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Sep 08, 2004, 02:40 AM
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zippiz's Avatar
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Question

Cut off question for 2cell lipo, 5.2 volts?


Hi!

I read that the Li-Po should never go under 6volt (3volts each)

The instructions for Jeti ESC says

"Li-polymer, cut off slow down, minimum voltage 5,2V / 2 cells, 8V / 3 cells"

Could someone explain, should I be afraid that this would damage my Li-Po cells?

Is this under loading... or am I just missing something?

Please help out, and sorry about my English
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Sep 08, 2004, 05:52 AM
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windface's Avatar
You don't have 6.V cut off? This is what you need.for 2S lipo.
Sep 08, 2004, 09:57 AM
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zippiz's Avatar
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But how come the instructions for jeti esc says 5.2v for 2 S lipo?
Sep 08, 2004, 10:17 AM
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My new FMA Direct 20 amp also is not set to 6 volts, but 5. something volts. I am guessing that they are looking for a cut off under load. I have flown some of my GWS esc,s that have a 5 volt cut off on 2 cell lipo's, and had them turn the motor off like they should. I have measured the cells after landing and they where always above 6 volts. I would not recommend doing this. I have some castle creations 20A's that I could not get to work at a 6 volt cut off, and had to put them a 5. Seems that the more amps you draw, the more the voltage will sag, and thus you can get away with a lower cut off. I think if you had a real low amp draw plane, you might be in trouble with something the cut out under 6 volts.

Dale
Sep 08, 2004, 10:56 AM
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zippiz's Avatar
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dwentz...

Thanks for the reply, that really makes sense... to me anyway

The plane I'm planing to use my ESC in is in fact a flying wing with a 300-motor, I belive its about 7 Amp.

What kind of GWS esc are you using?
Maybe I could use my ICS-480, ( http://www.gws.com.tw/english/produc...roller/480.htm ) don't know the cut-off volt for them tough...
Sep 08, 2004, 11:04 AM
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zippiz's Avatar
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found this info on this issue

26
"What is a cutoff?
To be frankly with you, a cutoff is a circuit that is added to an esc equipped with a BEC to try to prohibit the motor battery from being run dead and causing a crash when the input voltage goes below 5.25 volts. GWS speed controllers not only have a BEC but have a cutoff circuit. We‘ll highly suggest that you shouldn't use a car type controller in a plane with the BEC active."

http://www.gws.com.tw/english/produc...troller/50.htm

To me it sounds as it should be ok to use a GWS ESC with li-po?
Sep 08, 2004, 11:16 AM
Registered User

don't use 5.x cutoff


zippiz,
don't use the 5.7 volt BEC...you can be setting yourself up for a flaming LiPoly in the future. Most BEC have a "margin of error" that allows the underload voltage to go 10-15% under the nominal voltage. I have seen this many times with high cost units like Castle Creations (which are top rate manufacturers in anybody's book). So the voltage can be closer to 5.5 volts for a 2S1P pack. This WILL CAUSE PROBLEMS.

Try this, get a good digital voltmeter and put your plane on the workbench. Have someone hold it down while you measure the voltage across the LiPoly while you give it full throttle. measure the time from when you first turn on (assuming a full LiPoly charge) until you hit 6 volts. Then continue to run until the BEC kicks in. You will most likely see an increase in "flying time" on the order of 10-20%. Also note the voltage just as the BEC kicks in. That should give you some idea of how "over discharge" you are running those poor cells. In the future, just put a digital timer on your Tx and start it every time you launch. Be sure to land when the timer reaches that time you measured to get to the 6.0 volt level. That is the only way you can be safe.

Just look at some of the videos I have posted to see what danger you can be in for over discharging /over recharging a cell. It is simply not worth the risk.
Hope this helps.

