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Interpreting LiPo Bounce Back Voltage

Old 05-30-2008, 11:55 PM
  #1  
Swift428
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Default Interpreting LiPo Bounce Back Voltage

OK, we all agree the following two 3S packs do not have equally matched cells
It is a cheapy 2200 22C brand that is an example of – “You get what you pay”
Packs 1 & 2 were first balanced charged to 12.45v and discharged at 4C = 8.8A

Rreadings were taken immediately after LVC and 1 hour after the LVC of 9.0v the individual cell voltages were as follows –

3S Pack 1 = 3.51v / 2.98v / 3.16v = 9.65v = immediately after LVC
3S Pack 1 = 3.54v / 3.20v / 3.58v = 10.32v = 1 hour after LVC

3S Pack 2 = 3.09v / 3.25v / 3.26v = 9.60v = immediately after LVC
3S Pack 2 = 3.21v / 3.67v / 3.68v = 10.56v = 1 hour after LVC

Which cell of Pack 1 has more mAh capacity and which cell more internal resistance?
Which cell of Pack 2 has more mAh capacity and which cell more internal resistance?

For example does cell 1 of pack 1 have the most mAh or the highest internal resistance?
How does one go about interpreting these readings?
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Old 05-31-2008, 12:49 AM
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OK, OK, OK you may be thinking - "Another idiot thinking he's saving money. When will they ever learn?"

First off these aren't my packs, and if I had listed the individual cell voltages of my ParkZone packs the cell voltages would have been within 0.02v of each other after resting one hour from LVC.

I purposely listed these real life extreme examples hoping at least SOMEONE might be able to tell me how to interpret which cells have the most mAh capacity and which cells have the most internal resistance (or least mAh capacity). Anyone can see which ones have the greatest bounce back voltage, but I'm not sure if that necessarily means the ones with the greatest bounce back voltage have less internal resistance and therefore the capacity to be discharged at a greater rate (22C vs 12C).

Does ANYONE know for sure
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Old 05-31-2008, 07:59 AM
  #3  
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Im not sure the bounce back voltage is going to tell you much thats usefull.

Whats more important is the cell voltage under load.

The cell that hold the highest voltage under load have the lowest Ir and vice versa. The cell with the lowest voltage under load have the highest Ir.

As far as capacity, you need to do a discharge to cutoff and measure the capacity. I dont think the voltages will give you any meaningfull guide to capacity.

By the way - I agree - thats a pretty crummy pack
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Old 05-31-2008, 09:48 PM
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The cell that hold the highest voltage under load have the lowest Ir and vice versa. The cell with the lowest voltage under load have the highest Ir.
How do you apply that to 3.51v if its ir is actually higher -- preventing the cell from being depleted as fast as the other two cells? Another reason for getting a battery monitor to monitor each cell's voltages under load, but wouldn't the load voltage be nearly the same just before LVC as the no-load voltage just after LVC? Do you actually need an Astro Whattmeter connected in line with each cell in the pack? Doesn't the amount of bounce back voltage give us an indication of both the ir and mAh capacity rating (12C vs 22C)?

I'm thinking a battery monitor should be a good enough indicator, but maybe not?

3S Pack 1 = 3.51v / 2.98v / 3.16v = 9.65v = immediately after LVC
3S Pack 1 = 3.54v / 3.20v / 3.58v = 10.32v = 1 hour after LVC

3S Pack 2 = 3.09v / 3.25v / 3.26v = 9.60v = immediately after LVC
3S Pack 2 = 3.21v / 3.67v / 3.68v = 10.56v = 1 hour after LVC

Three cells of these new (but dysfunctional) 2200mAh 22C packs have the same amount of bounce back - 0.42v - Therefore, my garage logic suggests to me that they have less internal resistance(ir) and therefore may be the only 3 of 6 cells that deserve a 22C rating.

On the other hand my garage logic suggests to me that the higher red cell voltage of 3.51v with only a bounce back of 0.03v to 3.54v has more ir.

Does any one agree with this premise ?
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Old 06-01-2008, 01:09 AM
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Your making some assumptions with out having the data to back it up. You dont really know what the voltage of the cells was when the esc went into LVC. So, you dont really have any idea what the actual bounce back amount was for each cell. If its something your interested in you do need to measure it.

Also, since your pack is wired in series - each individual cell delivered the exact same mahr. Doesnt matter what the Ir is because they are connected in series. The differences in Ir would effect the voltage under load, but not the delivered capacity.

One other factor that is probably skewing your results and I should have mentioned this earlier, you actually over discharged that pack to some degree.

You should never allow the pack to have a resting voltage below 3.7 volts per cell. Its hard on the cells and 3.7 is the practical minimum voltage you will see at zero usefull remaining capacity.

