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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 03-15-2017, 08:09 PM   #26
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Originally Posted by dereckbc View Post
Well thanks for sharing, it does shine some light on the subject.

The website calculator does not reveal the formulas anywhere that I can find. Now from your spreadsheet it appears you do have the correct formula in Cell D8 for Maximum Continuous Current. However I do not see the relationship of power in the equation. Perhaps I am missing something.

SQRT (mah/Ri *6)
It's ages ago that I worked out the formula they use, and my brain is a bit foggy, but how it works is:

It's basically based on heat input ( P=I^2.r ) and factors that against how much energy it takes to heat the battery up.
The mAh is in there because it tells us how much mass of battery we have to heat up and also roughly how much surface area of battery there is to dissipate heat (it's not an exact science!).. The 'fudge factor' of 6 takes into account multiple variables and conversion factors and is largely empirically derived.

While it's not directly based on your voltage drop method it should produce similar results because it's resistance that creates that voltage drop and the formula takes that resistance into account in it's heat input calculation.
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Old 03-16-2017, 12:16 AM   #27
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After starring at the formula a while I figured it out, or at least I think I did. Appears to be based on 6 watts per Watt Hour of battery. As you indicate is a heat based on mass.

Again a thermal limit is what I am trying to get rid of make it strictly a performance spec. The formula only works to around 19 to 20C before you go over .3 volts per cell

If you take your spreadsheet and add the formula I added you can see the voltage go up with Ri. Once you go over 6 millohms voltage drop goes above .3 volts.

Anyway it was a good learning exercise and I know at .3 volts per cell maximum drop is withing thermal limits of any cell.
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Old 03-16-2017, 09:06 PM   #28
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Perhaps I can clarify this here as I was the originator of the formula in conjunction with Mark Forsyth and John Julian.

Some years back I was involved in lipo testing and carrying out full discharges at constant current in order to compare performances of different packs and to determine the real maximum C rating of each pack.
I always took IR readings before each test run and carried out further discharge runs at increasing C ratings until the final temperature indicated that the pack was operating at its maximum capability.
It became obvious that there was a close correlation between the IR taken at 25*C and the maximum C rating that the lipo was capable of. Obviously the limit is a function of heat dissipated within the pack which is current squared x IR so we decided that a max initial dissipation based on the IR would enable us to forecast the real C rating of the lipo.
Having surveyed many discharge results we decided that a conservative initial heat loss was 6watts/cell/Ah so that the Lipotool formula was born. Whilst we agree that it is empirical, it really does forecast the real CONTINUOUS max discharge current for most lipos with surprising accuracy.

There is no secrecy involved; this has been explained on RCGroups several times as a search will easily show.

I built myself the ESR meter and offered it on RCG in case others were interested expecting to make perhaps 5-10 but it took off and I gradually developed it until the final version incorporates the Lipotool. I have no axe to grind as I have recently given up making them and passed it all over to Rick Distler ("RAMPMAN" on RCG) who now makes them for ProgressiveRC.

The proposal,to use a max cell voltage drop of 0.3V/cell is interesting in that it is a similar limit but effectively limits the C rating to 20C on any lipo based on the 6W/cell/Ah criteria

Taking a 1000mAh cell with a 0.3V drop would dissipate 6W at 20A. This would equate to an initial cell IR of 15m.ohms which is reasonable but not outstanding for a 1000mAh cell.

The problem would be that a very good 1000mAh cell with an IR of 5m.ohms would dissipate 18watts which is quite unsustainable, equating to a C rating of 60 whilst the lipotool suggests a maximum C rating of 34 because it restrains the heat dissipation to 6W.

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Old 03-16-2017, 10:45 PM   #29
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Wayne thank you very much for jumping in here, I appreciate it.

I hope you were not offended as I was not discrediting the tool or formula. My whole frustration with manufactures which I am sure you share is LiPo C/Rate is just an arbitrary number they are comfortable with posting they feel is safe and CYA in any liability case brought forth. Its a thermal value, and greatly exaggerated as I am sure you are painfully aware of.

I finally figured out the formula, and I do not have a problem with it. None what soever. I found the RCF discussion after I figured out what the intent was.