Crazy Ted -- a battery junkie
Sep 08, 2004, 11:28 AM
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zippiz's Avatar
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tjcooper

Do you mean that the Jeti ESC is not safe to use?
I was planing on buying the JES 020, the instructions is availble here
http://www.jetimodel.cz/eng/hlavnien.htm

and it says 5.2volts for Li-po

what about the input from Dwentz?
"I am guessing that they are looking for a cut off under load. I have flown some of my GWS esc,s that have a 5 volt cut off on 2 cell lipo's, and had them turn the motor off like they should. I have measured the cells after landing and they where always above 6 volts."
Sep 08, 2004, 11:50 AM
Registered User
I just checked the manuals from Kokam, and they state do not run them down to below 2.5 volts per cell. There controller is set up to cut off at 5.3 volts +- 0.1. I have seen the 2.5 number before, and also the 3 volt number. I do not know what to believe. I always check my cells and make sure that they are at 3 volts per cell after each flight. I am wondering if the 2.5 volts is under load, and the 3 volts is at rest? See the manuals on the FMA site at https://www.fmadirect.com/site/fma.htm?body=Support

I also found some info from Graupner that uses the 2.5 volt number. I just got off the phone with Thunderpower and they say 3 no lower than 3 volts per cell.

At this point who knows what to use.

Dale
Sep 08, 2004, 11:59 AM
Registered User
Found this in the FAQ: at https://www.rcgroups.com/forums/showthread.php?t=209187
snip
-----

It important to use a programmable ESC since the correct voltage cutoff is critical to the life of your batteries. Use the ESC's programming mode to set the LVC to 2.5 volts per cell with a hard cutoff, or 3.0 volts per cell with a soft cutoff. If your ESC does not have hard or soft cutoff, use 3.0 volts per cell. You will know when flying that it is time to land when you experience a sudden drop in power caused by the LVC.
---
Dale
Sep 08, 2004, 03:07 PM
Registered User

Thanks tjcooper for advice!
I have been going to buy Jeti 020 like zippiz by the time now.
Is timer vital for using my 2S2P pack?
Isn't better LiPo saver, such as one from DIY Electronics Forum?
Will Castle Creatons Pixie 20P be enough to my pack (6.0V option)?

Thanks,
Szymon

PS: Sorry for language mistakes
Sep 08, 2004, 03:50 PM
Registered User

What is minimum discharge for LiPoly!


zippiz and dwentz,
There has been much discussion about the minimum discharge voltage for a LiPoly over the last two years. Originally it was set by Kokam and others at 2.5 volts / cell as the ABSOLUTE MINIMUM before damage occurs. This was originally defined for 1C discharge only. As R/C users pushed the envelop to 3C, then to get the 300 discharge cycles that left 80% remaining capacity, the new LVC was set to 3.0 volts.

I know that FMA still has an application note that talks about 2.5 volts/cell. This is old stuff and should be discarded! Look at the new page for Kokam (who is their maker at www.kokam.com). You will see that the new spec sheets clearly show 3.0 volts/cell as the LVC. In conversations with J.J. Haung, he has mentioned that by not charging a LiPoly to 4.2 volts, but rather to 4.1 volts, and by discharging only to 3.1 volts rather than 3.0 volts, that an average user can DOUBLE the useable lifetime of a cell.

So the bottom line is: don't go below 3.0 volts---every. There is enough error in the BEC's mechanism that gives the LVC that you may well be at 2.8 volts. When this happens, you cell is loosing part of its long term life. If you only want the cell to last one season (like racers), then you can push the LVC to 2.8 volts but the batteries die sooner. But there is a very big KICKER in this tradeoff. When you go back to recharge, do so at less than 1C. We have seen many cases where even slight over discharge can cause swelling upon recharge at 1C. If you keep the recharge to a lower amount, the potential for swelling is less (wish I could tell you the probability density curve for the C rate but I don't know it). And even then, since this is a statistical thing, watch those batteries like a hawk! You never know when a pack is the "weak sister" and will go thermal faster than normal.

