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best volt per cell for accurate IR reading???

Old 10-17-2015, 05:42 AM
  #1  
Flysohigh
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Default best volt per cell for accurate IR reading???

Just got an icharger and i noticed the IR throughout the charge was lowest (1.8 miliohms) at 4.0v per cell....anyone else getting best readings around this parameter? I always heard that the most accurate readings for internal resistance is towards the end of the charge?
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Old 10-17-2015, 07:35 AM
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My understanding was its better to have readings at two set points ... but the main point is to check at same voltage so any change in pack is detected.

I have chargers and a dedicated meter that measure iR ..... they do not give exactly same results. Anything such as leads / connection etc can affect the results. So I regard them as indications rather than exact. Given this - my use is to keep a check on pack to see change from 'new' ......

Nigel
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Old 10-17-2015, 09:25 AM
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Charge state makes virtually no difference to I.R.... But what does make a big difference is temperature. If the battery is warm I.R. reduces a lot.

When you charge a battery it warms up, even if you cant feel it on the outside, on the inside it warms. So you will always observe that a battery has low I.R. after charging. If you leave it an hour or so the I.R will return to it's previous value. You will also find that if you take a charged battery that's rested at room temperature and discharge it down to storage on the charger then the I.R. will also decrease. Again this is due to warming.

The trick to get repeatable I.R. readings is to always leave the battery to rest to constant room temperature. The other thing you might notice is the process of testing I.R. (which briefly charges and discharges the battery) also in itself reduces I.R. due to heating. If you repeat the test several times I.R. comes down.

I'm with Nigel though, it's easy to get too obsessed with I.R.... it's a good tool indicative tool to monitor battery health but the absolute value needs taking with a pinch of salt. Ultimately as long as the battery performs ok then it is ok.
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Old 10-17-2015, 01:05 PM
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Hi

One of the biggest problems with these readings and tools used to do the readings is that the vast majority are off the shelf, often low cost un-calibrated units, you could have 10 of them and get different readings off all of them.

At the low cost end of the market Ignorance is not only bliss but often the best way to go.

If your packs are giving you the sort of flight times you are happy with don't bother worrying what everything is reading, just go with the flow, it will be much easier on the mind and the wallet

If you can afford and have access to high end fully calibrated test equipment, then go for it.

Enjoyment before paranoia I say
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Old 10-18-2015, 04:40 AM
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Originally Posted by JetPlaneFlyer View Post
Charge state makes virtually no difference to I.R.... But what does make a big difference is temperature. If the battery is warm I.R. reduces a lot.
.
I've not tried my two Cellpro PL8's on LiPo batteries, but have noticed that there isn't much different in IR on my various A123 battery packs at different temperatures.

IMHO, the best way to check IR is by applying medium and high load currents to the battery pack, and use the resulting voltage and current values for the two different load currents to calculate IR.

IR = (Vmedium_load-Vhigh_load)/(Ihigh_load-Imediumload)

Typical currents would be perhaps 5C and 15C. Easily measured with the use of a wattmeter, while adjusting the throttle to different power levels.

For me, those calculated values are the most accurate, much more useful info that what my PL8's measure. At any rate, once you've determined how accurate your charger's IR values are by comparing the chargers reading to the calculated reading, that makes the chargers IR readings more useful.

Those modelers that are using the dedicated IR meters that do apply heavy loads to the battery packs have something much more useful than some of the IR readings made with AC testing.
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Old 10-18-2015, 04:45 AM
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Originally Posted by JetPlaneFlyer View Post
I'm with Nigel though, it's easy to get too obsessed with I.R.... it's a good tool indicative tool to monitor battery health but the absolute value needs taking with a pinch of salt. Ultimately as long as the battery performs ok then it is ok.
IMHO, the most useful battery measurement is to simply take an RPM reading with the battery pack and the model airplane right after a charge cycle. And repeat the RPM reading after landing. Write those numbers down on the battery pack along with the model/motor/prop info.

After a few months, or years, just repeat those measurements. Simple, and it provides what information is really needed.
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Old 10-18-2015, 06:41 AM
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Originally Posted by kyleservicetech View Post
I've not tried my two Cellpro PL8's on LiPo batteries, but have noticed that there isn't much different in IR on my various A123 battery packs at different temperatures.
The I.R. function on the PL8 isn't very good unfortunately. It tries to measure I.R. while the battery is charging but this usually doesn't give results that are very representative of discharge I.R.(which is what you are actually interested in).

