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LVC Cutoff --> "Soft" versus "Hard"

Old 11-19-2007, 04:41 PM
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Lieutenant Loughead
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Default LVC Cutoff --> "Soft" versus "Hard"

Can someone please educate me on EXACTLY how "Soft LVC Cutoff" works?

I've been flying with "Hard" cutoff for about 3 years, and I'm very comfortable with that. About a year ago, I bought some less expensive ESCs (I normally buy Phoenix 25's, but I tried to save a buck by buying Thunderbird 18's and Thunderbird 9's)...

I THOUGHT I understood how it worked, but I ended up ruining TWO 3s 800 mAh LiPos, after 48-49 flights. The only explanation was the Soft Cutoff on the ESC... (I have Lipos which are over 2 years old, and still 100% capacity -- and have only been used with "Hard LVC Cutoff" ESCs...)

So, last weekend, I performed a test. I used a BRAND NEW 3s 1000 mAh LiPo, and flew the airplane with my Watts Up meter... Guess what? The battery voltage was allowed to drop as low as 8.64 volts! (After flight, unloaded voltage was 10.59 volts -- I never would have known without flying with the Watts Up meter installed!)

So -- obviously, I'm doing something wrong... Please educate me! :o
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Old 11-19-2007, 04:42 PM
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Reprogram the T'birds for 10 or so volts! The new "beta" software permits this modification. As I recall, the T'birds come with a 9 volt lvc.
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Old 11-19-2007, 05:01 PM
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I thought hard and soft cut offs on the ESC were for the brake. Is there also one for the voltage? I'm interested too.
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Old 11-19-2007, 05:11 PM
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I believe cutoff and brake are two independent functions: you can use the brake with either type of cutoff. The voltage is variable. Hard cutoff simply means it stops at the lvc; soft simply means it slows as it approaches lvc so you get some warning (soft thrust).

I use the brake function for my motorgliders and "no brake" for my powered airframes.

If I'm wrong, someone correct me.

Hm, looks like CC knows the LVC is too low for the lipo crowd:

ATTENTION LIPO FLIERS - Always follow your
battery brand’s safety recommendations. Your
Thunderbird is set at the factory to use a 3.0
volt per cell cutoff voltage. You may change this
voltage setting using the Castle Link system (sold
separately).

As I recall, this can ONLY BE RESET with the link: I've used mine for several locals who don't have the cable.
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Old 11-19-2007, 05:14 PM
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Originally Posted by Dave Matison View Post
I believe cutoff and brake are two independent functions: you can use the brake with either type of cutoff. The voltage is variable. Hard cutoff simply means it stops at the lvc; soft simply means it slows as it approaches lvc so you get some warning (soft thrust).

I use the brake function for my motorgliders and "no brake" for my powered airframes.

If I'm wrong, someone correct me.
Makes sense. I just wasn't aware of it.
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Old 11-19-2007, 05:14 PM
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Originally Posted by Dave Matison View Post
Reprogram the T'birds for 10 or so volts! The new "beta" software permits this modification. As I recall, the T'birds come with a 9 volt lvc.
Well, this is VERY interesting.

However, I don't know that it will solve my problem... :o

Here's my understanding of how "soft" cutoff works: "When the ESC sees 9 volts at the battery (under load), the ESC reduces throttle to a point where the battery voltage (under load) is above 9 volts."

In theory, you should be able to fly until the airplane can't fly any more, and the battery will never fall below 9 volts. The ESC will simply reduce the throttle to a point where the airplane can't stay aloft.

However, after performing my test, I do not believe this is true... :o

If I set my LVC to 10 volts, it will still allow the battery voltage to drop below 9 volts -- it will just make the "throttle change" noise quicker.

I think my mistake is that I can't seem to hear the "throttle change" noise quickly enough -- I need a loud buzzer to tell me it's time to land... Does anyone make anything like this?
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Old 11-19-2007, 05:15 PM
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Originally Posted by Dave Matison View Post
I believe cutoff and brake are two independent functions: you can use the brake with either type of cutoff. The voltage is variable. Hard cutoff simply means it stops at the lvc; soft simply means it slows as it approaches lvc so you get some warning (soft thrust).

I use the brake function for my motorgliders and "no brake" for my powered airframes.

If I'm wrong, someone correct me.
This is correct. Typically, I use "no brake", and "hard cutoff".
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Old 11-19-2007, 05:26 PM
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Originally Posted by Lieutenant Loughead View Post
This is correct. Typically, I use "no brake", and "hard cutoff".
I use soft, 10 volts and no brake for my powered models.

I'm not sure about your position that setting the LVC to 10 volts wills still allow discharge to 9 volts.

Why not call Joe Ford at Castle Creations?

