Normal Battery Failsafe threshold

The question is a little bit difficult to answer as this depends on the internal resistance of the battery and the amount of current consumed in operation. When you connect the equipment without throttle, you'd typically see a voltage close to the actual voltage. Under load, due to internal battery resistance, there's a voltage drop of (usually) around 0.7V when you measure on the battery poles.

There's also some disagreement what the safe limits are for operating batteries. Some people say you shouldn't discharge under the nominal voltage if you want your batteries to last, others are happy to discharge down to 3.4V per cell.

10.5 is the limit under load (0.7V drop) which is close to nominal voltage discharge. So it's quite a safe limit. When you measure it back on the ground, you'd see the 11.1V left in the battery.

To understand the response better, you should also check with Google on lipo battery discharge curves. Then you notice how at the start and the end of the battery discharge you see steep slopes with high discharge rates (changes in voltage), but that inbetween you get a relatively flat linear area. If you study that graph as well, you understand the concerns of running the battery down a little bit too far.

Also given the fact that cells aren't necessarily balanced or if a cell isn't healthy, when you run the cells down including the bad ones it can suddenly plummet and kill the motors.

In my operations however I never use battery failsafe or cutout. The reason is that I'd rather risk losing a battery than the entire aircraft. If I do lose the aircraft because the battery is too low, I certainly ascribe this to my own stupidity.

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Gerard Toonstra February 8, 2014 at 2:42pm

The question is a little bit difficult to answer as this depends on the internal resistance of the battery and the amount of current consumed in operation. When you connect the equipment without throttle, you'd typically see a voltage close to the actual voltage. Under load, due to internal battery resistance, there's a voltage drop of (usually) around 0.7V when you measure on the battery poles.

There's also some disagreement what the safe limits are for operating batteries. Some people say you shouldn't discharge under the nominal voltage if you want your batteries to last, others are happy to discharge down to 3.4V per cell.

10.5 is the limit under load (0.7V drop) which is close to nominal voltage discharge. So it's quite a safe limit. When you measure it back on the ground, you'd see the 11.1V left in the battery.

To understand the response better, you should also check with Google on lipo battery discharge curves. Then you notice how at the start and the end of the battery discharge you see steep slopes with high discharge rates (changes in voltage), but that inbetween you get a relatively flat linear area. If you study that graph as well, you understand the concerns of running the battery down a little bit too far.

Also given the fact that cells aren't necessarily balanced or if a cell isn't healthy, when you run the cells down including the bad ones it can suddenly plummet and kill the motors.

In my operations however I never use battery failsafe or cutout. The reason is that I'd rather risk losing a battery than the entire aircraft. If I do lose the aircraft because the battery is too low, I certainly ascribe this to my own stupidity.

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Normal Battery Failsafe threshold

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