klhoard

My Feedback: (10)

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You are correct, Joe. You will just get to the flat part quicker. As long as your charger is showing at least 20% remaining after landing, you're good. Just like a glass of water, you're not filling it to the brim.
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My new Revolectrix 5000's are coming down after one Advanced sequence at 29-32%, so I'm sure I have enough margin for 4.1V charging. I'll know next week when I get home and try it.
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Jetdesign

My Feedback: (8)

The other thing I'm thinking about is what people consider 'over discharging' - all the data I see on Lipo batteries says to keep the voltage above 3.3V. I am not sure where the 80% rule came from, but I have a feeling it is a 'rule of thumb' and not technically based (like the 3x outlet:inlet for cooling, lol). Is the 80% a guideline to use so voltage does not drop below 3.3v in the air? I rarely land with less than 3.7v/cell and the lowest I've seen is about 3.63v/cell. This should be no issue for lipo, but I don't know that a vertical climb on a pack that low will stay above 3.3v in the air.

I fully respect the thoughts about doing what works. I personally see an opportunity to extend the usable life of my battery packs pending some good discussion here about the possible consequences.

I suck at organization, however I do have two chargers and could see trying to keep one set to 4.1V and the other at 4.2V and designating one new flight pack to each to monitor longevity. I can also see that not lasting much more than a week or two

nonstoprc

My Feedback: (90)

Several plotting of voltages vs. other parameters can be found [link=http://www.rcuniverse.com/forum/m_9492163/anchors_10333259/mpage_3/key_/anchor/tm.htm#10333259]here[/link] . The time duration in which the pack goes from 4.2v to 4.1v is about 1.5 minutes.

Jetdesign

My Feedback: (8)

Those plots all seem to show the voltage drops to 4.1v or below as soon as the real flight begins. The airplane is armed, sits around for a few seconds, has a takeoff burst, and by the time the sequence has started the voltage is well below 4.2v and likely below 4.1v.

You can also see that those with healthier packs never dropped below ~3.8v whereas the tired packs got down to 3.4-3.6V.

And that's kind of the point of this thread - 4.2v to 4.1v is not very noticeable in the air, happens almost immediately. Tired packs also happen, and would happen less often with less 'abuse' to the pack. 4.2v/cell is a harsh environment for the chemistry inside the battery, according to the technical articles.

bjr_93tz

I think the 80% thing is a general rule of thumb based on what others have said seems to offer good pack life. Plus if one cell is getting a bit dodgey and your LVC doesn't look at individual cells then it's an extra safety margin to prevent overdischarging one particular cell.

As for a cut off voltage? I've seen plots where the cut off voltage should be much higher than 3.3V/cell (under load) because the IR of the cell was very low. The dropoff at the end of the discharge curve was very sharp and not a place I would like to be operating. "Fuel gauge" technology is where it's at and LVC should just become a backup to this. Resting voltage or voltage under load is a cheap and nasty way of guessing how much capacity has been used and "fuel gauge" functionality could be easily incorporated into any logging esc just like LVC was put into "li-po aware" esc's.

nonstoprc

My Feedback: (90)

In the two plots, the voltage touches base with 40v for about 100 seconds (from 100-200 seconds). The assumption here is that it is with the load and the pack is fully charged. I would think if the initial voltage is 4.1v/cell, then the voltage logged for the duration will be less than 40v and this can translate to a loss in RPM.

So to settle the argument, data logging and plotting are necessary :-).

I personally have experienced with flying the plane when the pack not fully charged (e.g., charged a couple of days earlier and not topped off). The motor has less kick through the flight. The amount of the lost charge is about 5-7% as shown on cellPro 10s charger.

Jetdesign

My Feedback: (8)

I will try to remember to charge one pack to 4.1 and one to 4.2 and record the data this weekend.

I might not be looking at the same plots, but most I see have flat line at about 4.2v until some throttle is used, then drop below.