Little back ground here. I am an Electrical Engineer with Professional Licensure and have worked in Telecom and utility for 40 years or so, and one of my disciplines in batteries. I sat on IEEE Battery Committee for 8 years and have worked with all the big manufactures like C&D, Exide, Penn blah blah. I have access to lab grade battery test equipment so I can get some really accurate measurements.

C-Rate as you know has no BCI, IEEE, or ANSI definition. Pretty much a term used with Lithium Batteries and there is no defined test to measure it. We both know it is a marketing gimmick and exaggerated. I know the formula used for Max C. But what is the FOM used for?

So IMO I came up with a performance based test of maximum current to develop .3 vpc drop. Now it can be debated if that number were .2, .3, .4, .5 or whatever. As long as there is something anyone can easily measure. I chose .3 volt because .2 volts is a bit tight, and .4 is to sloppy. At .3 is a 8.1% voltage/power loss.

What I am driving at if you have say a 2000 mah battery rated at say C/40 and expect 80 amps on it is no good if the voltage sags from 4 to 3 volts or 25%.

I am sill missing something on the 6 watt/AH thermal limit. I assume it is a thermal mass relationship? Perhaps you could send me a PM. I would like to talk shop some time. I know what the Max C formula is, but what is the FOM used for?
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Old 03-16-2017, 11:12 PM   #30
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Dereck,

No offence taken at all. Just as interested as anyone to find a real answer to determining real C values.

If you look at this thread link, it will show how it all started some years back and explain FOM which is based on an idea that Mark Forsyth was already using to determine likely lipo performance based on IR values.

https://www.rcgroups.com/forums/show...cement-Overdue

I will send you a PM when I can figure out the procedure.

Wayne
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Old 03-16-2017, 11:20 PM   #31
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Originally Posted by Wayne Giles View Post
Dereck,

No offence taken at all. Just as interested as anyone to find a real answer to determining real C values.

If you look at this thread link, it will show how it all started some years back and explain FOM which is based on an idea that Mark Forsyth was already using to determine likely lipo performance based on IR values.

https://www.rcgroups.com/forums/show...cement-Overdue

I will send you a PM when I can figure out the procedure.

Wayne
I am on RCG, same user name.
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Old 03-16-2017, 11:28 PM   #32
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Some very cool analysis. A mechanical guy can always learn something from you EE's.

But I have to ask about a couple more variables that could factor in.

1). The conditions during assembly EG Ambient temp, humidity, pollen count(LOL) Seriously, many of the Chinese manufacturers assemble lipos in dusty, air-polluted outdoor environments. Others are made in more temp /humidity controlled lab environments.

2). The human factor- I can see where if there were a very consistent, repeatable process in place such as packaging automation, the lipos coming out of facility would also be consistent from one to next. But if Lucy does it one way, and Mary does it another way, there could be built-in inconsistencies in a manually driven process. Are lipos made on Tuesday better than the ones made on a Friday?

Just throwing these unknowns into your equations as I suspect that especially on the lower end, no two batteries from the same mfg are exactly alike. Just a suspicion with nothing to back it up.

-Hawk
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Old 03-17-2017, 01:40 PM   #33
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Thanks for chipping in Wayne. I didn't appreciate that you were involved in deriving the formula nor was I aware of the RCG thread.. It was posted on this forum one day and natural curiosity as to what was it's basis got the better of me.

FWIW I totally agree in that it seems to give pretty sensible results. The biggest wild card factor is the accuracy of chargers to measure IR, which seems to vary enormously... Not an issue if everyone used your ESR meter obviously.
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Old 03-17-2017, 08:36 PM   #34
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Originally Posted by JetPlaneFlyer View Post
Thanks for chipping in Wayne. I didn't appreciate that you were involved in deriving the formula nor was I aware of the RCG thread.. It was posted on this forum one day and natural curiosity as to what was it's basis got the better of me.

FWIW I totally agree in that it seems to give pretty sensible results. The biggest wild card factor is the accuracy of chargers to measure IR, which seems to vary enormously... Not an issue if everyone used your ESR meter obviously.
Thanks for that. John Julian did some comparative measurements using the meter and some chargers. From memory the i chargers were closest to the meter; generally reading slightly lower if I remember correctly, but most chargers were way off as you say.