In my experience, when cells are discharged to low levels the voltage alone is a very poor indicator of remaining capacity.

Packs that are in good balance at 4.2 volts per cell often show large imbalances when deeply discharged. They will return to being in good balance once the voltage goes up during the recharge - even if no balancer is used.

Once again - the only good way to test for Ir is to measure under load voltages. You can do that while charging as easily as discharging. Cells with higher Ir will show higher voltages while charging and lower voltages while dis-charging. That assumes a common voltage to begin with.

Resting voltages cant tell you anything about Ir. They are only good for use as a rough capacity guage and an indicator of imbalance.
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Old 06-01-2008, 02:58 AM
  #6  
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Originally Posted by Larry3215 View Post
Your making some assumptions with out having the data to back it up. You dont really know what the voltage of the cells was when the esc went into LVC. So, you dont really have any idea what the actual bounce back amount was for each cell. If its something your interested in you do need to measure it.
I had an Astro Whattmeter in line which verified that the low voltage cut-off of the ESC ocurred at 9.0v on one pack and 9.6v on the other pack. So, I know what the load voltage was just before cut-off at 9v and 9.6v on these two packs.

Also, since your pack is wired in series - each individual cell delivered the exact same mahr. Doesnt matter what the Ir is because they are connected in series. The differences in Ir would effect the voltage under load, but not the delivered capacity.
You are assuming that in a 3S 2200 pack each cell's capacity is 733.33mAh. I would agree if the pack's three cells are equally matched, but not all LiPos are of an Enerland, Kokam or even ParkZone quality.

One other factor that is probably skewing your results and I should have mentioned this earlier, you actually over discharged that pack to some degree.

You should never allow the pack to have a resting voltage below 3.7 volts per cell. Its hard on the cells and 3.7 is the practical minimum voltage you will see at zero usefull remaining capacity.
Even with a LVC of 9.6v a LiPo won't always rebound to 3.7v per cell = 11.1v even after resting for 1 hour. The amount of rebound depends on the discharge rate. So are you saying that a LiPo should be discharged at a rate and LVC that will allow it to rebound to at least 11.1v after resting 1 hour?

Packs that are in good balance at 4.2 volts per cell often show large imbalances when deeply discharged. They will return to being in good balance once the voltage goes up during the recharge - even if no balancer is used.
I thought LiPos are only to be charged to 4.15v per cell = 12.45v for a 3S

Once again - the only good way to test for Ir is to measure under load voltages. You can do that while charging as easily as discharging. Cells with higher Ir will show higher voltages while charging and lower voltages while dis-charging. That assumes a common voltage to begin with.
Yep, that is why I'm getting that battery voltage monitor to keep an eye on each cell's voltage.

Resting voltages cant tell you anything about Ir. They are only good for use as a rough capacity guage and an indicator of imbalance.
I thought perhaps there was some ir clue as it related to the bounce back examples(red & green) shown above. I know temperature is one indicator of ir and thought the higher cell voltage of 3.51v would be of some ir indication.
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Old 06-01-2008, 05:19 AM
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Lets see if I can get all the "quotes" in the right places

Originally Posted by Swift428 View Post
I had an Astro Whattmeter in line which verified that the low voltage cut-off of the ESC ocurred at 9.0v on one pack and 9.6v on the other pack. So, I know what the load voltage was just before cut-off at 9v and 9.6v on these two packs.
That still doesnt tell you what each individual cell was doing. The resting voltages varied by as much as .53 volts. The UNDERLOAD voltages may have varried even more right before cut off. Especially yhat one cell that had a resting voltage under 3. It may have been waaaaaaaay lower under load.

As I said - once you get on the down side of the discharge curve the voltages can drop dramatically.

You are assuming that in a 3S 2200 pack each cell's capacity is 733.33mAh. I would agree if the pack's three cells are equally matched, but not all LiPos are of an Enerland, Kokam or even ParkZone quality.
Actually, no. You have confused the way series and paralell mahr adds up.

In a 3S 2200 pack each cell has an individual capacity of 2200 mahr - NOT 733.33. You dont devide the total by 3 in a series wired pack. You would only do that if you had cells in paralell.

In a 1S3P 2200 pack you would devide 2200/3 = 733.33. However, in a 3S1P 2200 pack all cells are 2200.

Aside from that -what I said was that the delivered capacity would be the same.

In other words, in a series wired pack, each cell will always be discharged (or charged) by the exact same amount.

It cant be any other way. Thats because, in a series circuit, you always have the same exact current flowing in all parts of the circuit. Capacity is current times time, so if the current is the same everywhere, then the capacity in or out is also the same.

If you have a 3S pack and discharge 1000 mahr out of it, then each cell will have been discharged by exactly 1000 mahr. If you charge that pack and put back in 900 mahr, then each individual cell will have taken in exactly 900 mahr.
It doesnt matter how closely the cells are matched or not, it always the same.