Other factors would be noticed by flying a pack that has been sitting. A fresh charge gets things moving inside and lowers the IR a little, for one.

I'm not saying we should flirt with 3.3v/cell cutoff, just saying that if healthy packs are dipping to 3.7v during a flight, 3.6 would not kill the battery. But again, data speaks so I'll try to collect some this weekend at/before my contest.

DRC1

My Feedback: (10)

Interesting thread to say the least...

I'm not a chemist or a Lipo expert... Just basic knowledge of a few little things...

First, heat is the enemy... Reduce the heat in charge and discharge cycles = Extend the life of the battery.

My Hypothesis - The life of the battery is affected more by charge/discharge rates generating heat than the peak voltage a battery is charged to. Rates = mAh consumed as well as "C" rating for charge/discharge cycle...

What I haven't read in this thread is a discussion on how fast you get to 4.2V or 4.1V. My charger will allow, like most, the rate of charge. Perhaps its in the links I didn't look at...

The discharge rate is pretty much set by the flight sequence and predictable for Pattern...

Thoughts?

klhoard

My Feedback: (10)

If you get on RCG and look up posts by REVOJohn you will learn a lot about LiPos. He is the owner of a LiPo factory in Singapore and has discussed all of these topics at some point over "there".

Jetdesign

My Feedback: (8)

I have been assuming 1C charge rate. Most/all of what I have read indicates that is a safe charge rate for lithium batteries. Discharge rate is another topic. A lot of lithium battery studies are around smaller (electronics) batteries. I saw something about a 'standard discharge rate of 0.2C' which I think might be good for a charger cycle.

One interesting thing is that batteries come with a discharge rating - 20C continuous or whatever. They don't come with a voltage rating, just a nominal 3.7v/cell. This makes me think that (with a quality battery) discharging at 20C continuous vs 10C continuous has limited impact if any (just opinion here). But current discharge rate is how many electrons you can get to flow at one time through the battery. Give the electrons a larger pathway and you can flow more of them. Hence, higher discharge batteries are bigger and heavier.

If you gave the electrons more space during storage, it would just translate to decrease in voltage.

Increasing the peak charge increases the energy of the lithium ions. Lithium reacts with almost everything, so increasing the energy increases the likelihood it will react with it's surroundings, and this is why we are told to limit charge to 4.2v/cell - even at 4.2v/cell the lithium is reacting and oxidizing. Lower peak charge causes less oxidation and prolongs battery life.

Also, topping off the battery at 4.2v does create extra heat, which as you pointed out does nothing good for the health of the battery. The chargers stop charging when they notice a decrease in charge current. That extra energy has to go somewhere, so it turns to heat.

DRC1

My Feedback: (10)

Nice post Joe... Thanks...

klhoard

My Feedback: (10)

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OK, I've done a few flights using a 4.1V charge limit preset on my PL8. Batteries are Revolectrix 5S-5000's with a total of 10 cycles each on them. Airplane is an Integral with a 21 x 14E prop, Plettenberg 30-10 motor, and Castle ESC. Test flights were AMA Advanced sequence in both wind and calm weather. Charger is an FMA Power Lab 8.
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A few factual and subjective results . . . (Cue Dwight from "The Office")
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FACT - The PL8 showed 93% capacity when it terminated at 4.1V per cell. The % reading is from the default "fuel table" in the PL8. You can let the PL8 cycle the batteries and make a custom fuel table, but I haven't done that.
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FACT - The PL8 showed anywhere between 18% and 23% capacity left over after flying exactly the Advanced sequence and nothing more. Batteries were placed on the PL8 to charge immediately after landing and check remaining capacity. YMMV.
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SUBJECTIVE - Flight power levels felt exactly the same as they did with using 4.2V termination charges. The only difference was that I was now concerned about flying into the 20% reserve.
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CONCLUSION - I'm going back to 4.2V charging so I can have more flight time. My personal battery management technique minimizes the amount of time the batteries spend at 4.2V, so I'm just going to let the cycles play out and plan on buying new batts every 2-3 years.
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danamania

Interesting, how many Volts/cell does 18-23% capacity remaining refer to? Just for my info as I don't have that fuel gauge on my chargers. Thx.

