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Old Oct 22, 2013, 04:31 PM
Tim Lampe; Hobbico R&D
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I know nobody believes manufacturer's claimed C-ratings (especially I guess when they soar to around 65C, 70C), but I figured it was still relative - a 70C pack (whatever the true C value) was still better than a 50C pack. But from what I'm getting in the last few posts, looks like 30C, maybe 35C is about the max no matter what. Begs the question (been asked before), is a manufacturer that claims 30C any more truthful than one that claims 50C or 65C?

I guess only the ESR knows.

Hmm, maybe I shouldn't bother with 65C packs and start flying 45C or 30C packs (provided the ESR is lower).

Tim
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Old Oct 22, 2013, 06:08 PM
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Your suspicion is quite justified Tim, as I have some older packs that carry a manufacturer's rating of 25C yet have lower internal resistance and outperform some new '60C' packs from another vendor.

Since a good many modelers are none the wiser, the purveyors of lipolys with ridiculously high ratings make more sales, charge higher prices, and thus reap far greater profits than the more scrupulous vendors who are honest with the batteries they sell. It's truly insidious and precisely why I try to reward those who are honest with my business.

Mark
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Old Oct 22, 2013, 06:28 PM
Tim Lampe; Hobbico R&D
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Thanks again Mark.

Okay, we all know LiPos have lower IR at higher temps. So I wonder if the higher C batteries have lower IR at operating temps than lower C batteries do at said temps (justifying the manufacturer's C claims)? I guess that's the heat coefficient Wayne (and I believe yourself) spoke of.

Again, for my particular application (F5D), looks like I'll be pre heating my LiPos (around 100F if memory serves). Then measuring surface temps ASAP upon landing to see where they are. And plan on continuing to cycle new LiPos into my supply. Maybe also test out some 30C - 45C stuff too!

Tim
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Old Oct 22, 2013, 06:47 PM
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Quote:
Originally Posted by KRProton View Post
So I wonder if the higher C batteries have lower IR at operating temps than lower C batteries do at said temps (justifying the manufacturer's C claims)? I guess that's the heat coefficient Wayne (and I believe yourself) spoke of.
You got it sir. Unfortunately, this is somewhat of a mixed bag. My 60C batteries do indeed have a steeper temperature coefficient slope and will outperform my 25C lipolys at high temperature. Sadly, the temperature required to reap the benefit of the steep slope is also at a place that it's starting to cause damage to the cells.

The ideal lipoly will be one that has low IR at normal starting temps (70-80F), and contain the magic elixir that provides the steep slope. That would suit all modelers and may be able to survive harsh continuous discharge rates above 40C with little damage. I will broadcast quite loudly if I ever come across a lipoly with these characteristic.

Cheers,
Mark
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Old Oct 22, 2013, 07:28 PM
Tim Lampe; Hobbico R&D
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Ha ha, yes magic elixir.

Well, at 30-ish C (50A from 1800mAh cells) I'm not exactly killing my batteries, but I'll experiment with some lower C batteries when flying season comes back around next spring.

Mean time, I'll play with my ESR meter and continue to work numbers, read the forums and such.

Tim
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Old Oct 23, 2013, 09:02 AM
Tim Lampe; Hobbico R&D
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Checking 1800mAh and 2200mAh packs around the shop with my Wayne Giles ESR meter. Lowest cell IR I'm finding for the 1800mAh packs is around 9 Ohms, a few in the upper 8s yielding max discharge amps of 35A (true C of around 20) and FOM around .75 at best.

Brand new, never used 1800's were the ones around 10 - 11 Ohms. A relatively new 1800mAh pack - few flights - was the one that got the low cell readings around 8.8 Ohms.

Now, where the heck am I going to find some cells with some sixes (6 Ohms)?

Fun stuff.

Tim
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Old Oct 23, 2013, 11:32 AM
Tim Lampe; Hobbico R&D
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...and another thing...

After looking through several results posted of people finding the IR of their batteries and the actual max Amps and true C ratings, I'm seeing few much above 30A for cells in the 1800mAh - 2200mAh range and most true C ratings around 15C, 20C, 30C at most.

So I guess my results are pretty much in-line with the norm (as Wayne and Mark have already commented).