Paralell wired packs are a different animal completely and its a lot more complex figuring relative discharge amounts because they will NOT be the same. In that case the individual Ir does play a big roll.

Even with a LVC of 9.6v a LiPo won't always rebound to 3.7v per cell = 11.1v even after resting for 1 hour. The amount of rebound depends on the discharge rate. So are you saying that a LiPo should be discharged at a rate and LVC that will allow it to rebound to at least 11.1v after resting 1 hour?
Exactly. Thats why I never count on LVC settings to protect my packs and never fly down to LVC. If your just lazing along up there its very easy to over discharge a pack and do serious damage to it.

I thought LiPos are only to be charged to 4.15v per cell = 12.45v for a 3S
The nominal fully charged voltage is generally considered to be 4.2 volts. Most chargers take the cells up to 4.2 volts durring the cc stage of the charge and then hold it there durring the cv stage of the charge untill the charge rate drops to c/10. A few minutes after the charge stops, the voltage will drop to the 4.15 volt range - plus or minus - depending on the charge rate and cell condition etc.

Yep, that is why I'm getting that battery voltage monitor to keep an eye on each cell's voltage.
I got a cheep one of those a year or so ago. $10.00 IIRC. It cycled thru the individual cells on the display every second or so. The readings were way off - more than .5 volts - and the plug didnt fit hyperion or TP balance plugs. I still have it somewhere I think.....

I thought perhaps there was some ir clue as it related to the bounce back examples(red & green) shown above. I know temperature is one indicator of ir and thought the higher cell voltage of 3.51v would be of some ir indication.
Its possible, but I dont really think so. In this particular case at least, Im quite sure that your seeing those large variations - at least partly - because of the deep discharge.

If you had the voltages under load for each cell to compare, that would definitely tell you something about the Ir of the cells.

Keep in mind that the Ir of the cells varies depending on cell temperature AND charge state. The Ir of the cells goes UP at lower charge states and again when they are fully charged. The Ir is lowest somewhere around 1/2 charge state.

Thats probably nother reason your seeing large voltage differences because the Ir increases quite abit when fully discharged, so if one cell is closer to being empty than another its Ir will climb faster at the very end.

However, the only way to know is to measure the under load voltages and compare them.
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Old 06-01-2008, 07:41 PM
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lowest voltage = lowest mah.

since the cells are coupled in series, its unavoidable that each cell has has to discharge exactly equal amounts of ampere. (kirchhoffs laws).
so if one cell has a lower energy storage capacity (mah) than another, it cant compensate by delivering less apms. so the only way to compensate is to drop voltages, and thats exactly what the battery does as it becomes depleted of stored energy.

so for that part, its pretty easy. lower voltage avter a run is lower mah.
as for internal resistance, thats a bit more tricky. im afraid you cant tell the individual cells internal resistance just by looking at the voltages alone. (at least i dont se a way of telling). however, you can test this cell-for-cell. this requires that your pack has a balancing plug to it, and you can connect to each individual cell through this plug.

now remember, you cant just ohm them with your multimeter. ohming with multimeter is onyl correct when the circuit is at 0 voltage level. a battery is never, so that isnt giving you a correct reading. the best way in theory is to shortcircuit your cell and measure the amps going through. that lets you calculate ohms from the amp draw. however, of course DONT EVER do that with LiPo. Or any other battery for that matter.
What you can do, on the other hand, is to complete teh circuit with a resistance, then calucalte the resistance, and when you then got Rtot - Rresistor = Rbattery.
For an accurate reading, you need to use a relatively low resisitivity resistor, and wiring & measurement equipment with next to 0 ohm.
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Old 06-01-2008, 09:17 PM
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Originally Posted by Larry3215 View Post
That still doesnt tell you what each individual cell was doing.
I had two digital voltmeters – one reading cell one and the other reading cell two and the total voltage from Astro Whattmeter so I could get a readout of the load voltage disparity during discharge. A battery monitor will be better than trying to figure out cell two and three on the fly when your voltages are compounded (e.g. 3.70v / 7.40v / 11.1v) and constantly changing during discharge. The two LiPos were both balance charged at 0.8C with the cells being within 0.02v of each other with resting voltage at 12.44v & 12.45v

As I said - once you get on the down side of the discharge curve the voltages can drop dramatically.
No Kidding! It’s like 2-5 seconds from 9.9v to 9.0v depending on the discharge rate.