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Old 03-17-2017, 09:03 PM   #35
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Wayne perhaps you have not checked your email, but ESR indicates you might be measuring AC Impedance rather than Resistance.
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Old 03-17-2017, 09:43 PM   #36
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Originally Posted by dereckbc View Post
Wayne perhaps you have not checked your email, but ESR indicates you might be measuring AC Impedance rather than Resistance.
Dereck,

Sorry for delay was just about to answer your mail.

Re this question; ESR stands for "Effective Series Resistance" rather than the AC measurement of Equivalent Series Resistance. I used this name as the meter meaures IR only in CELL mode using a Kelvin 4 wire connection but includes the resistance of leads and connectors when measuring in PACK mode so calling it an IR meter would not be strictly correct.

It uses a single 15mS DC current pulse of 16A (all except the latest Universal version which uses a 8A pulse. The 1KHz AC meaurement system which is widely used gives lower values,as you say, but I cannot see how this is relevant to our application. It is fine for manufacturers checking production cells as it is a lot easier to apply and the lower values suit sales splurge but what we are interested to know is how much a lipo voltage drops when we apply a DC load.
I think the fact that the ESC takes current pulses is a red herring as there are capacitors taking a huge beating on the ESC input in order to ensure that the current take is nearly pure DC.

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Old 03-20-2017, 02:07 AM   #37
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Originally Posted by Wayne Giles View Post
The proposal,to use a max cell voltage drop of 0.3V/cell is interesting in that it is a similar limit but effectively limits the C rating to 20C on any lipo based on the 6W/cell/Ah criteria

Taking a 1000mAh cell with a 0.3V drop would dissipate 6W at 20A. This would equate to an initial cell IR of 15m.ohms which is reasonable but not outstanding for a 1000mAh cell.

The problem would be that a very good 1000mAh cell with an IR of 5m.ohms would dissipate 18watts which is quite unsustainable, equating to a C rating of 60 whilst the lipotool suggests a maximum C rating of 34 because it restrains the heat dissipation to 6W.
Yes I understand the relationship of 6w/AH to .3vpc is a 20C battery. This is where my interest lies is true 20C rating. Your 6 watt limit seems reasonable although I do see a catch. Say you have two 1000 AH batteries, one is 20C and once is 50C. The 50C battery will be larger and heavier from my observations. That changes the thermal mass.

Anyway my goal is two fold. One is to find the best value LiPo for my use. I know from my very limited testing TP seems to be the best in terms of performance, but not the best value. Example a 25C TP form my use is $28, or I can buy a Glazier 35C for around $22. When you test them both give you roughly 25C. Glazier exaggerated but I get what I want for less.

2nd reason is I have another expensive hobby. I like to build racing Golf Carts. I want to set the record. Today I have to beat 12.241 sec 1/4 mile @ 118.76 mph. Plum Quick Motors did it using a 500 pound 144 volt AGM battery and a DC motor.

I know that can be beat using an AC 3-phase Induction motor running 144 volt with a 500 amp Controller. So I need a LiPo that can deliver 144 volts and 500 amps for 20 seconds. Why? Because I can shave off 450 pounds of a 500 pound battery, and LiPo is the only battery that can do that. So I am looking at a 40S 30C 16 to 18 AH battery.

Most DIY EV's and golf carts use chi-com LiFePo4 large format prismatic cells. At best with a lot of voltage sag can only deliver 10C for 10 seconds, and a pack of those would way 175 pounds and not any good way to fit all of them without a custom chassis/frame.
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Old 03-20-2017, 06:59 AM   #38
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Originally Posted by dereckbc View Post
2nd reason is I have another expensive hobby. I like to build racing Golf Carts. I want to set the record. Today I have to beat 12.241 sec 1/4 mile @ 118.76 mph.
You must be in an awful hurry to finish your round of golf
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Old 03-20-2017, 12:38 PM   #39
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dereckbc
I would not look for a specific big LiPo but consider 'building up' one from the cheapest available.
Typically a 2200mAh 3s is the best value - Turnigy 25C around $13?

If I have done the sums right you would need 130 of them arranged in a 10 parallel, 13 series configuration to give you the require volts and amps but it would have to be treated a bit like an un-exploded bomb!

