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Battery Packs Internal resistance testing

Old 12-09-2015, 11:27 AM
  #1  
Paul80
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Default Battery Packs Internal resistance testing

Hi all

Here in the UK its another wet & windy day so no flying possible and nothing on the the building board, so I got a bit bored so thought I would play around with testing some of my battery packs.

As we know one of the ways we can test the health of our packs it to check the packs internal resistance and there are quite a few options to do this.

I have a few meters that claim to be able to read the individual cells internal resistance so thought I would sit down and log all my packs one each meter to see what the results would be.

The first method I used was that used by the YouTube user ExperimentalAirlines



which used a known load of 100watts and making a calculation based on the voltage drop and current drawn, this method only give the packs IR value

Next up I have my Turnigy Accucell 6 which has a IR Function and gives the results for individual cells



Then we have three cell meters, a turnigy IR Meter a Hobbymate cell Meter and a Smart Battery Meter cell meter



Now the results, I will only post the results from a few of the packs so as to not make it too long and boring if its not already

Battery Packs Tested are all Turnigy 2200mAh 3Cell 25C Packs that are all new this year and only been charged half a dozen times and never been crashed.

So first up the ExperimentalAirlines method

Pack A 47 mOhm for whole pack avg 15.66 mOhm per cell
Pack B 64 mOhm for whole pack avg 21.33 mOhm per cell
Pack C 51 mOhm for whole pack avg 17.00 mOhm per cell

Next the Turnigy Accucell 6 80W charger

Pack A Cell 1. 6 mOhm. Cell 2. 4 mOhm. Cell 3. 5 mOhm
Pack B Cell 1. 9 mOhm. Cell 2. 7 mOhm. Cell 3. 6 mOhm
Pack C Cell 1. 4 mOhm. Cell 2. 7 mOhm. Cell 3. 7 mOhm

Next the Turnigy IR Meter

Pack A Cell 1. 4.1 mOhm. Cell 2. 4.2. mOhm. Cell 3. 4.4 mOhm
Pack B Cell 1. 6.4 mOhm. Cell 2. 6.0. mOhm. Cell 3. 5.5 mOhm
Pack C Cell 1. 3.3 mOhm. Cell 2. 6.4 mOhm. Cell 3. 5.9 mOhm

Next the Hobbymate cell meter

Pack A Cell 1. 7. mOhm. Cell 2. 2 mOhm. Cell 3. 5 mOhm
Pack B Cell 1. 4. mOhm. Cell 2. 5 mOhm. Cell 3. 5 mOhm
Pack C Cell 1. 6. mOhm. Cell 2. 5 mOhm. Cell 3. 7 mOhm

Next the Smart Cell meter

Pack A Cell 1. 5. mOhm. Cell 2. 4 mOhm. Cell 3. 5 mOhm
Pack B Cell 1. 4. mOhm. Cell 2. 2 mOhm. Cell 3. 8 mOhm
Pack C Cell 1. 10. mOhm. Cell 2. 2 mOhm. Cell 3. 3 mOhm

one thing to note is that the last two gave totally different readings each time the were plugged in, even to the same battery so their results are totally worthless so if thinking of getting an IR meter I would avoid these like the plague.

Of the others although they gave quite a range of readings they were consistent so even though none may be accurate they will still be good for monitoring your own packs, but stay away from the last two as they were just all over the place.

Hope someone finds this of use.

Paul
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Old 12-09-2015, 03:54 PM
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dereckbc
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Unless you know what it was when new, not much to be gained from it. A much more useful number is Short Circuit Discharge Time.
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Old 12-09-2015, 04:08 PM
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solentlife
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Originally Posted by dereckbc View Post
Unless you know what it was when new, not much to be gained from it. A much more useful number is Short Circuit Discharge Time.
Yes ... but the one aspect that could be used ... whichever numbers you choose to be the 'starting point' .... you check periodically to see if any change / deterioration of cells. It's not the actual numbers ... its the change of numbers ...

nigel
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Old 12-09-2015, 04:31 PM
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Originally Posted by solentlife View Post
Yes ... but the one aspect that could be used ... whichever numbers you choose to be the 'starting point' .... you check periodically to see if any change / deterioration of cells. It's not the actual numbers ... its the change of numbers l
Well Duh! Unless you know what the baseline is when new, the numbers are meaningless. If you start with say batteries that are already 1-year old have already started to deteriorate significantly. All you are seeing is bad to worse on a battery that is already ready for retirement.