“So are you saying that a LiPo should be discharged at a rate and LVC that will allow it to rebound to at least 11.1v after resting 1 hour?”
Exactly. Thats why I never count on LVC settings to protect my packs and never fly down to LVC. If your just lazing along up there its very easy to over discharge a pack and do serious damage to it.
So are you saying that I’ll get more performance cycles from my LiPos, if theoretically I were to continuously discharge a *3S pack at 80% of its maximum continuous C discharge rating to a LVC of 3.3v per cell = 9.9v than discharging a 3S pack at 70% of its max cont C discharge rating to 3.0v per cell = 9.0v if the 9.0v LVC only results in a bounce back after 1 hour to 10.62v ??? Or is it a knit-picking tradeoff in which I may actually get more performance cycles at -- 70% C / 9.0v LVC / BB 10.62v -- than -- 80% C / 9.9v LVC / BB 11.1v. Is a BB after 1 hour to 10.7v OK or does it have to be at least 11.0v ? Can you give me any quess whatever as to the difference in performance cycles with these *two examples assuming this was the only, ONLY, only variable?


Here are three of my own 3S Lipos 1 hour after LVC to 9.0v per cell.
(listed according to age/use with the Electrifly having the most age/use)
10.78v = 3.59v / 3.61v / 3.58v = ParkZone 2200mAh 12C
10.62v = 3.53v / 3.57v / 3.52v = CommonSense 2000mAh 8C
10.30v = 3.42v / 3.49v / 3.39v = Electrifly 910mAh 15C – (60% -- 6 min. instead of 10 min.)


I got a cheep one of those a year or so ago. $10.00 IIRC. It cycled thru the individual cells on the display every second or so. The readings were way off - more than .5 volts - and the plug didnt fit hyperion or TP balance plugs.
I’ve never seen so many positive comments on a UnitedHobbies item as their battery monitor feedback. One person did mention the voltages were off on one, but he ordered three as they are only $4.95. The one I ordered from BP Hobbies comes with an adapter so it can be used with either TP or PQ pin spacing.


Thanks for your feedback. You have been very helpful.
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Old 06-01-2008, 09:50 PM
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Originally Posted by Moxus View Post
lowest voltage = lowest mah.

so if one cell has a lower energy storage capacity (mah) than another, it cant compensate by delivering less apms. so the only way to compensate is to drop voltages, and thats exactly what the battery does as it becomes depleted of stored energy.
Yes, I thought there had to be some correlation as VERY FEW packs have equally matched cells throughout their charge/discharge lifetime. They may be equally matched when new, but gradually become unmatched with more & more charge/discharge cycles.

so for that part, its pretty easy. lower voltage avter a run is lower mah.
as for internal resistance, thats a bit more tricky. im afraid you cant tell the individual cells internal resistance just by looking at the voltages alone. (at least i dont se a way of telling). however, you can test this cell-for-cell. this requires that your pack has a balancing plug to it, and you can connect to each individual cell through this plug.
Having a balancing plug is aleady a given! How would you interpret the three green cells with the most BB voltage of 0.42v as opposed to the high voltage red cell (3.51v-3.54v). What is the mAh implication with respect to the greater amount of BB voltage, and can we conclude that the 3.51v cell has the most mAh capacity and its excess mAs are being siphoned off by the other two cells during BB explaining why there is only a 0.03v BB?

What you can do, on the other hand, is to complete teh circuit with a resistance, then calucalte the resistance, and when you then got Rtot - Rresistor = Rbattery.
For an accurate reading, you need to use a relatively low resisitivity resistor, and wiring & measurement equipment with next to 0 ohm.
Is this even necessary as long as a LiPo doesn't get too hot (over 140F)? It seems like the handy little battery monitor is plenty enuf to check the balance/imbalance condition of your LiPos at home and in the field. Afterall, a pilot has some idea of the condition of his LiPos by any noticeable changes in flying time assuming that certain LiPos are used in certain planes with certain flying patterns.
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Old 06-01-2008, 10:50 PM
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Having a balancing plug is aleady a given! How would you interpret the three green cells with the most BB voltage of 0.42v as opposed to the high voltage red cell (3.51v-3.54v). What is the mAh implication with respect to the greater amount of BB voltage, and can we conclude that the 3.51v cell has the most mAh capacity and its excess mAs are being siphoned off by the other two cells during BB explaining why there is only a 0.03v BB?
i wouldnt look too much on the bounceback voltage. not beeing a chemist, im an electronican, so how to pretict bounceback voltages or reading them isnt my domain at all.
however, i do understand the electrotechnical aspects involved here, and what i can tell for sure is that the cell lowest on energy (mah), has to drop voltage in order to deliver less watts than the other cells, since the batteries is connected in series and hence all batteries delivers the same current.
in other words, the voltage of interest (at least from my point of view) here, is the voltage of the cell immediately after low voltage cutoff.

