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Author Topic: Rpm drop with Molicel packs  (Read 1703 times)

Offline Vitalis Pilkionis

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Rpm drop with Molicel packs
« on: May 17, 2023, 04:43:37 AM »
Has anyone noticed a slight rpm drop after a Molicel battery depletion (usually near the end of a pattern flight). My friend observes that with his Molicel P28A 6S packs before overhead eight maneuver. And I have the same phenomena with Molicel P42A 6S packs if I try to do prolonged training flight for 6 min. Both of us use Jeti 66 non-Pro Esc, Axi 2826/F2B with Igors 12x5 narrow props and Igors active timers. LVC on Jeti 66 Esc set to 2.5V.
It seems the Jeti Esc doesn't like the voltage drop of Li-Ion cells and can't keep rpm despite enough mah left in battery. Waiting for any thoughts from you "molicel guys".

Offline pmackenzie

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Re: Rpm drop with Molicel packs
« Reply #1 on: May 17, 2023, 05:34:17 AM »
No ESC governor can correct for voltage droop.
Once the ESC hits 100% (no chopping, only commutation), rpms will start to drop with battery voltage.
Solution is to either keep discharge level lower ( i.e. no "prolonged training flights").
Or perhaps increase motor kV to increase the motor's base RPM,or increase pitch to reduce the actual RPM required. Or add a cell.
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Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #2 on: May 17, 2023, 05:47:06 AM »
I understand what you mean. But we do not use full throttle in our timers. Also there are no problems with lipo packs.


No ESC governor can correct for voltage droop.
Once the ESC hits 100% (no chopping, only commutation), rpms will start to drop with battery voltage.
Solution is to either keep discharge level lower ( i.e. no "prolonged training flights").
Or perhaps increase motor kV to increase the motor's base RPM,or increase pitch to reduce the actual RPM required. Or add a cell.

Online Igor Burger

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Re: Rpm drop with Molicel packs
« Reply #3 on: May 17, 2023, 05:55:13 AM »
Pat explained it clearly. If you have once low voltage, governor cannot compensate it. Simply goes for 100% and that is all it can do, if you have low voltage, mottor cannot make wanted RPM. And even worse - if you reach voltage limit on ESC, it will lower delivered power to maintainn battery voltage limit, it is much better than immediate cut off and following crash. I will call it as advantage of Spin, not " Jeti Esc doesn't like the voltage drop of Li-Ion cells ".

The reason is clear, if you compare Volt-Ampere characteristic of LiIon vs LiPol you will see large volt difference (and since flight needs some value of power - Watts, lover voltage means higher drain in mAh). Simply LiIon battery gives less possible RPM on given motor at given load compared to the same LiPol battery. Solution is either:

- use motor with higher KV (but it will use higher urrent)
- use prop with higher pitch (but it will give less power)
- use smaller sensitivity on Timer, so that it does not ask for such strong boost (but it will limit the boost)
- and yes, do not drain your battery so much :- )))

Lions ale lighter, but as you can see it is something for something.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #4 on: May 17, 2023, 06:04:01 AM »
Thanks Igor for the explanation. Now I wonder, people like Orestes H. and Paul W. successfully use Molicel packs. Don't they experience the same problems or simply use less agressive settings to remain within usable voltage range of Li-Ion cells?



Pat explained it clearly. If you have once low voltage, governor cannot compensate it. Simply goes for 100% and that is all it can do, if you have low voltage, mottor cannot make wanted RPM. And even worse - if you reach voltage limit on ESC, it will lower delivered power to maintainn battery voltage limit, it is much better than immediate cut off and following crash. I will call it as advantage of Spin, not " Jeti Esc doesn't like the voltage drop of Li-Ion cells ".

The reason is clear, if you compare Volt-Ampere characteristic of LiIon vs LiPol you will see large volt difference (and since flight needs some value of power - Watts, lover voltage means higher drain in mAh). Simply LiIon battery gives less possible RPM on given motor at given load compared to the same LiPol battery. Solution is either:

- use motor with higher KV (but it will use higher urrent)
- use prop with higher pitch (but it will give less power)
- use smaller sensitivity on Timer, so that it does not ask for such strong boost (but it will limit the boost)
- and yes, do not drain your battery so much :- )))

Lions ale lighter, but as you can see it is something for something.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #5 on: May 17, 2023, 06:17:36 AM »
Thanks Igor for the explanation. Now I wonder, people like Orestes H. and Paul W. successfully use Molicel packs. Don't they experience the same problems or simply use less agressive settings to remain within usable voltage range of Li-Ion cells?