And never mind a single 1/4 mile it would have the capacity to go back and forth quite a few times.
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Old 03-20-2017, 03:35 PM   #40
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Originally Posted by JetPlaneFlyer View Post
You must be in an awful hurry to finish your round of golf
Nah, not so much. This all started about 13 years ago when I built a house on a lot I bought in a gated golf course community. 4th of July here is kind of a big deal as we have parades with floats, skydivers, carnival for the ids and contest. One of those contest is golf cart drag races.

The cart I have now will win, and has won the last 4 years. It tops out at 70 mph and takes all of 5 seconds to get to 70 mph. I have it speed limited via the motor controller limiting the motor RPMs . Trust me 70 mph in a golf cart is scary fast.

However I am looking to break and set Guinness World Record and right now Plum Quick Motors holds that record.
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Old 03-20-2017, 03:58 PM   #41
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Originally Posted by quorneng View Post
dereckbc
I would not look for a specific big LiPo but consider 'building up' one from the cheapest available.
Typically a 2200mAh 3s is the best value - Turnigy 25C around $13?

If I have done the sums right you would need 130 of them arranged in a 10 parallel, 13 series configuration to give you the require volts and amps but it would have to be treated a bit like an un-exploded bomb!

And never mind a single 1/4 mile it would have the capacity to go back and forth quite a few times.
Copy that, I was not looking for a specific size pack. When I said last at least 20 seconds was just the minimum so as to get one run and be as lite in weight as possible. A pack of 18 AH assuming 450 amp load in theory C25 and roughly 2 minutes run time. So yes one could make several 1/4 mile runs, but I would never do that as each consecutive run would be slower than the one before it.

Plum Quick motors set the record with Odyssey AGM batteries a great high end AGM. However they have 2 things working against them.

1. Using 12 12-volt AGM batteries is heavy around 500 pounds.
2. They use a Series Wound DC Motor they make for golf carts. They do not make AC motors.

I do not have those limitations. There is a company out there called HPEV that makes 3-phase AC Induction Motors for EV's. The motor I am looking at using produce 110 ft-lbs of torque from 0 to 6000 RPM, then a constant HP of 125 HP from 6000 RPM to 11,000 RPM with a 500-amp controller. It can do that for about 15 seconds. Otherwise it is a constant 35 HP motor.

I already use a smaller motor they make in my cart today, and my cart will beat just about any current ICE passenger car made today in 1/8 mile. It is quicker than even a Tesla Roadster. It will scare the crap out of you. In that cart I am using 32S Calb 100 AH LiFeP04 cells (96 volts) and that pack weighs around 250 pounds with a 40 to 50 mile range.
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Old 04-24-2017, 05:00 AM   #42
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Wayne Giles hope you are still following. Sems I lost your email address, so thought I would post here.

After taking some time thinking about the IR test has me thinking it might not be realistic or fair to both user and manufacture. Here is what I mean. The test is to be done on a rested battery at room temps. I understand why because IR depends heavily on temps, and you want a standard temp to test. So I get that. But it is not realistic or real world operating conditions. As soon as you take off and have 30 seconds of fly time, the battery temp has gone up significantly, and thus Internal Resistance has dropped significantly Resistance is inversely proportional with temp.

So the question becomes should it be measured at operating temps? Example say 100 degrees F or 38 degrees C? Seems to me that is more realistic and fair. I do not know what everyone else does, but when I go to fly my batteries are placed on the dashboard in the sun to get them good and warm before flight.
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Old 04-24-2017, 03:21 PM   #43
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Battery internal resistance reduces with temperature but for LiPo chemistry in particular the battery life (the point where the loss in capacity becomes significant) is reduced when operating much above 20 Celsius.
It has been suggested that operating above 40 degrees can result in its life being shortened by as much as 50%.

As a LiPo will warm up anyway during a rapid discharge is there not a danger that by starting 'toasty' the battery might reach a life degrading temperature earlier than one starting at room temperature?
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Old 04-24-2017, 03:51 PM   #44
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Originally Posted by dereckbc View Post
Wayne Giles hope you are still following. Sems I lost your email address, so thought I would post here.

After taking some time thinking about the IR test has me thinking it might not be realistic or fair to both user and manufacture. Here is what I mean. The test is to be done on a rested battery at room temps. I understand why because IR depends heavily on temps, and you want a standard temp to test. So I get that. But it is not realistic or real world operating conditions. As soon as you take off and have 30 seconds of fly time, the battery temp has gone up significantly, and thus Internal Resistance has dropped significantly Resistance is inversely proportional with temp.

