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.

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.

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.

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?

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.

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.

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.

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 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.

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.

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.

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

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.

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.

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.