To be of any real use, the battery needs to be new, cycled a few times, at a reference temperature, at a reference SOC level. I do log all my batteries Ri. I let them come to room temps for at least 24 hours, at 3.85 volts per cell (storage voltage). Temperature significantly effects Ri,

About the only thing you can tell in an older battery without a baseline is if you have a bad cell(s). Example in say a 4S 2600 mah cell you see 11, 12, 25, and 9. You know cell 3 is toast. At 50 amp draw you will loose 2.85 volts.

I use a Power Lab 8, and I know for certain they use DC Current/Voltage edrop to measure Ri which is what you want. What I do not know is what other manufactures use. I suspect some may use AC impedance which is not useful at all unless you have a baseline when new.

But the most useful of any of them is Short Circuit Discharge Time like ewe use on EV's. Makes no difference how old the batteries are. If the battery SCDT has gone higher than spec regardless of size, it is bad. However LiPo manufactures do not give you the Discharge Curves to determine SCDT like they do with LFP EV batteries.
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Old 12-09-2015, 04:57 PM
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Paul80
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Hi all

The post was no so much about the merits or otherwise of checking IR of your packs but more to point out just how bad some of our test equipment is at checking IR with some being just worthless as they cannot even give the same result twice on the same pack and by a long way as well in the case of two of them.

For me I suspect I will just stick with the Turnigy IR meter as that was very consistent and as all my packs are new this year with very little use on them yet the figures I obtained will be a good benchmark as I test them as the get older and used more.

Paul
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Old 12-09-2015, 08:10 PM
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Plus IR varies massively with battery temperature. To get consistent results you need to leave the battery resting for hours in a controlled temperature environment. Even then if you charger or discharge the battery it creates 'internal warming' which skews the results significantly. Even the brief charge /discharge cycle that occurs when IR testing heats the battery slightly. You will probably find that if you do a series of IR tests the measured IR decreased with successive tests due to this phenomena.

PS.. regarding the accuracy of Powerlab IR measurement. Unless I'm mistaken Powerlabs only measure IR during charging. This gives highly inconsistent results compared with any IR measurement that measures during discharge (it is after all discharge IR that you are interested in). Typically the IR measured with a Powerlab is in the order of half the value of IR measured by specialist IR meters such as the Giles meter.

there was a report on another forum comparing IR measurement between that calculated from discharge voltage drop, that measured with a Giles meter and that measured with Powerlab and iCharger chargers. It also showed a high of inconsistency but the Powerlab was by far the most inconsistent. If there is any interest i can probably dig the thread out.
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Old 12-09-2015, 11:16 PM
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Originally Posted by dereckbc View Post
Well Duh! Unless you know what the baseline is when new, the numbers are meaningless. If you start with say batteries that are already 1-year old have already started to deteriorate significantly. All you are seeing is bad to worse on a battery that is already ready for retirement.

To be of any real use, the battery needs to be new, cycled a few times, at a reference temperature, at a reference SOC level. I do log all my batteries Ri. I let them come to room temps for at least 24 hours, at 3.85 volts per cell (storage voltage). Temperature significantly effects Ri,

About the only thing you can tell in an older battery without a baseline is if you have a bad cell(s). Example in say a 4S 2600 mah cell you see 11, 12, 25, and 9. You know cell 3 is toast. At 50 amp draw you will loose 2.85 volts.

I use a Power Lab 8, and I know for certain they use DC Current/Voltage edrop to measure Ri which is what you want. What I do not know is what other manufactures use. I suspect some may use AC impedance which is not useful at all unless you have a baseline when new.

But the most useful of any of them is Short Circuit Discharge Time like ewe use on EV's. Makes no difference how old the batteries are. If the battery SCDT has gone higher than spec regardless of size, it is bad. However LiPo manufactures do not give you the Discharge Curves to determine SCDT like they do with LFP EV batteries.
so the CHANGE of numbers has no significance ?

The Baseline is when you start to check the packs.