Is this even necessary as long as a LiPo doesn't get too hot (over 140F)? It seems like the handy little battery monitor is plenty enuf to check the balance/imbalance condition of your LiPos at home and in the field. Afterall, a pilot has some idea of the condition of his LiPos by any noticeable changes in flying time assuming that certain LiPos are used in certain planes with certain flying patterns.
as for checking wich cell is in best/worst condition, yes you can tell alot with a battery monitor. actually, you dont even need a batteymonitor either.
but that was only half the question. the other half was internal resistance, and for that part you DO need the test i listed above.
internal resistance is absolutely neccessary to know in order to make sure your batteries is up for the job. for example, on my 18.5V, 62A hyperion LVX lipo pack, my speedregulator didnt even get hot.
on my 19.8v, 60A LiFePO4, i just fried my 80A speedregulator today, and the only difference is the battery pack.
how could that be? on paper, the packs are nearly identical?
its all about internal resistance. im sure the hyperion delivers 62A as promised, but the voltage drop is so significant that the total watts is still way lower than for the LFP (LiFePO4) cells. the LFP is famed for having very low internal resistance and purely hellish discharge characteristics, and as mentioned, these characteristics was just felt by my ESC.
The difference is that the LFP delivers the same 60A juice at a much higher voltage, due to lower internal resistance. and voila, even though both packs has same rated voltage and current, the LFP outperforms the lipo by lightyears.
this doesnt matter for lazy outrunners in a little funtana, that barely puts any strain on the battery at all, and hence the "ordinary" wattflyer doesnt even need to bother.
but when running the batteries to the absolute limit, then its of huge importance.
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Old 06-02-2008, 12:50 AM
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Originally Posted by Moxus View Post
what i can tell for sure is that the cell lowest on energy (mah), has to drop voltage in order to deliver less watts than the other cells, since the batteries is connected in series and hence all batteries delivers the same current. in other words, the voltage of interest (at least from my point of view) here, is the voltage of the cell immediately after low voltage cutoff.
Help me try to correlate what you just said with the 3.51v cell at LVC. If I'm hearing you correcty it would seem that this cell and the cells with the highest voltage immediately after LVC have more inherent mAh capacity. And because the 3.51v cell has noticeably more capacity than the others its BB (only 0.03v) won't be as noticeable because it's excess mAh is being siphoned off by the other cells having the less capacity -- which is being exacerbated by red cell 3.51v. The only problem with that theory is that for it to really make sense than cell one of pack 1 & 2 need to exchange places with each other. Do you see what I mean?

Is there any logical assumption/clue that you or anyone cares to venture a 'lithium chemically plausible' guess as to a cause & effect mAh & voltage relationship with respect to the amount of BB voltage with the two packs voltages as listed in the first post?

as for checking wich cell is in best/worst condition, yes you can tell alot with a battery monitor. actually, you dont even need a batteymonitor either.
Yes and No ? -- No and Yes ?

I would hope it would at least make be a better caretaker/giver of my electric arsenal.

As for your ir/impedance explanation my internal capacity has reached passed the knee and is nearing its LVC. I'll have to recharge my battery and cycle through it again tomorrow.

Thanks again for your taking time to guide me through the woods so I don't get lost.
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Old 06-02-2008, 01:05 AM
  #13  
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Why the BB voltage numbers don't add upp perfectly can possible be explained as simple as individual differences between the cells chemically. Again, im on thin ice when it comes to the chemical aspect of batteries, but i would like to think this is whats going on.
It's the only way i can think of that makes the electrical aspect add up.
The BB voltage of a battery can with some goodwill be compared to what happends with a battery when its charged at too low ampere (especially true with old nicads and also lead acid batteries), is that the voltage rises, but the chemical processes in the battery don't happend as its supposed to, and the voltage rises unproportionally with electrical charge. Thats why I'm leaning towards the belief that the bounceback voltage might be causing some confusion with numbers that dont add up.
I don't think that the cells is in some way sharing voltage or charge with each other.
Because the cells are connected in series, the current flow has to be equal in all cells at all time, and for there to be any current flow at all, there has to be a completed circuit. Wich there isn't when the cell is dosconected.
This can be demonstrated in practice by deep-discharging individual LiPo cells, and monitor their BB voltages cell by cell, not being connected within a battery package.
I'm quite sure that you will observe the same bounceback phenomena with individual cells as you do with cells within a pack. So in other words, there isnt any currents from cell to cell, wich lets the higher voltage cells "share its charge" with the lower voltage cells. This would happend in a parallell connectd battery pack, but not in a series.