Portlandflyer

I find this chart to be very accurate:

<o></o>http://www.rcgroups.com/forums/showp...57&postcount=7

<o></o>The bottom line is, 3.7 volts is the number we need to be concerned with. At 3.7 volts you have about 20% left in the pack. That number is at no load and right after landing.

<o></o>The nice thing about the ICharger is the ability to reduce the high end cutoff voltage by fractional amounts. I have been gradually bringing mine down and have it at 4.16. I am flying 5,000 Zippy Compacts and use about 3,200 to 3,600 for Masters. So far I have about 30% to 35% remaining.

Jetdesign

My Feedback: (8)

Keith, thanks for taking some data. It is good to see. What % life do you usually have left over after a flight? I charged one pack to 4.1v and one pack to 4.2v, but ended up flying IMAC that weekend and in a much longer, more power demanding sequence than I or my batteries were ready for, so I was not comfortable starting there to take data.

A few comments regarding the minimum voltage and the "20%" guideline, which ties into Keith's subjectives:

*Maintaining 20% capacity at the end of the flight is a conservative rule of thumb, there is nothing technical about it.

*3.7v is the nominal rating of a lipo battery cell. Nominal generally is a number somewhere between minimum and maximum operating limits. This implies that the minimum voltage you can safely run a lipo is below 3.7v/cell. A few of the reports I linked above claim that no damage happens to a lipo battery cell until you hit 3.3v/cell

I don't blame people for not wanting to take 'chances' on their battery packs. I don't really want to risk a good pack either. Hopefully I will get a little more settled with my living conditions and have some time and space and peace of mind to do some experiments. Definitely when my Integral is flying I can use my Yak and it's cheapo batteries to do some experiements (sounds nice, what are the odds of flying a banged up old Yak when I've got a brand new 2M pattern ship to fly?)

klhoard

My Feedback: (10)

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Scroll down to the How does temperature affect my battery FAQ. John Grzan explains where the 20% reserve comes from. Your batteries resistance increases substantially below 20%, and thus heat . . . and heat breaks down the separator in the battery. The separator is what prevents this . . .
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While you are collecting data, a worthwhile datalog file may be "%Capacity remaining vs. Battery Temperature" measured immediately after landing.
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SUBJECTIVE - I've noticed that my LiPo's are noticeable warmer when I run them down below 20%.
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Jetdesign

My Feedback: (8)

That is an awesome link! I definitely agree that heat is a killer for our batteries, but I don't see the 20% rule. I'll keep looking. I did find this though:

bem

My Feedback: (1)

Hi,
That is my experience also.

/Bo

Jetdesign

My Feedback: (8)

I think I have noticed the same too. I don't necessarily keep track of how much juice I have left in a pack, but if I fly a little longer, I think they come down wamer.

It might have something to do with the combination of remaining voltage and power draw. I bet most of the data regarding voltage minimums are not assuming 2500W bursts near the bottom end of the spectrum.

klhoard

My Feedback: (10)

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It is the chart right next to John's post titled "how does temperature affect my battery . . . I've copied a screen grab of his chart showing the "zone of temptation".
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danamania

Thanks. Interesting chart following the link, here in part...

After flying and measuring the voltage, see how your pack compares to the chart by calculating the used capacity from the mah put back into it during charging.

