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Old Jan 14, 2013, 01:56 PM
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Joined Oct 2005
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How to estimate flight times accurately, and safely.

Greetings all,

New Quad owner (you may have seen some of my other postings..) I wanted to ask about calculating flight times, and looking at the battery performance vs. just "guessing" and using a timer that is "safe".

For starters I am flying:

HT-FPV, Witspy build, GPS, Gopro and a 600mw vtx. with 9" props.

I have a 4S 3300MAh pack. and I currently set my flight timer to 7:30sec. There was an issue on the third flight at 9min, where it went down.. and there is speculation this could have been due to voltage? I dont know.. but, I know I am motivated to learn more.

The question is, how do I "check" the battery to determine if I can fly longer etc? The first thought is to buy a volt meter and check after I land.. but, then I was thinking this would only give me a "resting" voltage, and that whats more important is the voltage under load?

At first, I was just hovering until I saw the RED LED blink on the FC to let me know voltage has dropped.. but, again thease values can be changed.. and also, this was during a hover.. so not too helpful if I crank it around in the air.. as more power is needed?

Let me know how all of you with $$$ setups are coming to the determination of how long your actually flying.

FWIW< I just bought x4 TP-3300 4S packs. that I will be using.
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Old Jan 14, 2013, 02:04 PM
Z06 Tony's Avatar
United States, NY, Rochester
Joined Sep 2008
7,075 Posts
Set your timer and fly with a fully charged pack.
Fly for 6-7 mins since you know it will atleast fly that long.
Next charge your battery and when it is finsihed see how many mAh you put it.
Now you will know how many mAh you burned per minute.

Only use 80% of your pack and those nice batteries you just bought will last for many charges.

Keep running them dead and you will ruin them.

As I am sure you know if you add weight the #'s will change and flight time will go down.
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Old Jan 14, 2013, 02:28 PM
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Hi Tony! Thanks for the quick reply.

So, lets say I fully charge a pack, and go fly for 8min. I get home, and re-charge (as I just did). It retuned about 1830MAh.

This pack is 3300MAh, thus 80% would be 2640.. so I still had 800+MAh remanning in the pack of "safe usable flying time".

However.. thinking of MAh in terms of a "fuel tank capacity" this is all good info. What about volts? I am wondering if the issue I perhaps had with the crash, and a possible issue in general is voltage drop under load? So, although I may only being using less then 80% of the total capacity.. which is good for the battery.. I am wondering about the volts I am getting from it, especially under load where the quad maybe fighting wind, or in forward flight etc.
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Old Jan 14, 2013, 02:35 PM
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Joined Dec 2004
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Well a 1S is 4.2V fully loaded, and 3.7V as a safe low value, and 3.5V is often given as the line you should stay above to get the most life of the battery. So for a 4S thats 16.8V/14.8V/14V, I don't think there's a too big difference under load I'd say few 1/10th of Volt per cell ?

And yes counting the mAh is a good method, because the discharge curve for a LiPo is quite linear (in the safe range), so for your setup this value of 228mAh per minute is decently scalable.
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Old Jan 14, 2013, 02:41 PM
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United States, NC, Garner
Joined Mar 2011
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Or just buy a low-voltage alarm.

I have one and they work great and ALSO lets you know which cell tripped the alarm first by a set of LEDs that are red and green. . .if you land immediately after the horn goes off. My only slight complaint is the horn is too loud. I suppose it would be great if you're flying hundreds of meters distant. I stuffed the horn with some foam and is just right for flying within 100 meters.
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Old Jan 14, 2013, 03:08 PM
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Interesting. So, let me as a basic question.. when I look at the pack, and it says 14.8V what is this really telling me? 14.8V for what? Where I get confused is your right, when the charge is done it says 4.2V / cell which is 16.8V (which at a glance makes no sense to me, as I am here thinking oh it should be 14.8 as this is what the pack says)..
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Old Jan 14, 2013, 03:32 PM
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United States, MI, Rochester Hills
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Quote:
Originally Posted by Tharr62 View Post
Interesting. So, let me as a basic question.. when I look at the pack, and it says 14.8V what is this really telling me? 14.8V for what? Where I get confused is your right, when the charge is done it says 4.2V / cell which is 16.8V (which at a glance makes no sense to me, as I am here thinking oh it should be 14.8 as this is what the pack says)..
14.8V is what's called the nominal voltage of the battery, as the nominal voltage of a LiPO cell is 3.7V. Aside from theory, as they said, that 3.7V (or 14..8V on the whole set) are for a discharged battery. When you charge it, you are putting 0.5V each cell on top of that. It's not like a saline battery (like the ones you use in your handheld radios) that drop it's voltage down to almost zero while discharging, the good thing in LiPo's is they keep almost full voltage during the whole discharge, so they are still fully operational when they are almost out.
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Old Jan 14, 2013, 08:41 PM
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Tharr62,
I am flying a DJI 450 quad using a 4 cell 4900 mah pack. 80% of that is 3920 so I base all of my calculations on this lower number and basically forget that the full capacity even exists.

