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Old Aug 12, 2012, 07:03 AM
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3s Lipo, whats safe for LVC? 9.6v -9.9v? 11.0v?

Hello, Just wanted to know when you guys are ready to land with enough time for Low Voltage Cutoff (LVC).

I fly with 1 battery setup (3s-2200mah Lipo). I power everything on plane with it.

I use HobbyKing simpleOSD. What is the save LVC to safe land with enough time?

9.6v-9.9v? or too low? I've read some are landing when its 11.0v?
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Old Aug 12, 2012, 07:33 AM
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3.5/3.6V per cell is the minimum a cell should be discharged to.

It depends how far you fly. If I wanted another 10% of flight time after the alarm, I'd choose 11.5V.
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Old Aug 12, 2012, 10:27 AM
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I land at 11v, but I haven't really done any research into this so Zen09 might have a better idea.
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Old Aug 12, 2012, 10:34 AM
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It depends, open circuit or under load?

I can suggest a pretty good website: http://www.rchelicopterfun.com/rc-lipo-batteries.html
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Old Aug 12, 2012, 10:36 AM
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Quote:
Originally Posted by Zen09 View Post
3.5/3.6V per cell is the minimum a cell should be discharged to.

It depends how far you fly. If I wanted another 10% of flight time after the alarm, I'd choose 11.5V.
Zen09,

That is not true. You can safely discharge a lipo till 3.2V (or even lower but not lower that 3V). Just be careful to leave at least 20% of the charge in the pack. So, if the pack is 1000mAh, there should be no more than 800mAh charged back in, meaning that 200mAh remained at the end of the flight (20% of capacity). You should also store the battery with 60%~70% capacity (3.78V~3.92V resting voltage per cell). Never store the lipo fully charged. You should only fully charge it before the flight. Hope this helps.

Pedro
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Old Aug 12, 2012, 10:53 AM
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Beware that under load, there's a big difference in voltage, much lower.
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Old Aug 12, 2012, 12:05 PM
BEOWULF
North vancouver, B.C. Canada
Joined Apr 2008
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3volts per cell is ok as long as you stop using motor and land soon

Your video should cut in and out to tell you hit lvc

Batteries bounce back up in voltage so set lvc to 3 volts and when video craps out stop motor and land ASAP

This is one reason I like separate battery for fpv video because I can fly around and not have to land

Castle creations as an example set stockmsettingsmat 3 volts per cell

If you really want to be safe set lvc to like 3.8 volts and don't charge to 12.6

But really batteries are cheap these days so have fun and slightly abuse them

Some planes I shut off lvc so I can get high bursts and high watts without lvc

3 to 5 second burst with high powered systems , using a switch for 100 percent wot
Lvc has to be turned off, there is a reason why the esc can be programmed
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Old Aug 13, 2012, 02:34 AM
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Quote:
Originally Posted by p_daniel_santos@ View Post
Zen09,
That is not true. You can safely discharge a lipo till 3.2V (or even lower but not lower that 3V). Just be careful to leave at least 20% of the charge in the pack.
Pedro
I haven't said it's not safe. It just shouldn't be done, because there's almost nothing to gain doing it yet it would shorten battery life.

See this graph to understand what's going on. At 3.6V per cell you already have nothing left.


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Old Aug 13, 2012, 10:06 AM
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Quote:
Originally Posted by Zen09 View Post
I haven't said it's not safe. It just shouldn't be done, because there's almost nothing to gain doing it yet it would shorten battery life.

See this graph to understand what's going on. At 3.6V per cell you already have nothing left.


Zen09,

Your graph confused me! At 3.7V per cell (resting voltage), the lipo have more that 50% of its charge! So I'm not understanding the graph when at 11.1V (3s - 3.7V per cell) you write 0%.
In a normal battery pack, at 3.2-3.4V you should have more that 20% of the rated capacity. Discharging to 3.2V would not shorten the battery life. At least this is valid in standard quality lipo's.

Pedro
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Old Aug 13, 2012, 10:49 AM
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You're having it wrong, sorry. I haven't said 0%, but I meant it's nearly empty.