So the question becomes should it be measured at operating temps? Example say 100 degrees F or 38 degrees C? Seems to me that is more realistic and fair. I do not know what everyone else does, but when I go to fly my batteries are placed on the dashboard in the sun to get them good and warm before flight.

Dereck,

Measuring the IR at a spot temperature is quite deliberate and this is why it is always specified as 22*C. You should note that the reference to the 6W/Ah/cell is always specified as being the INITIAL power dissipation. This is to take account of the temperature coefficient of the IR of the lipo.

All lipos show similar, although not identical, plots of IR against temperaure and we must remember that the lipotool is an empirical equation based purely on practical measured results and makes no pretension to being a theoretically derived equation. Taking this into account, it is surprising how accurate it is at forecasting a real practical, if somewhat generally conservative max safe continous discharge current.
It also seems to work over a wide range of capacities from the tiny 100mAh single cells up to 10,000Ah+ packs.

Wayne
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Old 04-24-2017, 04:45 PM   #45
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Originally Posted by quorneng View Post
Battery internal resistance reduces with temperature but for LiPo chemistry in particular the battery life (the point where the loss in capacity becomes significant) is reduced when operating much above 20 Celsius.
It has been suggested that operating above 40 degrees can result in its life being shortened by as much as 50%.

As a LiPo will warm up anyway during a rapid discharge is there not a danger that by starting 'toasty' the battery might reach a life degrading temperature earlier than one starting at room temperature?
If the lipo is prewarmed the IR, and therefore the heat generated, is reduced so that the final temperaure is likely to be no higher.
This is because the plot of IR against temperature is very non-linear and IR rises dramatically at low temperatures ie below about 10 - 15*C. An undesirable result of this is the "Winter failure syndrome" which we (that's John Julian, Mark Forsyth and I; not the royal "we"!!) think is caused by the very high dissipation within the lipo at low temperatures.
At take off, with the lipo cold and IR very high, the heat dissipated is high and the lipo voltage low so that the pilot uses more throttle to compensate which increases the current and further increases the dissipation. This damages the lipo which is why so many die early in winter conditions.

Some suppliers of lipos with high IR try to argue that they will match ones with initial low IR values when they are up to operating temperature but this ignores the high initial heat dissipation and the fact that the initial performance is inferior. The heat generated in reducing the IR is energy lost which is shortening the life of the lipo instead of going to power the the motor.

I believe that preheating your lipos to ensure they are at least 25 - 30*C before use is a bit of a pain but a very good habit.

Wayne
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Old 04-25-2017, 03:58 AM   #46
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Wayne thank you for the feedback. I understand test conditions have to be made to establish a baseline so the numbers are comparable. I was not offering criticism, just observations.

I ran test on a few batteries and had repeatable results. Example I took 1 of my Gens Ace 3S 2200 mah 25C packs today that has roughly 10 cycles on it I purchased a few months ago. It stil test roughly the same as it did when new. Ri of all cells are 5 milli-ohms and that translates to 20C on a 25C pack.

I repeated the test except this time I put the cell in a test oven, yes I have access and brought the pack to 98 degree F, body temp, and Ri lowered to 3 milli-ohms. (3/2/3). That translates to 30C a fairly significant change.

It also occurred to me after looking at test result compilation spreadsheet, I see quite a few people are using the Icharger to measure Ri. As I am sure you know the resolution on the Ichargers is 1 mill-ohm. The lowest it can display is 1 milli-ohm, and with a 2200 mah cell there is not much meat left on the bone at 3 milli-ohms. It doe snot get much better.

So I am curious how low your meter can go and the resolution. Icharger is just a Ball Park meter. I have equipment that can go to 1 micro-ohm with 1 micro-ohm resolution.

All I was trying to point out test conditions do not reflect actual user conditions. Example for myself I know to warm my batteries up to 100 degrees before flying. For those that think that is crazy, think again, it keeps the battery cooler while operating. If you really want to hurt a battery put it in COLD.