Oh I see. They fly different (bigger) motors and perhaps UC props so their setups consume much less mah.

Online Igor Burger

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Re: Rpm drop with Molicel packs
« Reply #6 on: May 17, 2023, 06:18:47 AM »
Well, that is not what I can answer, but for sure I can tell you that I spent lot of time to optimize whole setup, so if you suggest me that you use battery with another parameter (or motor, or prop etc), then from logic of the thing I must tell you that you will not get better result - because that mine is simply best I found  VD~

So may be they simply found another solution with different components, but from what I have seen, I did not see them fly 6 minutes, and I think they use much (FAR) smaller sensitivity. If I want seriouse response of power train in first or second climbing in four leaf (for those who do not fly FAI - there are 2 climbings in 4L) wit my relatively aggressive sensitivity setting, then the battery must be able to deliver responsive voltage and so power also end of flight.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #7 on: May 17, 2023, 06:41:49 AM »
Well, that is not what I can answer, but for sure I can tell you that I spent lot of time to optimize whole setup, so if you suggest me that you use battery with another parameter (or motor, or prop etc), then from logic of the thing I must tell you that you will not get better result - because that mine is simply best I found  VD~

So may be they simply found another solution with different components, but from what I have seen, I did not see them fly 6 minutes, and I think they use much (FAR) smaller sensitivity. If I want seriouse response of power train in first or second climbing in four leaf (for those who do not fly FAI - there are 2 climbings in 4L) wit my relatively aggressive sensitivity setting, then the battery must be able to deliver responsive voltage and so power also end of flight.

Igor, I am not going to discuss about Maxbee and its setup performance. I'v built mine because I believe it's the best. Did only ~40 flights with mine to the date, still tinkering with CG and leadouts position. Thanks for your huge input.

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Re: Rpm drop with Molicel packs
« Reply #8 on: May 17, 2023, 07:05:01 AM »
I do not mean Maxbee, I am speaking about power train configuration. Means matching battery, motor, ESC, timer and prop.

But you know all necessary and now it is on you.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #9 on: May 17, 2023, 07:38:25 AM »
I do not mean Maxbee, I am speaking about power train configuration. Means matching battery, motor, ESC, timer and prop.

But you know all necessary and now it is on you.

Ok then. Speaking about power train configuration it for sure depends. It hugely depends of the motor and the prop. I successfully used 2250mah 6S Lipo packs on my previous model, which RTF weight was 1830g. It had the iFlight XING X4214 660kv motor and Kravchenko 13x5.5 3BL prop. It consumed ~1850mah for the entire pattern flight and cells voltage stayed to ~3.70V after a flight. Lap speed was 5.1 - 5.2 s on 20 meter lines. And my timers settings were quite an agressive.

Online Ken Culbertson

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Re: Rpm drop with Molicel packs
« Reply #10 on: May 17, 2023, 08:14:53 AM »
The Endgame III I am trimming now has this exact problem.  It is the first plane I have equipped with the Molicel batteries.  The pack shape was made to fit the nose for the Canard and going back to the boxcar shaped Lipo is not a realistic option if I want to maintain my CG.  So, I am committed to finding a solution.  My plane is heavier than I planned - 65oz and uses the SpinPro 66 ESC.  I am not using Igor's timer.  Some have suggested that the 3.2 low voltage cutoff suggested for the Spin may be an issue and have recommended setting it at 2.9.  A second recommendation was to charge the battery to 4.2v per cell.  I prefer preserving the plane to saving a battery and I have been told that the Li-Ion can stand a 25v charge.  I will be testing a battery today with that setting and charge.  I wonder if the 2800mah pack is just too small and I should be using something in the 3200 range.

In all cases so far the battery was low but not anywhere near empty.  Lowest per cell was 3.5 after landing, not even close to 3.2.  Others in my group are getting 3.6 or better with planes in the 62-3oz range.