And sorry for me being inaccurate with the battery monitor issue. What I meant is that "Yes, you can get a reading of a cells condition with a battery monitor, but you can also do it without. So it isn't strictly neccessary"
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Old 06-03-2008, 03:12 AM
  #14  
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Originally Posted by Moxus View Post
"Yes, you can get a reading of a cells condition with a battery monitor, but you can also do it without. So it isn't strictly neccessary"
OK, if I were just getting into electric flying now instead of March of 2007, I would be buying an iMax B5 charger/balancer (see attachment) with CC-CV instead of going the Dynamite Vision Peak Ultra-Astro Blinky route which doesn't offer provide CC-CV charging/balancing. However, if CC-CV only offers a 10-15% increase in performance cycles (e.g. 200 vs 230) I'm not going to change horses at this point.

As long as I use my Vision Peak Ultra charging to 4.14v-4.15v per cell (with the help of a Blinky balancer) so that my 3S LiPo cells are within 0.01v of each other at 12.43v-12.45v, set my LVC so I get a BB to 11v, and not discharge my LiPos above 140F I'm thinking I should get almost as many performance cycles as someone using an iMax B5 with CC-CV.

Thanks to you and others I now G-E-T the importance of matching each cell's ir (impedance) within a pack. I learned today that each of the hand picked cells of top-tier expensive packs have the same ir; which isn't true with less expensive packs. So thanks again for pointing out to me the importance of *ir as the primary factor in *equally matching LiPo cells within a pack.

I received my new BP Hobbies battery monitor today and it is calibrated exactly the same as my digital multimeter when checking individual cell voltages--so I feel it's a good investment for the money.

Unless someone you can explain another cause&effect reason, my logic tells me that the cells with the greatest BB (0.42v) have less internal resistance which is a good thing. In other words even though the cells are connected in series it appears that the BB voltage behavior gives some clue as to each cells ir. Therefore, my logic is telling me that the reason for the higher voltage of cell 3.51v compared to the packs other two is because the 3.51v cell's ir is greater. This would also explain why its BB voltage is only 0.03v from 3.51v to 3.54v because there is more internal cell resistance.

Why is this of any importance? Well, for one thing it's critical information when making up at least one good 3S 2200mAh 22C pack from two not-so-good packs.
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Old 06-03-2008, 07:56 AM
  #15  
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Well, I still think your chasing a wild goose with the bounce back voltage giving you any useful info - especially when your operating below 3.7 volts.

Your in no mans land down there

Your idea of bounce back is really not all that different from the more accepted way of just measuring the voltage drop. The difference is that your looking at the after effect when the voltage drop is the direct effect.

The voltage drop is the thing to check as it directly relates to the Ir of the cell - ohms law - and its not subject to time related variables - like how long to wait. The voltage drop is instant and gives the Ir directly via ohms law.

To do the tests properly, check the voltage drops under load at a couple of different amp rates and average them. You will find the readings can vary a lot.

Just be sure the cells have a reasonable amount of charge on them - well above 3.7 volts. That will give you a much more usefull answer about each cells Ir.


On second thought - I think your bounce back logic has some merit but - you have it backwards

The cells with the LOWEST bounce back should have the lowest Ir.

Think about how Ohms law applys here. A cell with a higher Ir will have the higher voltage drop. It will therefore also have the greatest bounce back when the load is removed.

Still, your looking at voltage ranges that are well outside the normal opperating range and deep into the down side of the curve. Re do your checks at a more normal opperating voltage and I think your results will be more usefull. Better yet - check the voltage drops instead
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Old 06-03-2008, 08:03 AM
  #16  
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By the way - were you able to measure each individual cells actual voltage when the pack hit LVC?

As I said - you cant assume they were all at the same voltage when LVC occured. Im quite certain they would NOT be the same even on a well matched pack.

If you havent measured each cells voltage under load at LVC, then you still dont know the actual bounce back amount.
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Old 06-03-2008, 08:10 AM
  #17  
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One other point - the Vission Peak charger does charge as CC-CV when its in Lipo mode.

The only major name lipo charger I know of that doesnt opperate on a CC-CV basis is the Astro 109 - but its close enough to CC-CV if you dont look tooooo close
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Old 06-03-2008, 11:59 AM
  #18  
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One problem:

Your logic holds water for all but 1 point:
If you was to explain voltage bounceback with ohms law, the bounceback had to be instantly after you remove the load. And thats pretty much how i described how he could tell a cells capacity (mah) above here.
But for internal resistance, i can't se any electrotechnical way of interprenting bounceback voltages for readings of IR.
So if there is any way at all, it has to be chemical. So forget ohms law for the bounceback voltages, and i go with larry at the accepted methods.
Measure voltage drop and current draw under load.
Its really quite easy, so I don't see any reason for trying another method than that, as long as we can't be sure of exactly what is happening.
The bounceback voltages was measured 1 hour after the load was removed. That has nothing to do with ohms law anymore. If ohms law could explain it electrotechnically, it would be instant. As the flick of a switch. There is no time variable in ohms law.
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Old 06-04-2008, 01:00 AM
  #19  
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I understand your explanations and reasoning for measuring 3S differences between cell voltages while under load; however, the intent of this thread was to see if it’s possible to determine which cells of a crappy LiPo are superior and which are inferior to their mate(s) based on the BB voltages after LVC.

