or slightly “dipped” and the temp rise is into the “hot” region.
Another pair of fairly new identical Zippy 3S 2200mAh 40C batteries came out at a recommended max. current of 48 Amps (which is 22C despite the 40C label).
40Amps is the maximum I can accurately test a 3S without a lot of hassle but at that current they were quite comfortable. Looking at the midrun cell voltage which is under 3.6V and temperature which was “warm” I would not want to push them a lot further however and 48A seems an entirely reasonable maximum number.
The results are in the first two graphs.
And finally for something that surprised me, given that the max. rating tool was developed largely by observational and measurement on “normal” 2200 mAh flight packs. I have had an interest in the small 1S cells used in micro fliers for some time and thought I would try out the calculator on a sample of three cells which I knew well.
I picked a 160 mAh Hyperion and Thunderpower which have similar characteristics and a Turnigy Nano which I know to be inferior at higher rates. From previous testing I know these “25C labelled” cells struggle to reach 20C in the case of the first two and 15C for the Turnigy. At these rates they get very hot and the voltage is just adequate.
Measured on a modified ESR meter at 1.6A rather than 16A I got the results in the table.
I then tested all 3 at 10C and got the results in the third graph which is consistent with practice. The Hyperion and Thunderpower are doing fine by 1S standards, the Turnigy is just making 3.4V mid discharge.
Then I compared them using 10C for the Turnigy and 15C for the other two which as close as I can get to the calculator predictions. See last graph. (There is some variation in the temperature curves – the Hyperion curve in the previous graph is incorrect and should be ignored due to poor sensor contact.)
At the recommended maximum currents predicted by the calculator all 3 cells are performing almost identically and at what I would regard as the limit of voltage drop and temperature rise. The Hyperion is showing reduced capacity – it has had a pretty tough life and I’m not surprised. However the CURRENT capability predicted by the calculator is quite remarkably accurate.
In short, this calculator developed from normal full size packs seems to predict surprisingly well at the tiny end of the battery world as well.
