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Old Aug 25, 2005, 12:45 PM
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WJ Birmingham's Avatar
St. Louis Metro
Joined Apr 2003
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A bad practice that should be nipped before becoming popular!

Ok,

I'm not sure who started this whole idea, but it's becoming more and more popular, and needs to stop before product starts self destructing.

Someone has suggested using a 3-cell pack to power a model, and then using the secondary ports that are intended for cell monitoring as a secondary power feed directly to the receiver.

THIS IS A BAD IDEA. It will cause an immediate imbalance in cell voltage due to the servos/receiver drawing more current from 2 of the 3 cells.

Buy a BEC or buy a speed controller with a proper BEC. Don't try using half thought out ideas and promoting them as good ideas without thinking of long term affects and the repercussions of them.

/steps off the soap box.

-WJ
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Old Aug 25, 2005, 12:52 PM
Southern Pride
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Haralson County GA. USA
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Very good advise. Even using a UBEC on one battery when there are two or more batteries in series is a bad idea IMO.

Charles
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Old Aug 25, 2005, 01:49 PM
Watts your motivation
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San Bernardino, CA
Joined Aug 2003
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Wow, I'd never thought of that. Thanks William and Charles for warning those who are followers and don't know any better (but really should).

Justin.
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Old Aug 25, 2005, 01:55 PM
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Joined Feb 2005
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Someone has actually done that? Yikes!
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Old Aug 25, 2005, 10:52 PM
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Chicago O, Illinois, United States
Joined Jan 2003
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Great advice. That is one of the worst things that you could do to Lipolys, and could ruin the hobby for a lot of people. It's totally irresponsible to abuse the cell taps. Rather than for balancing, this is unbalancing the pack! Even though I didn't know about this before, it's the stupidest idea I have ever heard!

Sponge
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Old Aug 26, 2005, 12:20 AM
X-Era Motors
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Newport, NC
Joined Jan 2002
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As eflight gets less expensive, more and more people are going to jump in feet first without having a clue. I was in a hobby shop the other day listening to someone tell me that a ubec would make the motor run cooler, and anodizing really made his frame stiffer. Get prepared for a lot of dumb stuff in the future of eflight. Just imagine what would happen if somebody offered a ready to fly 50 size E-heli for $200. I can't imagine the carnage.
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Old Aug 26, 2005, 08:19 AM
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Milton Keynes UK
Joined May 2003
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Whilst I agree that tapping off 2 cells in a 3cell pack is a very bad idea, using the balance connector as an auxilary power connector should be fine as long as the current is kept withing reasonable limits.

for example powering an onboard video system. maybe it will require a separate voltage regulator, maybe it will not. The important thing is to use the full pack voltage (most probably 1st pin and last pin on the tap connector).

A big advantage of using the tap connector would be the ability to conenct various accessories without modifying the wiring to the ESC or having a separate battery.

As always, if you are not happy doing this, or do not understand it, DON'T DO IT.

Stuart
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Old Aug 26, 2005, 11:45 AM
Fly
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Tempe, Arizona USA
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If the balancing taps are always used to charge the cells individually, then it does not matter that they are discharged asymmetrically. The only problems this is likely to cause (over and beyond normal LiPo issues) are 1) letting any cell go too low, 2) overcharging any cell, and 3) drawing too much current from the secondary tap.

1) The LCV is on the motor, which is measuring the sum of the voltages of all the cells. If the LVC is set to a higher voltage that will compensate for the more heavily taxed cells dropping more in voltage, but not allowing the higher voltage remaining cell to mask a low voltage condition on the other cells. If you set your LCV to 3.3 V/cell, that should be plenty of margin. For example, if 2 cells are feeding the receiver in a 3S pack, then the LVC should be set to 10 V instead of 9 V. Then the 2 extra taxed cells can get to 3 V while the 3 cell is still 4 V and you will get safe cutoff. In practice, I would not expect the difference to be near that large and more likely would be 2 cells closer to 3.2 V with the third cell at 3.6 V when cut off is reached. Keep in mind that the extra current through the 2 cells will be typically less than 20% more than the remaining cell for a 5A application, with no more than 10% difference likely in a 20A application. This will reduce the total capacity drawn from the pack, but that is the price for doing this safely. This will also reduce the power wasted in the BEC (and lower the temp of the BEC), so some gain here will compensate a bit for the higher LVC capacity loss.

2) So long as each cell is properly charged (that is, individually charged), then the imbalance is immaterial to the charge safety.

3) It is also important that the secondary tap for the receiver does not draw more current than that tap can handle. If you charge through that tap at 2 amps, then you should be fine, as most servos and receivers in a 3 servo small electric application will draw less than 2 amps total. Most of the current is still going through the main taps to the ESC for the motor.

Finally, by reducing stress on the third cell a lot, (less total current discharged per use), you should be increasing the total life of that cell. However, that will be of no use because the other two cells are still being used pretty much the same. No real gain here.

I don't see any real benefit from doing this except to reduce the stress on the BEC. That has never been much of a concern to me! I've run 4 servos, maxing out the current of the BEC on an ESC rated for 3S max, with a 4S pack without any problem. That is, I have a plane with 4 HS-55 servos on a Hackermaster 18 A ESC driving 10 A with a 4S pack. That is over current on the BEC, over-voltage on the ESC, and have not had any problems in months of heavy use. The ESC gets warm. As I am actually running two 2S packs in series, it would be trivially easy for me to tap one of the packs separately for the receiver, but I see no reason to do so. My only concern was the BEC, which is having no problems. Also, I rarely run these packs below 3.5 V/cell and never wait for the LVC, which is too low for 4S.

