There has been some discussion about lipos internal resistance on another thread and I felt it is important enough to give some explanations on the subject. Without going into the science which is not all that clear to me, IR measurement is an indispensable tool to determine your lipo batteries health. I bought a cheap Imax B6AC charger at Hobbyking for $21 US which measures each cell's IR. It is worth more than its weight in gold as it has saved me more than once from losing my planes. I check the battery's IR after each charging cycle.
https://hobbyking.com/en_us/imax-b6-...plug-copy.html
Measuring only the voltage can lead you to think that your battery is fine and fully charged until you take off or at some time during the flight when your plane suddenly loses power. You may think your battery is discharged but it's not, one or more cells may be dying and your voltmeter did not detect it. The IR measurement is instantaneous and real time and will let you know when a cell is starting to sag. With high C rating batts (50C to 135C), the IR should be 005 ohms or lower, lower C rating batts may have a higher IR of 010-020 ohms, but all cells should show about be the same reading. So for example, you have a 4S batt and three cells read .005 ohms and one cell at .020 ohms, that cell is going. This means that you will drop to 3 cell power at some time in the flight, possible on takeoff or at the worst possible time. I bought the copy Imax B6AC which is cheaper, you can get the original, more expensive but make sure you next charger can give you IR reading. You will never regret it.
Note: As batteries get older or mistreated the IR will go up! I never fly under 3.8 V each cell and always keep them in storage at 3.8V, charge them the day of flying or the day before. I used to puff and kill batteries, not anymore.