I try to run a stop watch or transmitter timer on each flight and then pay close attention to what my charger tells me. Then I enter those numbers into a simple spreadsheet which calculates my battery drain per minute.

Doing this allows me to safely gauge the length of my flights and it also let's me monitor the health of my batteries. See the attached screenshot of my spreadsheet.
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Old Jan 14, 2013, 08:59 PM
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Hi Fixed Wing! LOVE it. So basically, (for example) looking at the pack I just had a flight on.. (4S 3300).. I plugged into charger, and I got 15.47V / 3.83V (per cell). So, now it's charging.. and when done I can see what the end MAh's replenished are.. and then see where that relates to my 80% budget of 2640MAh to stay in the "safe" zone.

This BTW was an 8:00 flight, with mostly hovering and not too much forward action or fighting the wind etc.. so very tame flying.

So I guess what I am looking for then, is to trend an average for the flights and keep track of the packs.. until I can find the time / flying style that brings be close to the 80% mark... while keeping an eye on the end voltage per cell / total voltage too (assuming the voltage is still strong on the pack at the 80% level).

In the end, I can divide the MAh replaced by the actual flight time, and get a MAh / per min calculation to aid in finding the correct flight time?

LOL.
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Old Jan 14, 2013, 09:30 PM
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I use one of the cheap $5.00 low voltage alarms much like Cyberdactyl uses. The one I use is low voltage adjustable. I set it for 3.6 volts and of course it will go off at first when copter is under added power. Hover and it goes off. I use a 3 cell 2200 battery and landing within a minute after the first sound of the alarm usually leaves my battery at 11.2 volts under no load.
Before I stumbled on this item I was really paranoid about flying too long and we all know the copters glide like a bowling ball.
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Old Jan 14, 2013, 09:46 PM
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Joined Sep 2004
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If you want to estimate the effect of changing battery size, adding payload etc., have a look at

http://www.ecalc.ch/xcoptercalc_e.htm
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Old Jan 14, 2013, 10:02 PM
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Quote:
Originally Posted by Tharr62 View Post
Hi Fixed Wing! LOVE it. So basically, (for example) looking at the pack I just had a flight on.. (4S 3300).. I plugged into charger, and I got 15.47V / 3.83V (per cell). So, now it's charging.. and when done I can see what the end MAh's replenished are.. and then see where that relates to my 80% budget of 2640MAh to stay in the "safe" zone.

This BTW was an 8:00 flight, with mostly hovering and not too much forward action or fighting the wind etc.. so very tame flying.

So I guess what I am looking for then, is to trend an average for the flights and keep track of the packs.. until I can find the time / flying style that brings be close to the 80% mark... while keeping an eye on the end voltage per cell / total voltage too (assuming the voltage is still strong on the pack at the 80% level).

In the end, I can divide the MAh replaced by the actual flight time, and get a MAh / per min calculation to aid in finding the correct flight time?

LOL.
Yup, you got it. The last column of my spreadsheet Automatically calculates the mah per minute used based on the minutes flown/mah replaced and at the bottom of that column I have a formula that tracks the average.

In my particular case I see that I use an average of 263 mah per minute which technically would give me a little under 15 minutes of flight time. I purposely do not fly that long though. I set my transmitter timer much less than that giving me plenty of time to land after my time is up.
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Old Jan 14, 2013, 10:47 PM
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Now, I am thinking about a 4S 4400mAh batt.. or even a 5000. But, the added weight I guess would also have to be factored in..

I just ordered some TP-3300 packs. (4) So, it was already an investment ($100 each).. Do you think I should get more mAh? or, stick with the 3300's?
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Old Jan 14, 2013, 11:20 PM
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Tharr I just read your other thread where you described the crash. For what it's worth, you can see from my spreadsheet above that I've got a good handle on my battery performance. And yet, about eight minutes into my flight my Red LED will start blinking. My Naza Assistant is setup for voltages much lower than what my battery is delivering so it really should not be blinking. For this reason I too have disabled it in the Assistant.
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Old Jan 16, 2013, 07:58 AM
Hamburger
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Joined Jan 2010
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Quote:
Witspy build
if that implies your controller is running MultiWii, then you have several options to choose from or to combine. Start with attaching buzzer to MultiWii, then do one or more of the following
- atlach voltage reading from balancer plug to controller
- set armedTimeWarning to 7 minutes
- try the powermeter feature to monitor the mAh drawn
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