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LiPo Internal Resistance

Does a table of maximum IR for different mAhr's exist?

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Mike Freeman26/03/2019 11:27:24
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I was wondering if there is a consensus on LiPo internal resistance (IR) and if there is a recommended maximum for different mAhr packs? I know it depends on the likely current draw but does a table exist showing suggested max IR against mAhr rating?

I was testing my packs last night ready for the hammering (considered of course! wink 2) they'll be getting now the weather is improving. To be fair, I'm quite kind to my packs, they tend to operate at around 12-15C so there're not exactly pushed to the limit!!

Years ago I set myself an IR threshold of 130% so, when the IR of a pack reaches 130% of what it was new the pack gets retired. Now, several of my packs have reached my arbitrary 130% threshold this year but are still giving good service. They're not puffed, don't get hot and have good capacity so I'm thinking my threshold is too strict.

During my testing I noticed that the larger the pack the lower the IR. This makes sense but is there a recognised correlation between IR and mAhr size and a recognised maximum for each size?

Here are some examples from my packs:

  • 3300mAhr = 8mohm/cell
  • 2200mAhr = 11mohm/cell
  • 1800mAh = 20mohm/cell
  • 1600mAh = 22mohm/cell
  • 850mAhr = 35mohm/cell
  • 280mAhr = 140mohm/cell (these are UMX size packs)

I've been measuring the IR with my icharger 106b with the packs all at storage charge and at room temperature.

If such a table exists I can maybe adjust it slightly to recognise my conservative 12-15C discharge rate, ditch my 130% threshold and get a load more flights from my packs!

Many thanks,

Mike

PeterF26/03/2019 13:13:41
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Have a look at the following RCGroups thread on this subject and the calculator for determining max continuous current from cell capacity and IR. **LINK**

Nigel R26/03/2019 13:26:45
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I haven't got any more complex than "if it gets hot when you come down, and the flights have got shorter, its getting old and tired."

to answer your question a bit better, conductance is the reciprocal of resistance, and I would expect to find that, given a consistent C rating, the conductivity is more or less proportional to cell capacity.

Just checking your figures:

3300 mAh, 8 mOhm (1/8mOhm = 125 S) = 37.9 S/Ah
2200 mAh, 11 mOhm (1/11mOhm = 90.91 S) = 41.3 S/Ah
1800 mAh, 20 mOhm (1/20mOhm = 50 S) = 27.7 S/Ah
1600 mAh, 22 mOhm (1/22mOhm = 45.45 S) = 28.4 S/Ah
850 mAh, 35 mOhm (1/35mOhm = 28.57 S) = 33.6 S/Ah
280 mAh, 140 mOhm (1/140mOhm = 7.14 S) = 25.5 S/Ah

in brackets is the basic conversion to conductance (in Siemens, "S"

the last figure is the conductance normalised by cell size

they're all quite close!

The 2200 packs come out quite good, are they a higher C rating?

Mike Freeman26/03/2019 13:27:59
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Thanks Peter, I have seen that spreadsheet. MattyB pointed us in that direction a few days ago. It is very good at determining a maximum C rating for a pack. I'm after the reciprocal of this though, I have a known C rating (12-15C for most of my models) and would like to know what the max IR should be for a given mAhr size.

With this information I can either verify or scrap my self imposed 130% threshold.

Cheers

PeterF26/03/2019 13:39:13
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The equation was published at some point if you want to make up your own spreadsheet to calculate things backwards rather than working the on line calculator iteratively.

Max current = square root(6 * Capacity / IR )

Effective C rating = 1000 * Max current / Capacity

Max current is in Amps

Capacity is in mAh

IR is in mOhms and is the IR of the cell with the highest IR in the pack.

The milli in the battery capacity and the IR cancel each other out.

I believe that if you have good batteries that over time you could let the IR double if they are still working. I have some 2200mAh batteries, new 7mOhm, aged 12mOhm, the calcs above suggest that the C rating has fallen from 20 to 15 and they still work OK in planes that draw 25Amps, although they are warmer at the end of the flight and flight duration is down compared to the new ones. However, my 3700mAh batteries that are my mainstay now have IRs of around 14mOhm (no record of as new) giving effective C of 11 and in one plane that draws about 55 to 60A they are now very sluggish.