I have noticed that LiPos with ‘equally matched’ cells have almost identical BB after LVC. For example after resting 10-12 minutes the cell voltages might be 3.69v / 3.72v / 3.70v and after 1 hour with some fallback practically no difference between the three cells (e.g. 3.60v / 3.61v / 3.60v). This explains why top quality packs that are discharged with proper use and properly charged only need to be balance charged every 5-10 cycles. But how is one to know how in-balance or out-of-balance a run-of-the-mill LiPo is unless you can test each cell.

An accurate $12 battery monitor with individual cell readouts is a good investment.
  • You are suggesting the 3.51v-3.54v cell (“kirchhoffs laws”) definitely has a higher energy storage capacity(mAh) and the 2.98v cell has the least energy storage capacity(mAh). Can we agree this is a correct interpretation? Can we go a step further and say that the cells having the most similarity in bounce back voltage (3.16v-3.58v, 3.25v-3.67v, & 3.26v-3.68v) possibly have the most similarity in both storage capacity(mAh) and internal resistance.
  • Major differences in LVC bounce back voltages after 10 minutes should provide a clue as to which cell(s) are superior and which cell(s) are inferior to the vendors rated usage, even if we aren’t sure as to the exact—internal resistance to energy storage capacity(mAh) relationship.
  • The most noticeable cell variance (3.51v-3.54v) having both the highest voltage and lowest bounce back would suggest that it’s a superior cell to its two mates. Could we go so far as to opinionate that it may be the only cell that deserves to be in a 3S 2200mAh 22C pack.
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Old 06-04-2008, 08:08 PM
  #20  
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Interpreting Bounce Back Voltage Correctly

I have noticed that LiPos with ‘equally matched’ cells have almost identical BB after LVC. For example after resting 10-12 minutes the cell voltages might be 3.69v / 3.72v / 3.70v *and after 1 hour with some fallback practically no difference between the three cells (e.g. 3.60v / 3.61v / 3.60v). This explains why top quality packs that are discharged with proper use and properly charged only need to be balance charged every 5-10 cycles. But how is one to know how in-balance or out-of-balance a run-of-the-mill LiPo is unless you can test each cell.

*A good LiPo's BB voltage will continue to climb for several hours. For example if the 3S LVC is 9.6v it's bounce back will continue to climb beyond 11.0v and may reach 11.8v after several hours, even with a moderate discharge rate. The amount of BB depends on the rate of discharge and condition of your LiPos.
-
-
-
  • You are suggesting the 3.51v-3.54v cell (“kirchhoffs laws”) definitely has a higher energy storage capacity(mAh) and the 2.98v cell has the least energy storage capacity(mAh). **Can we agree this is a correct interpretation? Can we go a step further and say that the cells having the most similarity in bounce back voltage (3.16v-3.58v, 3.25v-3.67v, & 3.26v-3.68v) possibly have the most similarity in both storage capacity(mAh) and internal resistance.
**No it's not a correct interpretation! A BB of only 0.03v (3.51v-3.54v) in one hour is an indication this cell has a serious problem.
-
-
-
  • Major differences in LVC bounce back voltages after 10 minutes should provide a clue as to which cell(s) are superior and which cell(s) are inferior to the vendors rated usage, even if we aren’t sure as to the exact—internal resistance to energy storage capacity(mAh) relationship.
YES!
-
-

-
  • ***The most noticeable cell variance (3.51v-3.54v) having both the highest voltage and lowest bounce back would suggest that it’s a superior cell to its two mates. Could we go so far as to opinionate that it may be the only cell that deserves to be in a 3S 2200mAh 22C pack.
***DEFINITELY NOT! -JUST THE OPPOSITE
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Old 06-04-2008, 10:51 PM
  #21  
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I do see that you have some valuable observations here, but unless someone can explain them in a more scientific way, I think its more guesswork than interpreting them.

I did actually write a more detailed answer for you going into details with how this hangs together with Kirchhoff's law, but I'm not sure it was the answer you looked for, and since it got so goddamn long, I had second thoughts about posting it. If it has any interest anyway, give me a hint and i post it. Got it stored.
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Old 06-05-2008, 01:15 AM
  #22  
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Originally Posted by Moxus View Post
I do see that you have some valuable observations here, but unless someone can explain them in a more scientific way, I think its more guesswork than interpreting them.

I did actually write a more detailed answer for you going into details with how this hangs together with Kirchhoff's law, but I'm not sure it was the answer you looked for, and since it got so goddamn long, I had second thoughts about posting it. If it has any interest anyway, give me a hint and i post it. Got it stored.

