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Jun 09, 2019, 04:26 AM
ancora imparo
jj604's Avatar
cbedfellow is correct.

Here's what I know about model LiPo flight packs. Other Lithium chemistries may be different and you should not assume that "known" facts for other packs necessarily apply.

Measured IR values are VERY dependent on how you measure them. This cannot be over emphasised. Unlike "intrinsic" properties like weight and size, the values are not invariant. If I measure the weight of a LiPo pack then, assuming my instruments are accurate, I should get exactly the same weight (allowing for a miniscule variation in the force of gravity at a different location) as anyone else. IR is not like that. It is a "derived" value and depends on the simplifying model you assume for a real LIPo. A real LiPo is a very complex little electrochemical factory and I don't pretend to understand exactly what is going on inside in all its detail. Like most things in electricity however the behaviour can be simplified by a model that puts together a few well understood fundamental components that mimic the behaviour of the real thing. A battery model may consist of several ideal voltage sources, several ideal resistors, several ideal capacitors and several ideal inductors and possibly some complex temperature dependant components. The simplest practical approximation to a real LiPo is an ideal voltage source in series with an internal resistor. The more current you pull, the bigger the voltage drop over the internal resistor, the more the voltage drop at the pack terminals and the more the internal heating. BUT this is an oversimplification. Real LiPos don't behave like this. However adding more and more "corrections" makes it impossible to do any meaningful analysis and predictions so this very simple model is pretty good PROVIDED you keep all the other variables constant.

Here's the variables I know about:

1) Test method. IR is calculated either by measuring the DC voltage change between loaded and unloaded state or applying a known AC frequency and measuring the impedance. The latter is the standard industry way of doing it and is fine for Quality Assurance in LiPo manufacturing but it returns IR values that are significantly lower than DC values. Vendors like this as it makes their packs look better than they actually perform in DC applications like ours. DC load methods are very dependent on the time you apply the load for and that's why values from say the Wayne Giles IR meter which is a pure DC load method are sometimes different from values returned by some chargers which stop charging briefly and measure the cell voltage difference between the charging and unloaded state.

Even if you use the same measurement method without fail there are still significant variables:

2) State of Charge. The measured IR varies a bit with the state of charge of the LiPo but it's not actually a big variation. IR is slightly lower at full charge and rises slightly until you get to about 10% SOC at which point it shoots up.
3) Pack size. Pretty obviously the IR is inversely proportional to the capacity of the pack.
4) Age of the pack. IR rises as packs age. Provided there is no actual cell failure it seems as though this is again dependent on cell chemistry and construction. Reliable genuine cycle life numbers are very hard to come by but the collective wisdom is that "heavy" packs like the Turnigy Graphenes and Panthers have a much slower increase in IR over elapsed time and number of cycles than HV packs and lightweight graphene packs. It is plausible that this is related to internal temperature rise as heat is pretty much the enemy of any consumer battery. Note that this is not an absolute judgment as a pack that runs hot will have lower IR (see below) and may perform better even although it won't last as long.
5) Temperature. This is the biggy. The measured value of IR is VERY dependent on the measurement temperature. Different chemistries have quite different IR vs. Temperature curves but all show a very dramatic increase in measured IR as temperature drops below room temperature. The changes can be large. A drop of 10℃ can double the IR value. That's why Wayne Giles, Mark Forsyth and others are obsessive about emphasising that IR must be measured at a standard temperature - normally 22℃ or 72℉. It also explains the good performance of otherwise high initial IR packs. The large internal heat generation of a high current load rapidly reduces the IR and reduces the internal voltage drop. This is evident in high current constant load tests by a dip and then recovery in the cell voltages in the first seconds of discharge. If it doesn't destroy the pack prematurely it is almost certain it accelerates its demise. Such packs may "flare brightly but briefly". In practice they might still perform well for as long as required.