Tim
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Old Nov 07, 2013, 02:25 PM
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Just got my meter!

Thanks Wayne, just got the Hi-Res ESR in the mail. Been looking through these pages looking for some general numbers for hi capacity pack like mine, but don't see anything. I've been using a lot of HK Nanotech batteries but have heard a lot of bad things about them lol! I'm always trying to push the pack as much as I can as these are used for mapping projects. I've got about 10 of these 8ah6s Nanotoech in service right now, range from 2 years old the new within a couple of months.

Oldest packs 19.3 milliohms total and 2.66 milliohm per cell
Newest packs 13.5 milliohms total and 1.63 milliohms per cell

So at the recommended 6W of internal dissipation and the following formula:

Max Amps = square root of 6W X 8ah over .0135ohms

That gives me 59.63 max amp draw working out to 7.45C! That's nowhere near 25C. Are these just really bad packs or is this the norm coming from the marketing departments? Even removing the IR of the EC5 connector I only get 8.75C.
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Old Nov 07, 2013, 02:35 PM
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Max Amps is based on highest individual cell measured internal resistance, not aggregate pack internal resistance.

Check again and report back.
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Old Nov 07, 2013, 02:38 PM
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Hmmm, I think my math may be wrong. I wonder if the IR is for a single cell, not for all 6. That would put me in the 21C rating for the new Nanotech packs, which would make sense.
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Old Nov 07, 2013, 02:43 PM
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Just as I posted may math was wrong your reply came in lol! Looks like my packs range from 21C for the newest to 17 for the oldest. In reality I never draw more than 3.75C (30amps) from these packs. I'm running two in parallel and rarely ever see more than 60 amps in total with my 7.8kg hex multi.

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Max Amps is based on highest individual cell measured internal resistance, not aggregate pack internal resistance.

Check again and report back.
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Old Nov 07, 2013, 02:55 PM
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Lost capacity?

Is there any formula to ballpark actual capacity versus the rated capacity if you track the internal resistance? Getting you charger to do it takes all day with 8ah packs.
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Old Nov 07, 2013, 04:10 PM
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Quote:
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Is there any formula to ballpark actual capacity versus the rated capacity if you track the internal resistance? Getting you charger to do it takes all day with 8ah packs.
Not quite sure what you are asking.
Most lipos do deliver the claimed capacity in terms of mAh and it is not a direct function of IR.
Obviously if the IR is higher then more energy is lost in heat in the pack and the pack will deliver less energy in watt.hours, although only marginally.

As a pack ages the IR rises, as your results show, and the capacity in Ah does fall although I believe that the two are not directly connected. In a 100 full cycle test I was involved in some years back, the best lipos only lost about 3% capacity in 100 cycles whilst the worst lost about 13%

Wayne
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Old Nov 08, 2013, 03:24 PM
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Great info. As long as my calculated max current using the 6W formula stays well above my actual current flying I'm good to go. C ratings on the labels are a joke. Thanks!


Quote:
Originally Posted by Wayne Giles View Post
Not quite sure what you are asking.
Most lipos do deliver the claimed capacity in terms of mAh and it is not a direct function of IR.
Obviously if the IR is higher then more energy is lost in heat in the pack and the pack will deliver less energy in watt.hours, although only marginally.

As a pack ages the IR rises, as your results show, and the capacity in Ah does fall although I believe that the two are not directly connected. In a 100 full cycle test I was involved in some years back, the best lipos only lost about 3% capacity in 100 cycles whilst the worst lost about 13%

Wayne
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Old Nov 08, 2013, 06:01 PM
ancora imparo
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Just to bear out Wayne's comments.

Here is some capacity measurements I recently made on a range of 3S 2200mAh packs I happen to have. All were tested under identical conditions at a low rate. The absolute measured values are a bit too low as the mAh meter readings needed calibrating but they can be compared relatively.

"New" are all less than a few months old and have all had some flight use.
"Old" are over year old and have been used a lot.

The Flightmax is over 4 years old and wouldn't pull the skin off a rice pudding now - it is struggling to be a 10C pack. It has huge IR compared with the others.

But it still has the highest actual capacity at moderate rates of all these packs. It's also the lightest.

John
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