At this point I am fishing for a solution.  The first indication I get is in the 4th loop (2nd outside) of the OH8.  It seems to recover in the level laps before the clover but does it again in the 3rd loop.  Fortunately, our recent rule changes end the maneuver at 45 degrees, so I am not forced to make it to the top, into the wind, with a weak battery - Not Fun and it does not leave much of an impression to bail so early.

I defer to those who know more about this than I do.

Ken
« Last Edit: May 17, 2023, 09:57:53 AM by Ken Culbertson »
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Online Brent Williams

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Re: Rpm drop with Molicel packs
« Reply #11 on: May 17, 2023, 09:06:34 AM »
Pat explained it clearly. If you have once low voltage, governor cannot compensate it. Simply goes for 100% and that is all it can do, if you have low voltage, mottor cannot make wanted RPM. And even worse - if you reach voltage limit on ESC, it will lower delivered power to maintainn battery voltage limit, it is much better than immediate cut off and following crash. I will call it as advantage of Spin, not " Jeti Esc doesn't like the voltage drop of Li-Ion cells ".

The reason is clear, if you compare Volt-Ampere characteristic of LiIon vs LiPol you will see large volt difference (and since flight needs some value of power - Watts, lover voltage means higher drain in mAh). Simply LiIon battery gives less possible RPM on given motor at given load compared to the same LiPol battery. Solution is either:

- use motor with higher KV (but it will use higher urrent)
- use prop with higher pitch (but it will give less power)
- use smaller sensitivity on Timer, so that it does not ask for such strong boost (but it will limit the boost)
- and yes, do not drain your battery so much :- )))

Lions ale lighter, but as you can see it is something for something.

What characteristics do you look at on a chart to identify volt/amp characteristics for li-ion?  Can the voltage drop be anticipated, calculated or factored?

P28A https://www.molicel.com/wp-content/uploads/INR18650P28A-V1-80093.pdf

P42A https://www.imrbatteries.com/content/molicel_p42a.pdf
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Online spare_parts

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Re: Rpm drop with Molicel packs
« Reply #12 on: May 17, 2023, 09:56:03 AM »
What characteristics do you look at on a chart to identify volt/amp characteristics for li-ion?  Can the voltage drop be anticipated, calculated or factored?

Fundamentally it's internal resistance. You want to see the voltage as high as possible under load. A good 50-100C Lipo will have lower internal resistance.
Greg

Offline Fred Underwood

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Re: Rpm drop with Molicel packs
« Reply #13 on: May 17, 2023, 11:19:59 AM »
Note that the Molicel charts/graphs that Brent posted show the charge voltage to 4.2v.  Also note the discharge rate characteristics chart and capacity at 20 to 30 amp draw.  The in flight voltage gets low under load, for the needed rpm.  When you land and load is off, the voltage bounces back up and you can be fooled.  The discharge rate vs amp "capacity" is descirbing what is being observed.  If you follow tshe capacity out to where you are drawing, look at the voltage for a cell under that load, quite low.

Less drag helps a lot to save amp draw so less VG, zig-zags, and elevator wedges.  LiIon batteries start to determine how you fly more than we are used to. 
Fred
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Online Igor Burger

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Re: Rpm drop with Molicel packs
« Reply #14 on: May 17, 2023, 11:28:09 AM »
What characteristics do you look at on a chart to identify volt/amp characteristics for li-ion?  Can the voltage drop be anticipated, calculated or factored?

P28A https://www.molicel.com/wp-content/uploads/INR18650P28A-V1-80093.pdf

P42A https://www.imrbatteries.com/content/molicel_p42a.pdf

Compare with that on picture, it will tell the story. you can have easily 0,5V difference per cell. So if AXI with 700RPM/V works well wit Lipo at say 3.5V discharge voltage, then liIon with 3v will need kv=700*3.5/3=  816. It is because internal resistance of liion is approximately 20mo while lipol somewhere at 5. So the difference is 15mo and at current 30A in boost it makes 0.45V