By cell
4.00V84%
3.96-77%
3.93-70%
3.90-63%
3.86-56%
3.83-48%
3.80-43%
3.76-35%
3.73-27%
3.70-21%
3.67-14%

Frequently I find that after flying the full Intermediate sequence, I am landing with 3.80-3.85V/cell resting, so 43-56% capacity by the chart above. This suggests (to me) a reasonble reserve for flight safety purposes, if I chose to repeat a manuver or two, or need to go around due to some guy taxiing out ahead of my landing. The last manuver or two no doubt loads the packs, so my sense is that Voltage/cell is dipping down to an in-flight minimum of 3.6-ish: If we call it 3.67V for sake of conversation, we have 14% capacity at the very bottom of the flight. The chart ends right there for some reason, perhaps not reasonable for data to be collected below that point? Anyway, given all the unexpected things that can happen in-flight that can delay a safe landing; and for the sake of having the Voltage on-board to make the model do what needs to be done amid club airfield traffic and prevailing winds, my personal preference is to have as much juice in the packs as is reasonable on take-off. But that's just me. (OTOH if one has the luxury of flying solo for a complete flight in calm conditions, perhaps one can push the packs a little longer if arrival is assured?)

My Nano-Tech 5000s are holding up fine in the middle of this 2nd season so I am unworried about their ultimate longevity. I do what I can to let them cool between charges/flights and returm them to storage voltage, usually before leaving the field. Will continue to charge them to 4.2V/cell just prior to flying, and not deeply discharge them below 3.6-3.7V in-flight if that can be avoided. (Certainly in practice, but during a contest round in stiff winds may be another story right!?) Anyway, good discussion of our gear and how we use it; no getting around burning the fuel, eh?

bem

My Feedback: (1)

Hi,

I find it very hard to get 20% capacity left in mid to high wind conditions with my 8 cells (4 cell x 2 in series) 5000 mAh (40C TopFuel) in my Sebart MythoS 125E. My setup draw 78 amp on full throttle (Hacker A60-7XS motor and Falcon electric 19x12 V2 carbon prop).
To save some time I start and turn directly downwind and then at proper height a quick Split S and on track for first maneouver. On last maneouver I skip the straight flying and almost directly turn for a quick landing. This is at practice. It is very difficult to not add a little extra throttle in the headwind. I usually do not need to go below 10% capacity left even in windy conditions so it is not that bad but on no wind I usually have 17-20 % left in battery at landing.
Probably with my setup with 8 cells it would be better with around 5400 mAh packs, for example ThunderPower G8 4S 5400 mAh 25C Pro Lite Plus -that would give the extra capacity marginal I need. The little strange is that ThunderPower in specification list their 4 cell G8 25C 5000 mAh and 5400 mAh at almost same weight - 465 gram for 5000 mAh and 468 gram for 5400 mAh (just curious how that can be possible since it is 8 % more capacity).

/Bo

Jetdesign

My Feedback: (8)

I think the thing I didn't realize is that 20% capacity happens at as high of a voltage as it does. I thought 3.7v gave plenty of life left, but I'm learning from you guys that's not the case. I agree its not a great idea to get too far below the 20%.

danamania

Another take-away from this discussion, IMO of course, is that Voltage is the one direct measure that we can make which is most relevant to understanding the condition of our packs. What I have learned from other threads is that capacity remaining expressed as a percentage is a calculated amount, not a direct measure. The chargers I have do not display this kind of fuel gauge and so I have come to rely upon Voltage measurements pre and post flight to keep tabs on battery condition and performance expectations. I have come to pay attention to the deltaV between cells as an indication of overall pack usefulness, i.e., if one cell is noticeably lower than the other 4 after rebalancing, then that may be a weak pack to keep an eye on, take out of the mix, or return if bought new. The next most useful measure after Voltage is probably current over time, expressed as mAh during recharging, which we seem to take as a proxy for current draw during a flight. Thoughts?

nonstoprc

My Feedback: (90)

My cellPro chargers can display the IR numbers, voltage per cell, and total charged back capacity per pack. Normally, I can sense a pack is degrading badly if
A) some cell has a relatively high IR number than the rest
B) some cell does not or super slowly take more charge even though its voltage is not peaked

Most of time, a bad cell will exhibit both signs. Combined with the performance of the pack in the air, one can pretty sure know the pack is dying.