He is a great guy and you will have a definitive answer you can share with the rest of us!
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Old 11-19-2007, 05:28 PM
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No brake and soft cut on every plane I own. I like a little heads up that I'm about to lose power. Some of my planes _require_ a lot of room to land
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Old 11-19-2007, 05:33 PM
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Originally Posted by Old Fart View Post
No brake and soft cut on every plane I own. I like a little heads up that I'm about to lose power. Some of my planes _require_ a lot of room to land
I concur with the ancient, learned flatus!
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Old 11-19-2007, 05:41 PM
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LOL Dave

Try getting the Brio-10 downwind and hit hard cutoff - they land into the wind oh so well......
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Old 11-19-2007, 08:37 PM
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Originally Posted by Dave Matison View Post
Why not call Joe Ford at Castle Creations?
Who is Joe Ford (CC owner, tech, engineer?), and what's his phone number?
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Old 11-19-2007, 09:02 PM
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Go to castlecreations.com.
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Old 11-19-2007, 11:06 PM
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Okay -- I called and left a message. If he calls me back, I will post his response here.
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Old 11-20-2007, 01:31 AM
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Lt.

My understanding of soft LVC is the same as yours, however, when the voltage drops to 9.0 volts, you reduce the throttle setting, the voltage recovers to above 9.0, and the ESC thinks everything is OK and allows normal throttle usage. This cycle can repeat, causing the battery voltage to go lower and lower.....not good!

I think if you could detect the first soft LVC, reduce the throttle, and land immediately, you would be OK.

Gene
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Old 11-20-2007, 06:06 AM
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Originally Posted by Lieutenant Loughead View Post
Well, this is VERY interesting.

However, I don't know that it will solve my problem... :o

Here's my understanding of how "soft" cutoff works: "When the ESC sees 9 volts at the battery (under load), the ESC reduces throttle to a point where the battery voltage (under load) is above 9 volts."

In theory, you should be able to fly until the airplane can't fly any more, and the battery will never fall below 9 volts. The ESC will simply reduce the throttle to a point where the airplane can't stay aloft.

However, after performing my test, I do not believe this is true... :o

If I set my LVC to 10 volts, it will still allow the battery voltage to drop below 9 volts -- it will just make the "throttle change" noise quicker.

I think my mistake is that I can't seem to hear the "throttle change" noise quickly enough -- I need a loud buzzer to tell me it's time to land... Does anyone make anything like this?
Thats basically corect except for the part I have highlighted above. You cannot ever fly untill the plane wont fly any more or you will kill the packs.

When people talk about a 3.0 volt per cell cut off they are refering to an UNDER LOAD voltage.

If your flying at very hi loads, the cells will reach cut off voltage while they still have a safe amount of capacity left in them.

The problem is that if you continue to keep flying by letting the cells come up and go down you gradually reduce the load on the cells by reducing throttle or average throttle level.

At very low load levels (low amp draw) a cell wont reach 3.0 volts untill it is dangerously over discharged.

Its a good way to kill a pack.

What your suposed to do is land after the esc hits the 3 volt/cell LVC the FIRST time it happens

This LVC system is/was designed/intended to alert you that the pack is low and give you time to go around and make a safe landing while the packs are still alive and well.

It was never intended as a method of letting you run the packs all the way empty.

Larry
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Old 11-20-2007, 06:12 AM
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Another point here thats good to know.

Say you have an airplane that draws 5 amps on average.

Lets say your flying it with a 1000 mahr 10C pack. That means your drawing 5C on average. That pack will reach LVC while it still has plenty of capacity left to be safe and not damage itself.

Now lets say you put in a 3000 mahr 20C pack and go up and fly around at 1/4 throttle or less - just enough to keep it in the air - only drawing 2 amps on average.

If your lvc is set to 3.0 volts per cell, you will over discharge and damage that pack the first time you fly it that way.

Larry
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Old 11-20-2007, 02:24 PM
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Thanks for your responses, guys. Please allow me to continue this conversation:

Originally Posted by GeneH View Post
My understanding of soft LVC is the same as yours, however, when the voltage drops to 9.0 volts, you reduce the throttle setting, the voltage recovers to above 9.0, and the ESC thinks everything is OK and allows normal throttle usage. This cycle can repeat, causing the battery voltage to go lower and lower.....not good!
I don't understand this. Let's look at each step:
  • Battery is fine -- everything operates normally.
  • ESC measures 9.0 volts under load.
  • ESC begins to "hunt", which simply means the throttle is ramping up and down to keep the loaded battery voltage at (or above) 9.0 volts.
  • As the battery discharges, the throttle ramping ("hunting") will become lower and lower -- yet, still keeping the loaded battery voltage at (or above) 9.0 volts.
  • As the throttle is reduced, the airplane will descend, and the pilot will be forced to land.
  • The loaded battery voltage should have never dropped below 9.0 volts!
I don't understand how the battery voltage can get lower and lower, if the ESC keeps the loaded voltage at (or above) 9.0 volts.

Originally Posted by GeneH View Post
I think if you could detect the first soft LVC, reduce the throttle, and land immediately, you would be OK.
I think that's my problem -- the hunting is very difficult to hear in my application. :o (Or, it could just be my tired old ears. Does anyone make a very loud alarm, which sounds when 9.0 volts is measured?)

Originally Posted by Larry3215 View Post
Thats basically corect except for the part I have highlighted above. You cannot ever fly untill the plane wont fly any more or you will kill the packs.
See my bullited list of steps, above. I really don't understand why this would kill the packs.