So Axi 2826/12 with kv=760 is still not enough, but will be certainly better, and will probably work well with conservative setting of power boost in timer, but will need little higher current to run. Axi 2826/10 with kv=920 is already too much. That is exactly the same problem we had while developing power train for 6 lipos. Axi 2826/12 was too much, so we ordered AXI 2826 with 13 turns called now AXI2826/F2B as Vitalis wrote. It simply takes time, effort and knowledge to develop optimal configuration. Any change will lead to worse result, if not compensated by another part.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #15 on: May 17, 2023, 02:24:46 PM »
If the internal resistance is so much in charge here, then perhaps P28B version will help a bit. Also I had the possibility to obtain P42B, but was thinking I'll be fine because of 4200mah. Actually I am, just have no extra flight time, but that's ok.

Offline Fred Underwood

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Re: Rpm drop with Molicel packs
« Reply #16 on: May 17, 2023, 04:09:33 PM »
The new Molicel is P45b vs the P42a, so lower resistance and a bit more amps.

https://www.molicel.com/inr21700-p45b/
Fred
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Re: Rpm drop with Molicel packs
« Reply #17 on: May 17, 2023, 04:11:56 PM »
P28B does show about 0.1V more at 2100mAh discharge depth at 30A. The increased voltage should reduce current. By the end of the flight it will be further left on the discharge curve which will also help.
Greg

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Re: Rpm drop with Molicel packs
« Reply #18 on: July 28, 2023, 12:31:51 PM »
Under heavy load near the end, the cell voltage will be less that what a Lipo would be. If the cutoff voltage limit is 3.2 (typical for lipos), the ESC will limit the voltage to the motor during that high liadvperiod. I experienced this also.
Simply change the min voltage limit in the ESC. Lipos are usually 3.2 cutoff. For Liion , change the min to 2.7 volts. (They havevdifferent discharge curves) If it still happens, you need a somewhat higher KV motor to avoid this.

This is assuming you are only using less than 2200 mah in the 26A battery. If more than 2200, try the 28A or 28B batteries.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #19 on: July 28, 2023, 01:07:09 PM »
I flew with cutoff set to 2.5V on my Jeti ESC. That did not help. If voltage drops below 3.2-3.3V per cell, rpm sag appears whatsoever. And that affects last 3 maneuvers on standard Maxbee setup (Axi 2826/F2B, 12x5N).

I have not try P45B, just switched back to Lipo - so far so good.


Greetings,
Vitalis P.

Online Igor Burger

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Re: Rpm drop with Molicel packs
« Reply #20 on: July 28, 2023, 01:26:32 PM »
Natural RPM drop because of low voltage and artifical RPM limitation because voltage drops under ESC voltage limit are two different things. Means it is necessary to recognize what of those two are cause of the problem. Spin has "measure/error code" menu which can be hlepfull to debug the problem.

BTW AXI2826/F2B, 6 cell lipo and 5" prop combo with my timer was developed and tuned during years to get most of those components, so clearly if you change one component and you do not compensate by other, you you will probably get worse result.

Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #21 on: July 28, 2023, 01:31:40 PM »
Natural RPM drop because of low voltage and artifical RPM limitation because voltage drops under ESC voltage limit are two different things.

That's right, Igor. And in my case the cause of the problem was the natural rpm drop, which is much bigger on Molicel cells.

Axi 2826/F2B / 12x5N combo works very well with Lipo.

Offline Dennis Toth

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Re: Rpm drop with Molicel packs
« Reply #22 on: July 28, 2023, 03:17:23 PM »
Just curious, what is the advantage to the Molicel cells? Price? Weight? Don't know what you have access to, but the HobbyStart lipo packs are very good and close in weight to the ThunderPower packs at a much better price (https://www.rcjuice.com/lipo-batteries/6s-22-2v.html).

Best,   DennisT

Online Ken Culbertson

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Re: Rpm drop with Molicel packs
« Reply #23 on: July 28, 2023, 06:17:24 PM »
Just curious, what is the advantage to the Molicel cells? Price? Weight? Don't know what you have access to, but the HobbyStart lipo packs are very good and close in weight to the ThunderPower packs at a much better price (https://www.rcjuice.com/lipo-batteries/6s-22-2v.html).

