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What is the minimum voltage I can safely discharge a LIPO battery during a flight?
For example, if I have a 3S LIPO battery, should I be aiming to land my RC plane before the battery gets below a certain voltage?
Related to that, is it the voltage under load that matters, or the open/resting voltage once it is disconnected?
The short answer is that you don't want to run batteries below 3.0V ever, and ideally want to avoid large current draw below 3.5V. For a 3S battery, once you start seeing voltages in single digits (3.33V/cell is 10V, below that), it's time to land as a quick reference point - although you can set more precise values of around 10.6V as the warning point through telemetry at a transmitter and/or OSD on an FPV display.
The real issue you're trying to avoid here is that in the Lithium-CobaltOxide (LiPO battery chemistry), below 3.0 volts (can be 2.8V or so in Li-Ion batteries), the battery starts plating lithium permanently onto the anode reducing the capacity and discharge performance of the pack. Doing this greatly harms the battery, so avoid that at all costs.
The reason I reference the 3.5V point is that typically you have another 10% of battery capacity left in order to affect a safe landing approach pattern without being under time pressure to land quickly, and make a good safe landing - also under load, the battery that is at 3.5V in flight will tend to recover to 3.7-3.85V depending on craft and setup, which is more appropriate for storing a battery as you travel back home to the bench charge the battery to its proper storage voltage (3.8V).
The answer to this varies depending on who you ask.
It is useful to think of it as voltage/cell.
I’ve seen estimates that say you should stop discharging when each cell in the battery reaches 3.3V as the lowest value. Some people, for example in Joshua Bardwell’s Wizard TS215 video (or possibly the Wizard X220HV video), say that you should stop when the battery sag recovers to 3.7V.
I personally fly my batteries to the minimum safe voltage as stated above, however as far as I am aware there is no set limit.
I set up my equipment/telemmetry to warn me at 10.7V on 3s and report low voltage alarm at 10.5v. At that point the pack is pretty much dead. After landing your "unloaded" voltage should rise to about 11.1v. I also have my radio (taranis) report my absolute minimum voltage since the start of the flight every minute or so. If you spike the throttle or pull a hard manoeuvre momentarily at may dip down for a split second. If it gets close to 10V, time to think about landing.
Those combined options can give you a very good sense of how you pack is doing while flying.
Also, many "store bought" or those 1s single cell toy grade quads will have protection for low voltage built in. Most will just land and shut down right around storage voltage (11.4v) so they're nearly fool proof. The lights will flash when your getting low, and about 30s to minute after, it will land and shut down. Airplanes however, may not have those options so you'll have to look to your radio gear for telemmetry etc.. Or just learn to time you flights and keep track of your lipo voltages the old fashioned way.
Bottom line: Never let them go below 10V loaded and you should be fine. Cheers!