John

Quote:
Originally Posted by MCSGUY
JJ604 is our resident IR expert, he monitors this thread I believe so he may respond soon. Otherwise you can read through his IR thread:
https://www.rcgroups.com/forums/show...rformance-tool

Joe
Quote:
Originally Posted by cbedfellow
IR increases as the batteries are used and as they age, that is pretty much how you interpret the readings, it aligns with a performance drop.
Last edited by jj604; Jun 09, 2019 at 08:06 PM. Reason: Changed ENTIRELY to VERY to avoid any misunderstanding.
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Jun 09, 2019, 10:30 AM
Donate Platelets
Thank you John, jj604, for such a good overview of LiPo internal resistance (IR.)
Jun 09, 2019, 11:55 AM
Registered User
Dennis C's Avatar
Joe
I want to thank you for all the work you have done, I have been following you since you started and based on some of your early work got some Gens Ace packs followed by Glacier when Gens Ace was not available, they were some of the best purchases I ever made. Just lost a set of the Glaciers to the bottom of the lake that were still doing good duty that I purchased March of 16. 6s 3700mAh. I got 4 packs that I use in 5 airplanes as either 6s or 12s. I field charge and those packs have a lot of cycles, several hundred.

I fly mostly 3D and some sport look for 6+ minutes of flight time. I want 2 flight packs and they go on the charger at the field as soon as the flight is over charge in +- 13 minutes fly all day.

I give this to let you know what I look for.

Real c, I don't push my packs very hard but I think a higher c used lightly will give better longevity.

Weight The HK Graphines are to heavy for my needs so W/g is a good metric, and unanswered is "am I better off getting higher capacity pack at the same weight as a higher c pack of the same weight.

A pack that will deliver through the full charge with low voltage sag

a supplier that is consistent with QC, I don't want to have to buy 3 packs to make sure I get 2 good ones

Price

longevity, those Glaciers would still be in use if they were not at the bottom of the lake

Based on some of older tests I also got a set of Dinogy packs 6s 30c 3700 mAh that are nearly as old and still going.

And after your last test I picked up a set of Dinogy 6s 65c 3700 mAh packs, time will tell- great weight, good price and great supplier. They will replace the packs at the bottom of the lake.

Again thank you for the work and effort that you put into all this.

Dennis
Jun 09, 2019, 01:57 PM
aka JetMan Joe
MCSGUY's Avatar
Thread OP
Your very welcome Dennis. I’m glad the work has served you well.

I recently adopted high capacity/low C for my eBandit. Mainly because the 10,000mah Pro’s actually do an honest 15C and I only need 125A WOT. Never thought I’d make the trade off but the Graphenes changed my thinking in that case.

Joe
Jun 09, 2019, 02:07 PM
aka JetMan Joe
MCSGUY's Avatar
Thread OP
John Julian! That was the quintessential explanation on IR. And if you don’t mind I’d like to add a descriptive link to it at the top of page 1 when I do the update after the next test series. IMO it couldn’t be any more clear, succinct and complete.

Joe
Jun 09, 2019, 02:11 PM
aka JetMan Joe
MCSGUY's Avatar
Thread OP
Wayne, your logic is right on. If you would wring out one of the 1450’s and post your findings here that would be great! I’ll work with Jack to get a pack to you.

Joe
Jun 09, 2019, 02:35 PM
Frankenstein recycled packs
rampman's Avatar
Numerous times since the Turnigy Graphene line was released I have proven that the lower the IR the more cycles possible out of a battery. Batteries that are testing out on a Wayne Giles ESR meter at 30C and higher are well capable of 1000 cycles or more if you live by the ESR Maximum continuous amps discharge displayed.
Lately I have tested a few brands nearing 2000 cycles at 30c discharges.

Rick
Jun 09, 2019, 02:36 PM
Registered User
Quote:
Originally Posted by MCSGUY
Wayne, your logic is right on. If you would wring out one of the 1450ís and post your findings here that would be great! Iíll work with Jack to get a pack to you.

