how can I understand if a battery is dead?
Here is my experience with that and what doesn’t convince me too much
I have a couple of batteries that are pretty new and work well. After each flight, I charge them with my IMAX B6 and read the amount of milliamps loaded into the battery. Normally, in order to go from 11.1V to 12.6V on a 3S 1500mAh I need about 1300 mA … that is quite correct I would say. I never discharge it completely, but only up to 85/90%.
I have also a battery that is almost dead. It was the first battery I used, I have discharged it a couple of times to 7 Volts or less … it was so low that the charger didn’t even recognized it as 3S but only 2S. Here the strange fact: when I charged it this morning with my IMAX B6, the number of milliamps I needed to pump-in in order to go from, let’s say 10V to 12.6V, was 1600mA
This sounds strange to me … If a battery is broken, I would expect to deliver less current while used and so, in order to charge it from flat to full, I would also expect to pump-in less current in order to raise the voltage up to the 12.6V.
Is my expectation fair or what is wrong in my assumptions?
Each time you over-discharge your lipos, they get damaged. A part of the cell chemistry enters a irreversible state and the lipo looses a part of it’s capacity. The internal resistance of the lipo will also rise and the C rating will get lower.
You can not meassure internal resistance directly. You need to build a test setup with a constant load. 12V halogen light bulbs are often used. Use them in series for higher voltage lipos.
You meassure the resting voltage of your fully loaded lipo. Then you connect a multimeter/wattmeter to meassure current/voltage and connect the light bulb. The difference between resting-loaded voltage divided by the meassured current is the lipos internal resistance.
This is no absolute meassurement, you need to meassure your lipos after each load and keep note of the results, to see the development over time.
Puffy lipos and lipos where cells do not load as fast as the others or do not reach full voltage are also broken.
The puffing causes the cell layers to be seperated, degrading the overall performance of the lipo.
Puffing occurs due to overcharging or exceeding the C-rating of the lipo.
I often notice that some lipos seem to have a higher capacity than advertised. Some of my 4000mAh 2s lipos take around 3900mAh during charging without beeing over-discharged.
That’s interesting, thanks! I will search more about it…
But how do you explain the fact I described above? In different words … how is it possible that two LiPo battery, one healthy and one dead, in order to be charged from 11V to 12.6V require the same amount of milliamps, more or less (based on the number that appears on IMAX B6) but then during flight … the healthy one lasts 15 minutes and the dead one only 4 minutes.
I have anyway pumped in - more or less - the same amount of current, where is this energy gone?
While charging, the dead battery doesn’t get particularly hot to justify that this energy is dissipated due to the increased internal resistance. Maybe that happens while discharging? It should then get particularly hot while used…
The only way to know, would be to meassure the lipo’s temperature after the landing. The only way to go for the energy, if it is not converted to mechanical energy, is heat.