-- Alan
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Old Aug 26, 2005, 11:54 AM
Registered User
Bellingham WA
Joined Jan 2005
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Hello WJ,

I am not sure I fully understand your concern...

If you use a 3S pack and tap off two of the cells to power something else, at the end of the run there will be some imbalance in the total pack.

However, if you charge using an independent channel charger or use cell balancing every time you charge, the pack will always be in balance when it is fully charged.

I understand your concerns if you are series charging, but then your concern should not be with the issue of tapping power from single or double cells, but should take the form of a warning that if you do this, you need to alter your charging methods because the pack will be out of balance as a result of doing this.

Am I missing something?

Tom
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Old Aug 26, 2005, 12:04 PM
jumping trees
bgabric's Avatar
keller tx
Joined Dec 2003
317 Posts
unless you have a cut off you could drain some of your cells below safe levels causing damage. even if you aux cuts off the main may still think it has enough juice to keep going and compleatly drain one or more of the cells. what worse if your powering your recever like this your recever could lose power!!!! (very bad) no this is a horrid idea. sooner or later you or someone will lose an expensive battery. Never misuse li-polys if you cause enough damage you could ruin it for the rest of us.

Bryan
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Old Aug 26, 2005, 12:23 PM
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WJ Birmingham's Avatar
St. Louis Metro
Joined Apr 2003
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Quote:
Originally Posted by SilverFoxCPF
Hello WJ,

I am not sure I fully understand your concern...

If you use a 3S pack and tap off two of the cells to power something else, at the end of the run there will be some imbalance in the total pack.

However, if you charge using an independent channel charger or use cell balancing every time you charge, the pack will always be in balance when it is fully charged.

I understand your concerns if you are series charging, but then your concern should not be with the issue of tapping power from single or double cells, but should take the form of a warning that if you do this, you need to alter your charging methods because the pack will be out of balance as a result of doing this.

Am I missing something?

Tom

Yes, you are missing the big picture. Only about 2-3% of the ENTIRE userbase of lithium cells understands how to properly use them. To 'propose' that this is a safe practice isn't a good idea.

We get call after call after call asking the same questions, which the answers to are posted on the web site under the usage guidelines. I feel like a parrot spitting out the same answers day after day.

It's amazing how many people don't even understand the principle of charge rate. They think that because the charger doesn't have the exact charge rate the pack requires, the pack won't be fully charged.

If you want to experiment on your lipos, using them outside of the recommended tolerances, and you know the consequences, that's your perogative. We would, however, appreciate it if you would educate other novice users on proper usage, and not experimental usage that will void the warranty as well as possibly cause harm to property, themselves, or others.
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Old Aug 26, 2005, 12:27 PM
Southern Pride
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Haralson County GA. USA
Joined Oct 2004
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Alan and Tom,
Yes it can be done safely by someone with enough knowledge/ experience etc. but how many percentage wise posses such knowledge?

I like to think I kinda know what I am doing however this morning I flew a 3S pack down to 10.4 open volts and never heard,felt noticed the LVC point. I was flying alone in my backyard with no distractions but still flew almost 2 minutes past my normal flight time.
There have been cases where using a UBEC which was connected to one battery only of a series battery setup has damaged a LiPoly. If you always leave 20% in the cells like I do ?? then no worries.


Charles
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Old Aug 26, 2005, 12:29 PM
Negative altitude hurts
m00se's Avatar
Litchfield Park, AZ
Joined Feb 2005
392 Posts
Quote:
Originally Posted by Fly
If the balancing taps are always used to charge the cells individually, then it does not matter that they are discharged asymmetrically. The only problems this is likely to cause (over and beyond normal LiPo issues) are 1) letting any cell go too low, 2) overcharging any cell, and 3) drawing too much current from the secondary tap.



-- Alan

That's a big IF.

If someone is aware of the issues, can deal with charging the cells individually,
and can set a LVC that will prevent the tapped cells from going below 3V, then it's not a problem. Otherwise, it could be a huge problem.
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Old Aug 26, 2005, 12:59 PM
Registered User
Bellingham WA
Joined Jan 2005
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Hello WJ and Charles,

Thanks for clarifying the "big Picture."

I was thinking that there was a technical reason, but totally understand that the issue involves "educating the masses," and the fact that from time to time, we humans occasionally have less than total concentration on the task at hand...

Tom
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Old Aug 26, 2005, 01:13 PM
Watts your motivation
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San Bernardino, CA
Joined Aug 2003
6,512 Posts
Quote:
Originally Posted by WJ Birmingham
Yes, you are missing the big picture. Only about 2-3% of the ENTIRE userbase of lithium cells understands how to properly use them. To 'propose' that this is a safe practice isn't a good idea.
And of that 2-3% that have the knowledge, do they actually even charge the cells individually? How new are the chargers to do this, of how long has respetable juys like William and Charles advocated this? I mean, to make the cells "safe" to use the charger port to power something else you have to be constantly checking and testing and balancing (or just plugging I guess )- a bigger hassle than a nirmal charge.

The other thing that concerns me is the cycleing of the batteries individually. As soon as they are used normally, they all will have individual characteristics. Why even bother buying a perfectly matched cell 3s or 4s pack if your going to intentionally muck with that characteristic?

I think the "cutting edge, creative" users of Lipos that ARE doing or advocating this practice should make a statement beforehand stating the downsides as well. We get fires from normal useage with a little carelessness. Try adding abuse to that mix and we fliers just might lose this awesome advancement in bateries.

*end rant*

Justin
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