C'mon Derek ...

Nigel
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Old 12-10-2015, 12:43 AM
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dereckbc
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Originally Posted by solentlife View Post
so the CHANGE of numbers has no significance ?

The Baseline is when you start to check the packs.

C'mon Derek ...

Nigel
Nope Garbage In = Garbage Out. A bad battery is a bad battery. You cannot detect a bad battery if you do not know what a good battery looks like to start with. So wise up.

Now if you use batteries like I do, all same model from same manufacture, then you already know what Ri is when new. My Glacier 2200 mah 35C when new measure 10 to 14 milli-ohms across all three cells. When they hit 20 to 25 milli-ohms they get pulled OOS and given away. But if you do not know what GOOD is to start with, it just numbers with no meaning. Very few manufactures publish Ri. Those that do are the ones you want to buy, but they will be pricey.
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Old 12-10-2015, 04:22 AM
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Default Calculating Battery Internal Resistance

Originally Posted by Paul80 View Post
Hi all

Here in the UK its another wet & windy day so no flying possible and nothing on the the building board, so I got a bit bored so thought I would play around with testing some of my battery packs.

As we know one of the ways we can test the health of our packs it to check the packs internal resistance and there are quite a few options to do this.
I've done some of this my self with the "simple" volts/Amps formula. The procedure is to apply a load to the battery pack, perhaps around 5 Amps. Measure Voltage and Current. Then increase the load to around 20 Amps, and repeat the measurement.

Putting the numbers into a simple Excel spreadsheet really speeds up the calculation if a batch of batteries is being tested.

This will produce accurate results for the entire battery pack as a unit.

http://www.wattflyer.com/forums/showthread.php?t=59266
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Old 12-10-2015, 06:50 AM
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Originally Posted by dereckbc View Post
Nope Garbage In = Garbage Out. A bad battery is a bad battery. You cannot detect a bad battery if you do not know what a good battery looks like to start with. So wise up.

Now if you use batteries like I do, all same model from same manufacture, then you already know what Ri is when new. My Glacier 2200 mah 35C when new measure 10 to 14 milli-ohms across all three cells. When they hit 20 to 25 milli-ohms they get pulled OOS and given away. But if you do not know what GOOD is to start with, it just numbers with no meaning. Very few manufactures publish Ri. Those that do are the ones you want to buy, but they will be pricey.
My point is not garbage ... my point is that iR CHANGE indicates deterioration or change of the pack / cell.

I'm not arguing that new pack readings are not best. Of course they are.

I did not have iR checking charger or Meters to do it BEFORE buying LiPo's. I progressed like most do to obtaining them after I had a number of LiPo's. Those LiPo's can still be monitored for change even though I do not have their 'new' state figures.

Nigel
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Old 12-10-2015, 10:31 PM
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Originally Posted by kyleservicetech View Post
Putting the numbers into a simple Excel spreadsheet really speeds up the calculation if a batch of batteries is being tested.
I admit I do this, but I think there is a batter way and have started doing it.

Rather than a Spread Sheet, take some wide Masking Tape, and stick it on the battery covering all of one side for a log

I wait until th ebattery has had a few cycles. Give it a good slow C/2 charge, and then Discharge noting Mah. Then charge to Storage SOC, and note Ri.

Then enter something like this
121015 2275 13

Which indicates:

Date mah Ri (in milli-ohms)

You know instantly the battery health and age without having to resort to using a spread sheet.

For me since I use nothing but 3S 2200 mah batteries and my 4 models use either 50 or 60 amp ESC once Ri goes above 20 milli-ohms it is time for retirement. At 20 milli-ohms max power voltage drop is 10%. Idealy you want it at 5% or less.

So Nigel if you want to know if a particular battery will work or not, does not mean it is good or like new, determine if the Voltage Drop will work for the application. In my case 20 milli-ohms @ 60 amps is 1.2 volts or 10% on a 3S LiPo. IMO that is retirement time to surface applications.
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Old 12-10-2015, 11:14 PM
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Originally Posted by dereckbc View Post
...once Ri goes above 20 milli-ohms it is time for retirement. At 20 milli-ohms max power voltage drop is 10%. Idealy you want it at 5% or less.
Which would be fine if you could have any faith in the accuracy of IR measurement. The problem as noted in this thread and elsewhere is IR measurement is all over the place depending on what you measure it and battery temperature (and charge rate if using a Powerlab)

According to those who have tested, Powerlabs are very inconsistent in measuring IR when compared with the most trusted type of specialist IR meter (Giles meter). This is probably because the Powerlab attempts to measure IR during charging, rather than during discharging, when of course it's discharge IR that you are interested in.