Another graph:

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Old Aug 13, 2012, 11:41 AM
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'An 80% discharged LiPo cell, will give an approximate open circuit voltage of 3.75 volts. A 3S LiPo pack therefore would show about 11.25 volts after a flight when it's 80% discharged, a 6S pack would be about 22.5 volts.'

http://www.rchelicopterfun.com/rc-lipo-batteries.html

Might be able to run down to 3.2V, but there won't be a lot of run time available from when the cells are at 3.7 until 3.2

I am no expert. I prepare to land when the OSD says 11.2V, even though that is voltage under load (maybe .1A to the motor and 0.1A to the video system when I cut the throttle to check it) and probably well above 20%. When I check on the ground afterwards I typically have 3.8V measured, which means I had a little safety in reserve for FPV flying. Makes it possible to fight some unexpected wind or do a couple go-arounds as needed.
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Old Aug 13, 2012, 12:07 PM
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Quote:
Originally Posted by Zen09 View Post
You're having it wrong, sorry. I haven't said 0%, but I meant it's nearly empty.

Another graph:

Zen09,

I'm not wrong. I have performed discharge tests in Zippy, Turnigy and Hyperion, both under high discharge and low discharge.

At 3.3V the lipo is not empty as your graph show. I've seen some competition model aircrafts to cut-off at only 3.0-3.1V/cell. I don't recommend this, but they could draw out more 5-10% of energy (but reducing the battery lifetime).

The storage voltage of lipo packs is 60%~70% capacity (3.78V~3.92V resting voltage per cell). So looking at your graph the data does not match. Where did you took the graph from? 333V pack is huge. Probably a very low discharge lipo that performs completely different from the high discharge lipos you use. That could explain the differences.

I propose you a test. Fully charge a battery pack. Discharge a pack till 3.7v per cell (resting voltage). Then charge back it again. Then you know how much energy you have consumed.

Pedro
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Old Aug 13, 2012, 12:34 PM
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The graphs and interpretations of them just don't match reality.
My practical "must land" voltage is 3.466V/cell (10.4V for a 3S pack) and
there's quite a lot of juice to be had between 11.1V and 10.4V.
After 10.4, the pack voltage curve goes over the "knee" and
starts to drop pretty quickly. If you want to always land with
20% of your battery capacity unused, then by all means land
at nominal voltage.

Looking at the OSD stats from a recent flight where I got caught
out downwind on the return leg, voltage was down to 11.0
at cruise throttle (little over 1C) 3.7 miles away, having used
3850mAh out of 5000. I made it 2.7 miles (against the headwind)
on 900mAh before hitting my normal must land voltage of 10.4V.
Only then did the voltage start dropping quickly under cruise
load and I made it all but the last 150 yards home
using another 100mAh or so and chopped the throttle at 3.2V/cell.

The way I fly, I can land almost anywhere without power, so if I have
the height to glide home, I'll do so from 10.4V (on 3S). If I need a
reserve for a go around or two, then maybe 10.8V or so. I set my ESC's
LVC around 3.1V/cell to protect the pack itself.

ian
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Old Aug 13, 2012, 01:23 PM
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Birmingham, Alabama
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1) dont fly FPV with a low voltage cutoff enabled
2) batteries in fpv planes are inconsequential. the question isn't what's safe for the battery, it's how far will it drop before your video goes out.


do like Daemon. be down by 10.4. if you fly around like a LOS plane until it hits 9-9.4v you'll get bit by it in fpv eventually. one day youll be flying on a hot day & suddenly that VTX that use to work fine at 9.Xv cuts out at 10...

and now you're a half away without video...

ask me how i know. been there. got the plane back off shear luck that it was a very windy day and i was flying up-wind.
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Old Aug 13, 2012, 01:26 PM
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Quote:
Originally Posted by p_daniel_santos@ View Post
The storage voltage of lipo packs is 60%~70% capacity (3.78V~3.92V resting voltage per cell). So looking at your graph the data does not match.
Pedro

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries


"For long-term storage, manufacturers recommend a 40 percent charge."

That matches very well with 3.85V on the graphs.
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