Anyway all I am trying to say is don't let the test results lead you to believe a particular battery will not perform well. Example in the above test indicates the battery does not meet spec now or while new. It measures 20C and spec is 25C. However when warmed up to operating conditions, it exceeds specs at 30C.

As for me, I have been in the battery biz so to speak for just over 40 years. From an industry POV IEEE and manufactures recommend a battery be replaced when any cell Ri measures 150% of new. That is when I retire them. At 150% Ri the capacity wil have lost roughly 20%, and cannot produce the high peak currents reliably anymore.
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Old 04-25-2017, 10:37 AM   #47
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Originally Posted by dereckbc View Post

Anyway all I am trying to say is don't let the test results lead you to believe a particular battery will not perform well. Example in the above test indicates the battery does not meet spec now or while new. It measures 20C and spec is 25C. However when warmed up to operating conditions, it exceeds specs at 30C.


'
Derck,
I agree with most of what you say, particularly about preheating lipos and i - charger resolution. The meter has a resolution of 0.04milliohms. To be fair to the i - charger, when they first arrived a resolution of 1 milliohm was adequate but the improvement in lipos over the years has made the resolution inadequate now that larger lipos are nearing cell values of 1milliohm.

I have quoted the above from your response because it suggests that you have missed the point of the Lipotool.
The Lipotool says that it will give you a conservative estimate of the real continuous max C value of a lipo if you insert the cell capacity and the value of the cell IR at 22*C. The estimate it produces allows for the heating of the lipo during discharge so measuring IR at other temperatures and putting it into the Lipotool equation is meaningless.

Remember that the Lipotool was derived from comparing IR measured at 22*C with the real results obtained from full constant current discharge results. It was only after much testing that I realized that it was possible to correlate the two results.

(BTW I started measuring IR at 25*C using an environmental oven but we later changed to 22*C as that is more likely to be nearer to room temperature; it just means that the results are slightly more conservative)

Here is a classic example.
https://www.rcgroups.com/forums/show...45&postcount=1

I had just bought two Turnigy Nanotechs 3S 2200 25C and was disappointed with their IR figures so I ran a full set of discharges on them increasing the C value by 5C each time until it became obvious from voltage sag and excess temperature rise that they were overstressed.
If you read the post you will see that I came to the conclusion that they were only really capable of 15 - 18C. The total of the cell IRs was 27.6milliohms equating to a cell IR of 9.2 milliohms. Putting that value into the Lipotool gives a real Max current of 38A = 17C.
I carried out these tests 10 years ago; way before we came up with the Lipotool, but the results help to confirm that the Lipotool gives us a surprisingly accurate estimate of real continuous discharge capability without flogging through long power discharge testing.

Once we had derived the Lipotool, I naturally compared the Lipotool figures with the real discharge results and found the correlation consistently close so we decided to publish on RCG as it helps to demolish the BS claims which so many lipo vendors make about their products.

Wayne
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Old 04-25-2017, 03:21 PM   #48
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Originally Posted by Wayne Giles View Post
The Lipotool says that it will give you a conservative estimate of the real continuous max C value of a lipo if you insert the cell capacity and the value of the cell IR at 22*C.

Wayne
This right here is precisely why I love my ESR Meter. I don't want to know what the peak power is that my lipo can deliver, but the continuous power. If I have a lipo that reads 105A continuous safe discharge, then I know it will fly my 80-100A setups without any issues.

And my real-world experience seems to prove this out. I used to use sub-standard lipos in some of my jets (Turnigy blue label for example) thinking I could get by on the C rating, but they always came down very warm or hot, and started puffing after not many flights. But now that I'm matching the packs to the planes, the lipos come down cold or barely warm, even in the hot summer here in Arizona.

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Old 04-25-2017, 11:03 PM   #49
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Thanks for feedback.

If the Lipotool suggests 105A as a max current which you correctly understand as the max safe continuous cuurent for a full discharge, then it is safe to use a surge rating occasionally. The 105A is the real C value that the manufacturer should have written on the pack as a continuous max C rating.

As a purely personal view I would suggest that a reasonable max surge would be
double the maximum heat dissipation for 10 seconds. As this equates to a current increase of just over 40% because the heat generated is a function of current squared, then a sensible surge rating would be 148A, say 150A.

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Old 04-26-2017, 05:26 PM   #50
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Wrong thread. Deleted.

AMA #959089
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