I cannot comment on Jeti controllers----I don't own any. But if they cannot set the voltage for LVC at 3.0 volts/cell, then you will have to time how long you can fly from the test setup I described earlier. I have many Castle Creations controllers -- which I really love -- and even these units when set to 3.0 volts/cell do not cut off exactly at 3.0 volts.....I have had some go as low as 2.8 volts/cell under load before they cut off. So take the time to measure what your equipment really does. Put on digital meters and see in a controlled environment what really happens. It is oftentimes much different that the "product spec". Steve Palmer of CC once told me: "Whats it worth to you to not have a fire? So buy two $37 LiPoly packs and just go easy on them by doing shorter flights. Your flights will be more powerful when you are doing them....and your batteries will last longer". And to this I would add: you garage/car will remain useable much longer, too.

If you have followed any of my "vent and flame" videos, you will notice that the newer breed of LiPoly are much more flammable than the original ones. I personally feel that at 8-10C, you have a battery that is a hand grenade ready to go off. So be smart.....don't mess with the firing pin....keep the discharge to 3.0 volts or higher....your life will be much simpler.

Crazy Ted -- a battery junkie...who still thinks LiPoly is better than sliced bread!
Sep 08, 2004, 03:53 PM
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kitfoxdrvr's Avatar
Lots of confusion about how the Jeti's set their voltage cutoff-some kind of proprietary hardware/software that figures out the number of cells using unpublished parameters. I have one of the 18 3Ps and it says the same thing as yours, zippiz-I have measured my voltage on 2s and 3s after run to lvc and in both cases resting voltages were over 3 volts per cell. I am not sure how the Jeti line works, but it does actually cut off the motor before the resting voltage reaches under 3 volts.

BTW, I have tested this esc by initializing with a 50% discharged pack, and it still got the lvc right. Give it a try and see what it does for you-do it in flight, though-don't run a motor static for too long or you could overheat it, particularly a brushed motor, due to lower airflow on a bench.
Sep 08, 2004, 04:05 PM
Registered User
I have a Jeti 30 amp with a speed 400 drawing only 8.7amps. When the ESC cut in it left a 3s1p 2100 TP lipo at 8.59v. This caused problem with my Schulze 330-d charger which said wrong cell count when I tried to charge as 3 cell. Once I got the voltage to 9v all was fine. In summary I cant trust the ESC to leave the battery at 3v per cell or above. I landed within 20 seconds of the ESC cutting in.
Sep 08, 2004, 04:40 PM
Registered User
I think the big question here is: should be the minimum resting voltage be 3 volts, or should be be looking for 3 volts under load. My castle creations had to be set to lower then 6 volts to keep from cutting out with my eps350C drive per castle creations recommendations. I checked the cells and they where always over 7 volts when taken out of the plane. (2S1500's)

Even though by mistake I let my GWS ESC's cut off the motor (Error in reading my time flown) when I checked the cells they where still at more than 3 volts per cell when removed from the plane. I would not want to run down a cell to bellow 3 volts per cell. If I thought for any reason that there could be a problem I always test my cells with a meter when removing them from the plane.

I think the goal here is to understand what is a safe way to use the cells. The manufactures are not clear on what or where an ESC should be set for a cut off voltage, so how are we to be sure. It is a good thing that we are 2nd guessing the manufactures, and there is not anyone other then us modelers to test their products. I was a little concerned with my FMA 20 amp controller after reading the manual, as they advertised (Don't believe anything you see in an ad!) it would auto adjust for NiCD, NiMHI, and LiPo. I purchased this for a plane that I will fly multiple battery types in, and thought it would be nice not to have to reprogram each time. With that said, I have not had a chance to use LiPo's with it yet, but will do some bench testing before I do.

I have recently done some testing with a LiPo pack that ballooned, and my Triton charger. I will post a video and some test results when I get a little time later this week. These are things not to be mistreated!!!!!! Use care, and if you suspect that you might have a problem, pitch the thing it is not worth taking the chance. I love the LiPo's due to their weight and power, but do not become complacent using them!

Dale


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