Most of the time these days I only do very occasional IR checks mainly to detect if any cell in a battery is showing markedly different IR to the rest, which is a sure sign of problems. Otherwise I just fly the battery until the voltage sag under load becomes too great. The way I look at it lately is if the battery is performing ok then it is ok, and vice versa.. simple. Life is too short to spend much time IR testing dozens of batteries.
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Old 12-11-2015, 01:17 AM
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To repeat again and again

My overweight undervoltaged A123 batteries maintain nearly the same resistance even after 5 years and 500 flights.

They are worn out when they drop to 75% of rated Amp Hours.
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Old 12-11-2015, 08:29 AM
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Originally Posted by JetPlaneFlyer View Post
Which would be fine if you could have any faith in the accuracy of IR measurement. The problem as noted in this thread and elsewhere is IR measurement is all over the place depending on what you measure it and battery temperature (and charge rate if using a Powerlab)

According to those who have tested, Powerlabs are very inconsistent in measuring IR when compared with the most trusted type of specialist IR meter (Giles meter). This is probably because the Powerlab attempts to measure IR during charging, rather than during discharging, when of course it's discharge IR that you are interested in.

Most of the time these days I only do very occasional IR checks mainly to detect if any cell in a battery is showing markedly different IR to the rest, which is a sure sign of problems. Otherwise I just fly the battery until the voltage sag under load becomes too great. The way I look at it lately is if the battery is performing ok then it is ok, and vice versa.. simple. Life is too short to spend much time IR testing dozens of batteries.
I totally agree with that and is what I basically do.

My only addition to that is that I do not retire a LiPo until its really 'dead' and no use at all.
Even a 4S that one cell is zero has a use ... as example.

From High power demand applications ... I drop a sagging pack down to a lesser demand application ... I have so many different models that I could probably divide them into 4 or 5 levels of demand ...
Once they drop of the model flight use ... they become bench power such as installation power ... servo tester power ... motor test rotation etc.
As some know - I use LiPo's as Rx flight pack power ... the amp draw being very low - even a shagged out LiPo is far more than needed.... one model has had a 3S 1500 pack in it for over a year now ... previously was retired from a low demand Cessna Skyartec ... That pack is still delivering excellently for Rx flight pack use even though for motor use it's basically finished.

Nigel
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Old 12-11-2015, 10:18 PM
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If you really want to know the Ri accurately is extremely simple using Delta Voltage and Delta Current measurement.

Charge the battery to Storage Voltage or around 50% SOC. Use any charger with a Discharge function and a volt meter. Draw 1C current and measure Voltage and call it V1, and note current as I1. Draw 2C, 3C, 4C or as high as you can and record the voltage, call it V2, and the current as I2

Delta Voltage = V1 - V2
Delta Current = I2 - I1
Ri = Delta Voltage / Delta Current

Example a 2200 mah 3S battery would look something like:

At 1C current of 2.2 amps voltage = V1 = 11.3 volts
At 3C current of 6.6 amps voltage = V2 = 11.2 volts

Dv = 11.4 - 11.3 = 0.1 volt
Iv = 6.6 - 2.2 = 4.4 amps
Ri = .1 volts / 4.4 amps = .023 Ohms

A more useful number when evaluating new batteries is Short Circuit Discharge Time. It is what engineers use to design EV's and to choose the best battery. Both Ri and SCDT can be extracted from Discharge Curves using the 1C and 10C curve lines.

SCDT in seconds = 3600 * voltage [V] / specific_current [1/h] / cell_voltage [V]

If you already know Ri then SCDT in seconds = 3600 * capacity [Ah] * resistance [Ω] / voltage [V]

The closer to ZERO seconds, the better. Is is a easy Graphical chart that shows 24 batteries for EV's .

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