Originally Posted by Larry3215 View Post
When people talk about a 3.0 volt per cell cut off they are refering to an UNDER LOAD voltage.
Yeah, I understand that. If the ESC automatically backs off the throttle, the load is reduced, and battery voltage will rise to at (or above) 9.0 volts.

Originally Posted by Larry3215 View Post
If your flying at very hi loads, the cells will reach cut off voltage while they still have a safe amount of capacity left in them.
Well, that's just it -- this is a very LOW load application. I'm drawing 6.68 amps at full throttle, on a fully charged pack.

Originally Posted by Larry3215 View Post
The problem is that if you continue to keep flying by letting the cells come up and go down you gradually reduce the load on the cells by reducing throttle or average throttle level.
Again, I don't see why. I didn't know there was a maximum mAh you could draw out of the pack -- if it's an 800 mAh pack, and I draw 650 to 700 mAh out of it, but the battery "at rest" voltage is 11.01 volts, is it overdischarged?

Originally Posted by Larry3215 View Post
At very low load levels (low amp draw) a cell wont reach 3.0 volts untill it is dangerously over discharged.

Its a good way to kill a pack.
Again, I thought it was all about VOLTAGE. :o

Originally Posted by Larry3215 View Post
What your suposed to do is land after the esc hits the 3 volt/cell LVC the FIRST time it happens

This LVC system is/was designed/intended to alert you that the pack is low and give you time to go around and make a safe landing while the packs are still alive and well.
I guess this is really my biggest problem. I simply can not HEAR the "hunting" on this airplane. In order for me to hear it, the airplane must be within 10 feet of my ear! (I don't normally fly that close to my head!)

I guess what I need is a really loud, audible alarm when the battery voltage hits 9.0 volts -- does anyone make something like this?
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Old 11-20-2007, 02:58 PM
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I have a solution to this problem......

Get a TX with a timer.... or put a timer around your neck like i do!!!!!!!

Then, estimate your flying time at full throttle, and fly the plane for that time how you would normally fly it.. land when the timer goes off.. see how much mAh you put back in the pack... adjust timer appropraitely..

I know that seems redundant... but.. each plane I have has its own timer setting and uses its own battery(s).. I always set them to Hard cut in case i screw up, I didn't hit lipo cut all summer and haven't roached a battery since last year while flying on soft cut without a timer!!!

and YES... a guy sells "screamers" on ebay that will alarm at set battery voltages. THey work very well
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Old 11-20-2007, 03:01 PM
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Hmm: is there general agreement that "soft cut-off" will usually lead to over discharge unless immediately detected and the model landed?
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Old 11-20-2007, 03:06 PM
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Originally Posted by BuzzardBait View Post
I have a solution to this problem......

Get a TX with a timer.... or put a timer around your neck like i do!!!!!!!

Then, estimate your flying time at full throttle, and fly the plane for that time how you would normally fly it.. land when the timer goes off.. see how much mAh you put back in the pack... adjust timer appropraitely..

I know that seems redundant... but.. each plane I have has its own timer setting and uses its own battery(s).. I always set them to Hard cut in case i screw up, I didn't hit lipo cut all summer and haven't roached a battery since last year while flying on soft cut without a timer!!!
Unfortunately, this solution does not work. I've been using a timer for the last three years. I've kept track of how much mAh I put back into each pack for the last three years. I thought I was doing everything properly.

Many of the lesser expensive ESCs on the market today do not have a "Hard Cutoff" setting -- it's "Soft Cutoff" or nothing.



Originally Posted by BuzzardBait View Post
and YES... a guy sells "screamers" on ebay that will alarm at set battery voltages. THey work very well
Good to know -- I'll give that a try!

Originally Posted by Dave Matison View Post
Hmm: is there general agreement that "soft cut-off" will usually lead to over discharge unless immediately detected and the model landed?
It sure seems that way, doesn't it? I'm still waiting on the return call from Castle Creations... :o
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Old 11-20-2007, 03:13 PM
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Originally Posted by BuzzardBait View Post
and YES... a guy sells "screamers" on ebay that will alarm at set battery voltages. THey work very well
Well, I'm having trouble finding these -- can you please post a link? :o
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Old 11-20-2007, 03:14 PM
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Well, if you can't find a "screamer," how about a "moaner?"
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Old 11-20-2007, 05:27 PM
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Lol!!!
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Old 11-21-2007, 04:28 AM
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Originally Posted by Lieutenant Loughead View Post
Unfortunately, this solution does not work. I've been using a timer for the last three years. I've kept track of how much mAh I put back into each pack for the last three years. I thought I was doing everything properly.

... :o
I dont understand why this isnt working for you. Thats exactly what I do.

How much are you putting back into the packs? Perhaps thats the issue.

Its best to never fly more than 80% out of a pack. Resting voltage after a few minutes should always be 3.7 volts per cell at the absolute minimum after a flight. I much prefer to see 3.75 - 3.8 on my packs.

Larry
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