Best,   DennisT

I have two reasons I am using LiIon.  Shape and Charge.  They are round cells so you are not restricted as to shape and they hold a charge much longer than a Lipo and do not need to be put into storage mode as often.  This is a 6s 2800 I am using.  It allows for quick changes to both the vertical and horizontal CG.

Ken
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Offline Dennis Toth

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Re: Rpm drop with Molicel packs
« Reply #24 on: July 30, 2023, 07:28:16 AM »
How do they compare on weight?

Best,    DennisT

Online Ken Culbertson

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Re: Rpm drop with Molicel packs
« Reply #25 on: July 30, 2023, 10:52:24 AM »
Slightly lighter.  At least mine are.  About 45g lighter that the TP 6s 2800 and about 10g heavier that the 5s 2800.  300-310g

I might add that I have experienced the slight sag in the clover some have mentioned.  It presents itself as a toned down boost in the 2nd vertical but I still have more than enough drive to finish the maneuver.  Only happens on hot days or with little or no wind.  Having the maneuver end at the center lets you bail, if necessary, instead of going over the top which gives me enough confidence to finish even if it is sagging a bit.

Ken
« Last Edit: August 04, 2023, 02:39:19 PM by Ken Culbertson »
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Online Ken Culbertson

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Re: Rpm drop with Molicel packs
« Reply #26 on: August 04, 2023, 02:38:14 PM »
Just an FYI for those of you who are just trying Li-Ion for the 1st time.  The Low Voltage Cutoff for a Lipo is 3.0.  It is 2.5 for Li-Ion.  Most of us set it to 3.2 and 2.7 respectively.  My point is that if you try a Li-Ion with the Lipo low voltage setting you may get some disturbing surprises towards the end of the pattern.  This is most likely a *DUH* to folks using them for a while but not knowing this has driven me Bat-Sh** crazy trying to get my new ship trimmed and there may be someone else about to try a Li-Ion who doesn't know.

Ken   
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Offline Vitalis Pilkionis

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Re: Rpm drop with Molicel packs
« Reply #27 on: August 12, 2023, 09:53:13 AM »
The Low Voltage Cutoff for a Lipo is 3.0.  It is 2.5 for Li-Ion.  Most of us set it to 3.2 and 2.7 respectively.  My point is that if you try a Li-Ion with the Lipo low voltage setting you may get some disturbing surprises towards the end of the pattern. 

Setting cutoff to 2.5V will not help. As Igor mentioned, there is natural RPM drop because of low voltage of a pack, which can not be bypassed.

Online Ken Culbertson

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Re: Rpm drop with Molicel packs
« Reply #28 on: August 12, 2023, 10:45:13 AM »
Setting cutoff to 2.5V will not help. As Igor mentioned, there is natural RPM drop because of low voltage of a pack, which can not be bypassed.
That well may be but changing it to 2.7v completely eliminated my problem.  My plane is rather high drag so I am expecting a little more usage on hot calm days.  I fly normally at 5.4-5.5.   I use 2300mah out of a 2800 pack on those days, normal is around 2100.   I am flying 5:30 on the timer and get about 5 laps after the clover.  With it set to 3.2 I started noticing a drop in the V8, becoming dangerous in the clover then running out the timer at about 6.5 - 7.0 lap times.  Observers said it actually started in the H8, I just didn't feel it.  With it set to 2.7 it powers through the clover and I am still flying about 5.8 when it quits.  This is acceptable since the plane will do a respectable full pattern at 6.1

If I check the battery immediately on landing I am showing an average of 3.1 per cell.  After a few minutes it recovers to 3.4.  If I show 3.1 how could it not have hit the 3.2 "cut off" in flight?

Ken
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Offline John Rist

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Re: Rpm drop with Molicel packs
« Reply #29 on: August 12, 2023, 12:58:17 PM »


If I check the battery immediately on landing I am showing an average of 3.1 per cell.  After a few minutes it recovers to 3.4.  If I show 3.1 how could it not have hit the 3.2 "cut off" in flight?

Ken

In any digital measuring system the least significant bit (LSB) is always +- 1 bit. So it depends on the measuring device.  However under load I would expect the reading to be lower so I don't know why it doesn't "cut off".
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