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I built 5" quad and I'm using 4S 1300mAh LiPo batteries. My question is: at what voltage should I land? Let assume that I'm trying to land when my battery is as low as it should go without causing damage to the pack.

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I start my landing approach when the average cell voltage reaches below 3.5V on a stable flight (not counting voltage dips on short, aggressive maneuvers).

At that point, you should still have enough authority to land safely. You can go somewhat lower without damaging the cells, but at that point, the voltage curve starts to be very steep and it is increasingly easy to slip below dangerous levels.

I wouldn't advise landing on 3.2V simply because it's too easy to overshoot and to me, it doesn't give meaningful airtime over landing at 3.5V.

Also, note that there can be a low voltage cutoff set up in the ESC that you probably don't want to trigger while in the air.

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I have found that landing based on voltage is inaccurate. For example, if you look at the voltage in the OSD while you have the stick at full throttle, the Voltage will be lower than if you had it at half throttle. So, do you land any time full throttle goes down to 3.2V/cell, or when you are at half throttle and 3.2V/cell? Instead of relying on voltage as the indicator, I've gone to looking at mAh usage (Current in the OSD). Note that if you choose to use the current sensor, it must be calibrated because there is no guarantee that it will come calibrated from the factory. An current sensor that is not calibrated is as effective as landing based on the amount of time you are in the air.

With a 1300mAh LiPo, you can use up to 1040 mAh (80% capacity of the LiPo used, where 1040 is the critical point at which your LiPo will likely start to see damage to some degree). I tend to set warnings at 910mAh used (70% capacity of the LiPo), and then try to land by 975mAh used (75% capacity of the LiPo). This puts my batteries at, or near, storage charge. Using mAh (current), instead of voltage, makes it easier to gauge when you should land.

Here is a chart I made that I keep in my drone bag as a reference, so I don't have to do the math every time I use batteries of different capacities.

enter image description here

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    $\begingroup$ I have found the opposite to be true. Landing based on mah used is inaccurate for me. Batteries degrade over time and don't carry the same mah. Usable mah is also affected by temperature. Voltage is the important stat when it comes to damaging your batteries. In my experience, when my battery is around 3.5 volts per cell at a gentle cruising speed, I land. After landing the voltage usually comes back up to 3.7 volts per cell. And I am comfortable with that value. However how aggressive you fly can affect when you should land. Punching out with 3.5v cells can drop your battery too low. $\endgroup$ – 5zero7rc Apr 19 at 17:34
  • $\begingroup$ I agree with @5zero7rc Most times the ampere-measurement is just a guesstimate and will not reflect the actual used mA/h. I like to make sure that I'm near my landing spot when I hit around 3.5V/cell during stable flight/hover. You can go lower, but most times I end up with the packs at storage voltage. $\endgroup$ – GreenT Apr 20 at 10:45
  • $\begingroup$ @5zero7rc, everyone has a different tolerance for when a battery can no longer be used. For me, once it stops holding a charge for longer than two minutes of race style flight, it's no good to me. I have been landing based on mAh consumption for over three years and my batteries always come down at or very near storage voltage. What I forgot to mention in my post is that the current sensor must be calibrated. We can't assume that they will be accurate from the factory. I hope that clears things up a bit. $\endgroup$ – Schome1 Apr 20 at 12:42
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My personal strategy is to use both voltage and mah as guides. For a 1300mah battery, I'll fly around and if I notice I've used up 1000mah I'll come in and land. However, if I'm flying around and I notice my voltage osd warning blink is happening more and more often during even nominal throttle use I'll come in and land. In the latter case, the battery is surely old and has a diminished capacity.

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You want to keep the battery voltage above 3.2 volts per cell (so 12.8V for a 4S pack) to maximise the lifespan of the battery. Remember that's the voltage once you've landed; you'll need to plan to land before this point.

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