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Old Jun 09, 2011, 01:03 PM
Tim Lampe; Hobbico R&D
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Champaign, IL
Joined Dec 2009
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Min. cutoff Voltage vs. min. capacity

Maybe I should have posted this over in the "Batteries and Chargers" forum or whatever, but seems like there is more activity over here and I know some of the experts who frequent this forum and might help me out. Plus, my question may be particular to "high performance" models...

Anyway, why am I running up against my LiPo’s minimum discharge capacity (which is suggested to be 20% of its rated capacity) before I run up against the minimum cutoff Voltage (which is suggested to be 3.0V per cell)?

After a flight with my Rifle, my 1800mAh battery takes around 1400 – 1500mAh on average to charge (which is right around where I want to be), but the Voltage is still all the way up to around 3.7V per cell. Were I to continue flying until the Voltage was down to the “allowed” 3.0V, I would have possibly destroyed the battery by draining the capacity all the way down to virtually nothing.

If both values are correct (3.0V, 20% capacity), shouldn’t the rule then be “fly until the batteries Voltage is 3.0V per cell or when you get down to 20% capacity, whichever comes first”?

Thanks for any clarification you can provide.
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Old Jun 09, 2011, 10:32 PM
The Kid
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Detroit, MI
Joined Dec 2007
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I'm guessing your 3.7V/cell reading is on the ground after the flight?

If this is the case, you can rest assured that the voltage is much lower in flight, most likely close to 3V/cell, depending on the aggressiveness of the discharge.

That being said, the 3V/cell value doesn't hold as true anymore. It was a good guide at one point for 80% discharge. But with modern, high "C", lipos the voltage stays so high throughout the whole discharge, that 3.0V/cell is likely to result in greater then 80% discharges. This obviously all depends on how rapidly you are discharging the cell. THe higher the discharge rate, the more likely the 3.0V/cell value is valid.

For instance, if you take a 2200mah 65C battery and put it in an application drawing 15A, your cut-off would have to be placed at 3.7V/cell (I usually strive to 3.8V/cell personally) in order to assure you didn't discharge too far. I seem to recall from charles' testing that a modern lipo discharged at 10C could drain upwards of 90% of its capacity before reaching 3.0V/cell.

Kid
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Old Jun 10, 2011, 06:05 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Yes, static reading after the flight.

Of course then, with the motor running the output Voltage would be much lower. I can check with a Watt meter.

Then, the guideline should be "never allow your batteries to discharge lower than 3.0V/cell read under the load with a Watt meter."

I lost you a little bit on the rest of what you wrote...I understand what you said about needing a higher Voltage/cell for higher C batteries that can hold their Voltage better during a discharge. But then you wrote "the higher the discharge rate the more likely the 3.0V/cell value is valid." Maybe you meant the less likely the 3.0V/cell value is valid?

Sounds like maybe forgetting about the Voltage/cell limit and just sticking with the 80% capacity extracted from the battery (don't take it down to any less than 20% of its rated capacity)?

How do you do it?

Thanks kid.

By the way, you going to Muncie?

Tim
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Old Jun 11, 2011, 02:43 AM
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Originally Posted by KRProton View Post
then you wrote "the higher the discharge rate the more likely the 3.0V/cell value is valid." Maybe you meant the less likely the 3.0V/cell value is valid?
The Kid got it right. Take a battery and put it on a motor that draws 15A (this is your discharge rate) and you might take 90% capacity before 3v/cell is reached. Increase the discharge rate to 50A and maybe you take 85% capacity before the cutoff. 100A and maybe 80%...and so on. The higher the discharge rate the more likely the ESC will cutoff at 3V/cell in flight. The lower the discharge rate the more likely you'll go way beyond 80% discharge.

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How do you do it?
The simple answer is to time your flights, measure what capacity you put back in and adjust flight times to suit the 80% rule. A more elegant solution is telemetry. I have this with the Jeti 2.4g system and use it to measure the capacity taken from my receiver batteries to make sure I don't risk anything...very reassuring. Other telemetry systems are now available but whether they are really worth it is questionable when a simple timer can achieve a similar result.

Mike
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Old Jun 11, 2011, 09:44 AM
The Kid
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Originally Posted by KRProton View Post
I lost you a little bit on the rest of what you wrote...I understand what you said about needing a higher Voltage/cell for higher C batteries that can hold their Voltage better during a discharge. But then you wrote "the higher the discharge rate the more likely the 3.0V/cell value is valid." Maybe you meant the less likely the 3.0V/cell value is valid?
No, what I ment was the higher the discharge rate (the more current you are drawing), the more likely the 3.0V/cell limit holds true. This is because the voltage sag will be greater as you draw a higher "C" rate from your battery.