Joe
Thanks Joe, I can test a 3S 1450 pack up to 75C at constant current and would be pleased to take a bit of the load off you. I donít do much testing now but welcome the chance of the odd pack to keep my hand in.
Canít remember if you have my address but will e mail it to you if you need it.

Wayne
Jun 09, 2019, 02:45 PM
big ignore list/drama is dumb
brushless55's Avatar
Quote:
Originally Posted by rampman
Numerous times since the Turnigy Graphene line was released I have proven that the lower the IR the more cycles possible out of a battery. Batteries that are testing out on a Wayne Giles ESR meter at 30C and higher are well capable of 1000 cycles or more if you live by the ESR Maximum continuous amps discharge displayed.
Lately I have tested a few brands nearing 2000 cycles at 30c discharges.

Rick
That is good info!
Jun 09, 2019, 02:48 PM
Registered User
fairplane's Avatar
MCSGuy! A huge thank-you for all the work you are doing here! Since I came across your thread I have been sharing it with all our club members. When purchasing new lipos I always come back and review your cliff notes before I purchase. So if I understand right, you will be updating sometime in June??? I am curious to see what the test results will be on the Hobbystar lipos.
Jun 09, 2019, 02:58 PM
HAL... Open the damn doors!
jfetter's Avatar
Quote:
Originally Posted by Wayne Giles
Thanks Joe, I can test a 3S 1450 pack up to 75C at constant current and would be pleased to take a bit of the load off you. I donít do much testing now but welcome the chance of the odd pack to keep my hand in.
Canít remember if you have my address but will e mail it to you if you need it.

Wayne
What's your preference on connector type?

BTW, you'll be happy to know they are also "100% waterproof" (from their web site), so feel free to test in the shower if you want to multi-task :-o

Jack
Jun 09, 2019, 03:29 PM
Registered User
Quote:
Originally Posted by jfetter
What's your preference on connector type?

BTW, you'll be happy to know they are also "100% waterproof" (from their web site), so feel free to test in the shower if you want to multi-task :-o

Jack
Jack,,

XT60 ideally but I can always change it if necessary.
Thanks for waterproofness info but I struggle to single-task nowadays; in any case you have just missed the boat as I had my annual shower last week.

Wayne
Jun 09, 2019, 03:30 PM
aka JetMan Joe
MCSGUY's Avatar
Thread OP
Quote:
Originally Posted by fairplane
MCSGuy! A huge thank-you for all the work you are doing here! Since I came across your thread I have been sharing it with all our club members. When purchasing new lipos I always come back and review your cliff notes before I purchase. So if I understand right, you will be updating sometime in June??? I am curious to see what the test results will be on the Hobbystar lipos.
Your welcome, fortunately itís turning more into a group effort which should improve the testing process and add more depth as it continues.

As soon as I add the 3 new packís data to the 4 others in the < 800g series Iíll unveil a new rating system for review and discussion. Iíll post it to the top of page one as the June 2019 update and move all the graphs into a separate post with a link in order to trim the page length.

Joe
Jun 09, 2019, 03:32 PM
Hamburger
hamburger's Avatar
Quote:
Originally Posted by MCSGUY
If in fact we can document truly fraudulent conduct (as in labeling a 20C pack as 175C) then I would be very comfortable approaching the RCG authority and making the case for bouncing MaxAmps.
Maybe this defacing of the most blatant marketing lingo would deserve a separate (sticky) thread?

Intentions and target audience are somewhat different from this thread? Though I suspect some frequent readers from here will enjoy and participate in that other field. What is more important, that other thread could cater to the needs of newbies in a concise entertaining way.

May I propose "lipo marketing on the test stand"
Jun 09, 2019, 04:16 PM
Registered User
+1 for hobby star test. Quote really low weight. A buddy of mine has the 8,000 mah pack for very large edf running 14s at 170A. Said they showed very low IR on recharge so may actually be decent despite low price and low weight


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