If we start a flight at 100% and fly in absolutely zero wind, would you be half the total distance when the battery indicator shows 50% remaining (akin to a gas tank indicator in your car)?  That's what I would hope it indicates and how I think most people interpret it.  If not, how is the value determined?  What use is it to me if it doesn't directly tell me how much farther I can go?  I'm aware that there would be other factors at play in a real-world situation like wind, etc. which affect how far I can go.  I'm asking about how the battery percentage is determined in an ideal/pure situation.

It sometimes seems like my last 50% doesn't take me as far as my first 50%.  

Any thoughts (and facts to back them up)?

Most batteries, and LiPo in particular, have no mechanism for measuring the state of charge. There is nothing you can measure directly that will give you an accurate indication. Therefore designers go to great lengths to try and calculate the battery's capacity. This is done by measuring the current in Amperes that is being put into the battery during a charge cycle, and the time for which it is applied. The battery does have the ability to signal when it is full, because at that point, the voltage across the battery has been steadily rising during the charge process. When full, the voltage stops rising and begins to fall very gradually.

At this point, the battery is deemed to be fully charged. Also at that point, the calculations are made to determine how much current was applied for how long, and the value is then related to the known ampere/hour capacity of the battery. Then, when the battery is being used to power the aircraft, the reverse process is applied, the amount of current being used is measured against the time over which it is used.

The resultant calculations are used to indicate the state of charge of the battery at any given time. This is the best the designers can do to indicate remaining battery capacity, but it does have some flaws, in particular if the individual cells were not charged to equal capacity. For this reason the processor contained in the battery also measures the cell voltage of each individual cell to try and reveal any unbalance, and if it finds such an imbalance, it may revise the indicated charge percentage. Therefore, it is wise to keep an eye on both the indicated capacity and also the individual cell voltages.

It is not an exact science, and it is made very much more difficult because of the high current drawn by the aircraft from the battery and wildly variable current consumption. The wise pilot looks upon the battery capacity value as an indicator of remaining battery capacity, and does not place great reliance upon it. Always return to home with plenty of reserve capacity, just in case, because while the indicated balance is worked out quite carefully, at times it can be a lying son-of-a-bitch.

Geebax Posted at 6-20 17:28
Most batteries, and LiPo in particular, have no mechanism for measuring the state of charge. There is nothing you can measure directly that will give you an accurate indication. Therefore designers go to great lengths to try and calculate the battery's capacity. This is done by measuring the current in Amperes that is being put into the battery during a charge cycle, and the time for which it is applied. The battery does have the ability to signal when it is full, because at that point, the voltage across the battery has been steadily rising during the charge process. When full, the voltage stops rising and begins to fall very gradually.

At this point, the battery is deemed to be fully charged. Also at that point, the calculations are made to determine how much current was applied for how long, and the value is then related to the known ampere/hour capacity of the battery. Then, when the battery is being used to power the aircraft, the reverse process is applied, the amount of current being used is measured against the time over which it is used.

Awesome reply.  Thanks.  I didn't realize it was that difficult from a physics perspective.  

Your first 50% is in fact 50%,  assuming you started with a fully 100% charged battery.

Your last 50% is usually,  if you're being safe.....only 30% because you should be landing at 20% ish.  Most people won't fly to zero....at least no one I know.

Randy
Aerial Traffic

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rwynant V1 Posted at 6-21 20:45
Your first 50% is in fact 50%,  assuming you started with a fully 100% charged battery.

Your last 50% is usually,  if you're being safe.....only 30% because you should be landing at 20% ish.  Most people won't fly to zero....at least no one I know.

15% I usually stop flying.I try to play it safe