The iCharger on the other hand works in a similar way to dedicated I.R. meters in that it measures I.R. while briefly discharging the battery and it performs calculations similar to what you suggest in your post.
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Old 10-20-2015, 01:15 PM
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So is it best to measure IR discharging the pack? If so at what rate is best 1C?
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Old 10-20-2015, 02:29 PM
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Its best to charge to a set level... let the pack rest an hour at a known temperature. Then measure the IR at a fixed discharge rate.

do it the same way for the same pack every time. As it ages the IR will slowly go up then suddenly start going up rapidly. When it goes up rapidly its not going to deliver the same capacity or current any more.
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Old 10-20-2015, 02:56 PM
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Is it ok from storage voltage to check the IR each time? I hear people say its best to check towards the end of a charged pack?
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Old 10-20-2015, 03:40 PM
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Just do it the same every time.

If all of your measurements are at 1C discharge rate and 80 F and storage voltage you get from your charger, then do that every time.
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Old 10-20-2015, 04:01 PM
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Originally Posted by fhhuber View Post
Just do it the same every time.

If all of your measurements are at 1C discharge rate and 80 F and storage voltage you get from your charger, then do that every time.
You beat me to it ...............

As already said by many ........ the trick is to test at same conditions each time - to monitor decline in battery condition.
The general Charger based iR and over counter meters results are to show you TREND ... not specific exact numbers.

But to be honest even though I have 2 chargers with iR capability and a dedicated iR meter ........ I now just rate my packs by flying them.

For my high power stuff like EDF / Pushers ... the packs get relegated when they start to fall off in performance - often shown by puffing or getting hotter than usual. I then scribe in permanent red ink a single line round the pack. It then gets used in less demanding models. As performance further reduces - a second red ring is scribed on the pack ... and it goes into such as glider use etc. These packs are actually good for Rx power on wet fuel models as amp demand is low and you step them down with BEC anyway.

The really poor packs get put to bench use where I need a short power test of a motor direction or installation check of a model. Saves using my good flight packs.

So iR checking really is not that necessary ... flight performance is the best indicator ...

Nigel
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Old 10-20-2015, 04:07 PM
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Ok thanks, ill check from a 1C discharge rate every time
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Old 10-20-2015, 04:08 PM
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Like I said in my first reply... Charge state makes virtually no difference.
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Old 10-20-2015, 04:35 PM
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Nor pack voltage either?
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Old 10-20-2015, 05:14 PM
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Pack voltage is going to depend on charge state... While the voltage curve is very flat for most of the charge, you can still relate % charge to voltage.
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Old 10-20-2015, 05:43 PM
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Originally Posted by Flysohigh View Post
Ok thanks, ill check from a 1C discharge rate every time
Again, it makes no difference what rate you charged the battery at. The iCharger (unlike the Powerlab) doesn't check IR while charging.

The only important thing for consistent IR measurement is battery temperature, so just leave the battery for an hour or two in a place with consistent temperature (e.g. room temperature) for the battery temperature to stabilise.

Dont check immediately after charging or discharging as both processes effect inner temperature of the battery.

For the purposes of this discussion battery voltage and charge state are one in the same, neither make any significant difference to IR.
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Old 10-20-2015, 06:58 PM
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Originally Posted by JetPlaneFlyer View Post
Again, it makes no difference what rate you charged the battery at. The iCharger (unlike the Powerlab) doesn't check IR while charging.

The only important thing for consistent IR measurement is battery temperature, so just leave the battery for an hour or two in a place with consistent temperature (e.g. room temperature) for the battery temperature to stabilise.

Dont check immediately after charging or discharging as both processes effect inner temperature of the battery.

For the purposes of this discussion battery voltage and charge state are one in the same, neither make any significant difference to IR.
Ok so you say it doesnt check while charging but when you start a charge/discharge within 10 seconds it shows an IR reading?
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Old 10-20-2015, 09:43 PM
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The charger does a brief discharge check of the pack's voltage to confirm the settings.

If it comes out too high or too low it will give an error message instead of starting the charge.
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Old 10-20-2015, 11:14 PM
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Spoke to progressive rc they stated it doesnt matter if you charge or discharge the pack as long as theres a load you can measure the IR when the charge starts. I was just confused on the difference between the line resistance and the pack reaistance, he stated it had to do with circuitry...
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Old 10-21-2015, 04:30 PM
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Originally Posted by Flysohigh View Post
Ok so you say it doesnt check while charging but when you start a charge/discharge within 10 seconds it shows an IR reading?
None of the three iChargers I've owned does that?