I've never used a timer or anything of that nature. At first I would fly until I heard a pitch change from the propellor. But this was back in the days of flying with 2200mah 25C batteries. Now that most of my planes fly on more powerful cells, its just something I go by feel with. I usually cut my flights pretty short. I have plenty of batteries, theres no point in overly stressing them when I have plenty more I can throw in and fly.

And no, unfortunately I won't be going to Muncie this year. I won't be able to get the time off, and the NATs go wednesday-saturday this year. The "mid-week" aspect of it was a deal breaker. The real-world really stinks when it gets in the way!

Kid
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Old Jun 11, 2011, 05:04 PM
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Joined Dec 2004
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Your safest bet would be to install a logger with limiter setup. Might seem like a big expense but it will pay for itself in time, less chance of blown motors and controllers, and less chance of destraoyed battery.
Second best would be a timer on throttle, which works really well too.

Regards,
Gerben
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Old Jun 13, 2011, 11:42 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
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All six of my Rifles (with various motor/battery/prop combinations) are on a throttle stick-activated timer. As are my other EP models. Then, I log the flight time (which is, as expected, right up to my maximum time within a few seconds usually). Then, I log the mAh that goes back into the battery after charging so I can confirm, and modify if necessary, the flight time based on how much was left in the battery after the flight. I shoot for 20% (remaining).

I have also dabbled with in-flight data acquisition using the EagleTree system. I used it to find out the current I was pulling in-flight back when I was doing motor testing so I would know how much I was stressing the motor. But now that I've got all my motors/props figured out, I no longer really need to gather much data. Sure, it would be nice to monitor more data for every flight, but we're talking about a Great Planes Rifle which is already a little tight on space for the existing gear (servos, ESC, receiver). Then, there's the weight issue as well.
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Old Jun 13, 2011, 11:48 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Originally Posted by Mike Seale View Post
The Kid got it right. Take a battery and put it on a motor that draws 15A (this is your discharge rate) and you might take 90% capacity before 3v/cell is reached. Increase the discharge rate to 50A and maybe you take 85% capacity before the cutoff. 100A and maybe 80%...and so on. The higher the discharge rate the more likely the ESC will cutoff at 3V/cell in flight. The lower the discharge rate the more likely you'll go way beyond 80% discharge.

Mike
Again though, the 3V/cell figure is in-flight, while the motor is loaded, not the reading you get when you check the battery after the flight, right?

If so, that's the source of my confusion (and possibly other people's too?). I've read several places that you can take your batteries down to 3.0V/cell without doing any damage. But I never recall reading that this was the in-flight reading while the motor was loaded (which, of course, would require data acquisition).

Less this data, it's just best to tell pilots not to use any more than 80% of their LiPo's capacity during a flight, yes?
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Old Jun 13, 2011, 01:56 PM
f5b-uk
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Originally Posted by KRProton View Post
If so, that's the source of my confusion (and possibly other people's too?). I've read several places that you can take your batteries down to 3.0V/cell without doing any damage. But I never recall reading that this was the in-flight reading while the motor was loaded (which, of course, would require data acquisition).

Less this data, it's just best to tell pilots not to use any more than 80% of their LiPo's capacity during a flight, yes?
Many ESCs can be set to cutoff when the pack reaches 3V/cell...so no need for telemetry. Using the ESC vow voltage cutoff is fine for sport flying but at high current it will cut too soon. And, yes, when you land the battery will bounce back to over 3V/cell.
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Old Jun 13, 2011, 05:13 PM
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For aerobatics I find the standard Castle ICE settings work fine provided you do plenty of vertical climbs towards the end of the time. When the motor cuts on a climb you close the throttle and start a landing pattern Then you can open the throttle and there is plenty for a power on landing and taxi back.
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Old Jun 14, 2011, 09:57 PM
Spoooooon!
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San Fernando Valley, California, United States
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Charles posted a chart in the battery section for capacity used at different voltages at rest. Can't find it, but 3.7 was pretty low. Under load, the pack would be much closer to 3v/cell. From my understanding, lipo's last much much longer when discharged no lower than 50% and the lower you go, the less cycles you'll get.
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Old Jun 19, 2011, 10:52 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
Joined Dec 2009
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Found this post over in the batteries and chargers forum.

Yes then, the 3.0V cutoff Voltage is UNDER A LOAD. This gentleman posted the cutoff Voltages (and their link to cutoff capacity) for the pack at rest...

http://www.rcgroups.com/forums/showp...1&postcount=11

So then, if, after a flight, my battery's Voltage is around 3.75 V/cell I should be near the cutoff capacity of 20%. (I always check the capacity that goes back in after a charge so I know how much I took out, but now I know how that forumla correlates to the min. Voltage--either under a load (3.0V/cell) or at rest (around 3.75 - 3.8V/cell).
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