I tend to agree.

I think if you did more testing you will find more variables than you like.

You will find larger variances at lower static voltages - as I said - anything below 3.7/cell things get more and more dicey. At higher charge states the variations will be much less appearant.

You will see more variations depending on the discharge RATE.

You will see differences dependijng on cell capacity and "C" rating as well as age and temperature.

All of these will be meaningless unless you know the voltage of each individual cell under load before the bounce back. If you dont know that then any references will have no basis for comparison.

Of course, if you know the voltage under load of each cell, you already have all the information you need to determine relative cell quality

Larry
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Old 06-05-2008, 06:14 PM
  #23  
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Originally Posted by Larry3215 View Post
All of these will be meaningless unless you know the voltage of each individual cell under load before the bounce back. If you dont know that then any references will have no basis for comparison.

Of course, if you know the voltage under load of each cell, you already have all the information you need to determine relative cell quality

Larry
Remember that the examples I listed were from two crappy 3S packs that aren't worth a @*#z. Both packs are not even suitable for field use. I purposely choose them as opposed to an Enerland or Kokam pack that may only need to be balanced every 5-10 cycles. If all LiPos were as good as Enerlands there wouldn't be any need for this thread.

The two crappy packs got us too far afield than necessary. So it is not necessary that an electric pilot need "know the voltage of each individual cell under load before the bounce back." Puffing and poor performance are evidence of crappy LiPo packs that need to be discarded or returned for exchange/refund.

The need to know the load voltage or attempting to balance the cells during the discharge load isn’t necessary. If it were then some pilots would be using the little .05oz Ultra Balancer plugged into the LiPo balancing connector during discharge while flying their plane. I doubt if its ever been done or ever will be done. But if you think it's important you may want to get an Ultra Balancer to use while flying your plane.

Ultra Balancer LEDs --
Red = 4.15v - 3.7v
Green = 3.7v - 3.2v
Yellow = 3.2v and below
Ultra Balancer instructions: http://www.commonsenserc.com/product_instructions/Ultra-balancer_instructions_final.pdf

You don't want to have any imbalance on your pack at all. Imbalance is defined as a difference of 50 millivolts between cells, or 0.05v. Anything greater than that needs to be balanced right away.

So, all you really need to know is how much imbalance you have between cells during bounce back before charging again. That alone will tell you all you really need to know. Using the Ultra Balancer during bounce back may even eliminate the need to balance during charging. The new Pro-1 from CommonSenseRC is both a cell voltage tester and cell balancer. The Cell Spy is just a cell voltage tester.
http://www.commonsenserc.com/product_info.php?cPath=45&products_id=549
That’s what I’ll be using with my Vision Peak Ultra charger and what I’d recommend to anyone charging via the discharge connector with a stand alone LiPo charger.
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Old 06-05-2008, 06:57 PM
  #24  
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Originally Posted by Swift428 View Post
...snip...

The two crappy packs got us too far afield than necessary. So it is not necessary that an electric pilot need "know the voltage of each individual cell under load before the bounce back."
..snip...
You don't want to have any imbalance on your pack at all. Imbalance is defined as a difference of 50 millivolts between cells, or 0.05v. Anything greater than that needs to be balanced right away.
..snip...
So, all you really need to know is how much imbalance you have between cells during bounce back before charging again. That alone will tell you all you really need to know. Using the Ultra Balancer during bounce back may even eliminate the need to balance during charging.

....snip...
.
I dont mean to be constantly disagreable - I hope Im not coming across that way.

However, I think you have made some incorrect assumptions and you've misunderstood the reason I think you need to know the individual cells cut off voltage.

Ive highlighted the statement above that are the key points.

Im at work at the moment and dont have time to go into details but I will get back to you later.
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Old 06-05-2008, 07:41 PM
  #25  
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Originally Posted by Larry3215 View Post
I think you have made some incorrect assumptions and you've misunderstood the reason I think you need to know the individual cells cut off voltage.
It's more important to know what LVC to use--depending on your flying pattern and battery condition (e.g 9.6) than knowing the individual cells voltage under load just before LVC.

I do at least 1 bench static condition charge/discharge cycle of a new LiPo. The bounce back voltage after 2 minutes, 15 minutes, 30 minutes and 1 hour is more than sufficient evidence as to whether:

1 - The LiPo is crappy having major BB cell voltage differences

2 - If the cells are within 0.05v of each other during BB

3 - Whether or not to use an Ultra Balancer during BB.

4 - If the cells are greater than 0.05v of each other during BB

5 - The amount of BB voltage after 2 min., 15 min., 30 min., & 1 hr.

6 - If the LVC was high enough to allow a BB to 11v after 1 hour.

____________ .

We don't need to make this more difficult than is necessary
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