To do an IR check on my iChargers you have to select the IR function, which is independent of charge and discharge options.
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Old 10-21-2015, 07:19 PM
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Originally Posted by JetPlaneFlyer View Post
None of the three iChargers I've owned does that?

To do an IR check on my iChargers you have to select the IR function, which is independent of charge and discharge options.
Have you used a 308 or higher? They show IR on load
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Old 10-21-2015, 10:19 PM
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Originally Posted by Flysohigh View Post
Spoke to progressive rc they stated it doesnt matter if you charge or discharge the pack as long as theres a load you can measure the IR when the charge starts. I was just confused on the difference between the line resistance and the pack reaistance, he stated it had to do with circuitry...
Impossible and complete BS.

There are two ways to measure IR and only one of the two is accurate.

One is using DC, and is very simple, and extremely accurate. You measure the open circuit voltage of the battery and record it. You then apply a load to the battery, and then measure the voltage of the battery and the load current. Then just a simple math calculation which all hobby chargers use.

Ri = (Voc - Vl) / Current.

So let's say you have a 3S battery and Voc = 12 volts, Vl = 11.6, and load current = 10 amps. (12 - 11.6) / 10 amps = .04 Ohms.

The other method which is meaningless unless you have a baseline measurement, is to use an AC signal typically done at 1000 Hz. It gives you the Impedance, not resistance. AC measurement can be done while charging, discharging, or idle. But tells you nothing unless you know what the Impedance was when the battery was new.

So unless they use AC to measure impedance, they are telling you BS that it can be done while charging or discharging because they use DC measurement. DC can only be done with a Load Test which is Discharge only.

Now what they can do is interrupt a charge or discharge cycle, but is meaningless.

To answer your question the best voltage is not a voltage Per Se. It is State of Charge aka SOC. LiPo Ri is very flat from 100% SOC to 0% SOC unlike all other battery chemistry. You want to know the Ri at 100 and 0% SOC.

Having said that whatever Ri is is meaningless unless you know what it was when new. Temperature also greatly affects Ri. A cold battery can measure 10 times higher than when warm. So when you make your measurements should be on a well rested battery at room temps. Otherwise you are just chasing ghost.
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Old 10-22-2015, 12:35 AM
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Originally Posted by dereckbc View Post
Impossible and complete BS.

There are two ways to measure IR and only one of the two is accurate.

One is using DC, and is very simple, and extremely accurate. You measure the open circuit voltage of the battery and record it. You then apply a load to the battery, and then measure the voltage of the battery and the load current. Then just a simple math calculation which all hobby chargers use.

Ri = (Voc - Vl) / Current.

So let's say you have a 3S battery and Voc = 12 volts, Vl = 11.6, and load current = 10 amps. (12 - 11.6) / 10 amps = .04 Ohms.
IMHO, DC current measurements are as accurate as you're going to get with measuring the IR of our battery packs.

Even more accurate is my posting #5 in this thread, where two different current levels are pulled from the battery. This eliminates any "Floating" voltage that might be present on the battery under no load conditions. That floating voltage is quite significant on the A123 battery packs. The calculations follow:

"The best way to check IR is by applying medium and high load currents to the battery pack, and use the resulting voltage and current values for the two different load currents to calculate IR.

IR = (Vmedium_load-Vhigh_load)/(Ihigh_load-Imediumload)

Typical currents would be perhaps 5C and 15C. Easily measured with the use of a wattmeter, while adjusting the throttle to different power levels."


So, is this worth while? For me, I just measure RPM on the same motor with the same prop with a brand new battery pack, and recheck it later on. A few months, or even a few years later.
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Old 10-22-2015, 02:02 AM
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Originally Posted by kyleservicetech View Post
[B] "The best way to check IR is by applying medium and high load currents to the battery pack, and use the resulting voltage and current values for the two different load currents to calculate IR.
That is why I said RESTED BATTERY. You have to bleed off the surface charge which is easily done by just applying a load for a few seconds.

But in a Lab or test facility, they use Open Circuit Voltage, thus is why iCharger, Progressive, and Mickey Mouse chargers use OCV method.

It can be done in reverse on the charge side. I am fortunate being in th epower biz, I have access to industrial battery test equipment that can measure internal resistance down to .0000001 Ohms with +/- 1% accuracy. You have to use a 10 amp current source.
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