Edited By PeterF on 26/03/2019 13:40:38

Mike Freeman26/03/2019 13:39:24
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Thanks Nigel,

I agree with your philosophy "if it gets hot... etc" but I was thinking about putting some science behind it and your conductance idea is certainly something to explore, thanks.

The 2200 packs are Gens Ace 25C's I have three packs and they all have similar results. A good case in point actually because when new they had IR's of around 7mohm/cell so, on my 130% threshold I "should" be retiring them BUT on your conductance argument above they appear to look good. They are good in reality too - they work a treat!

Cheers

Mike

PeterF26/03/2019 13:43:45
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Mike,

I believe that your arbitary 30% increase in IR is very conservative and will result in you dumping batteries prematurely. Use the calcualtor or the equation and see how things pan out for a 30% increase in IR. Because the formula is square rooted, this make a 30% increase in IR about a 15% reduction in C rating. I believe that you can normally absorb a much larger reduction in C rating before seeing issues.

Nigel R26/03/2019 14:11:05
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Interestingly (and quite reassuringly from my point of view) the equation posted by PeterF effectively converts to conductance first...

"Max current = square root(6 * Capacity / IR )"

The "capacity / IR" term is the last column in my post.

The square root operation is the reverse of working out dissipated heat by using current squared, or another way of putting it, this looks like a rule of thumb for "max heat per unit weight of battery"... and working backward to get the current which will cause that amount of thermal energy. I'd assume "6" is an arbitrary value worked out from "if it gets hot" type checks...

"Effective C rating = 1000 * Max current / Capacity"

and then turn that current into a C rating.

So I'm not sure I'd explore my post too much because it seems that exploration is already done

PeterF26/03/2019 14:39:53
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Nigel,

I knew it was somehow related to heat generation, taking the equation apart explains it well. The constant 6 will effectively be the combination of battery mass per mAh, specific heat capacity and an allowable temperature rise. There are a range of battery weights for a given mAh rating, so a better equation would be based on that, but the one shown seems to be sufficient.

Nigel R26/03/2019 14:53:31
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Heat is pretty much the prime dictator of how fast battery chemistry changes (well, degrades, more specifically) so it's a sensible limiting factor to use. IIRC about 40 degrees at the end of a flight is about as high as you'd want to go.

Taking battery weight would add more complexity to the equation - IIRC the weight per unit capacity is quite constant across a reasonable range of "C" ratings, so it would make a good enough proxy for a rule-of-thumb type calculation.

Mike Freeman26/03/2019 15:10:06
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Peter and Nigel,

I'd like to thank you both because between you you have given me a rule of thumb I can use, along with Nigel's anecdotal "If it's hot when you land..... etc" mentioned above.

Here's my thinking:

I like the look of Nigel's S/Ahr ratio shown in the last column of his post above. I know my 2200 packs are good and Nigel's figure of 41.3 is a good reference point. I also know I've been getting suspicious of my 1800 packs and they show 27.7S/Ahr in Nigel's table so, I'm thinking of setting a threshold of 25S/Ah and by rearranging Nigel's calculation to:

IR = 1000/(Ah x S/Ah)

For my conservative consumption of around 15C I'm starting off using an S/Ah ratio of 25:

Max IR (in mOhms) = 1000/(Ah x 25)

Max IR = 40/Ah

So, working with my packs I need to consider retiring my packs when the worst cell IR gets to:

  • 3.3Ahr pack - 12mOhm
  • 2.2Ahr pack - 18mOhm
  • 1.8Ahr pack - 22mOhm
  • 1.6Ahr pack - 25mOhm
  • 0.85Ahr pack - 47mOhm
  • 0.28Ahr pack - 142mOhm

That's just what I was after, thank you. But, thinking about it, it should be relatively easy to factor in different consumption C values using Peter's calculation above. My maximum IR's are based on my max consumption of around 15C. Those who push their packs harder would need to use a different S/Ahr ratio to my 25!!

Cheers,

Mike

Allan Bennett26/03/2019 20:54:54
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I read some time ago that a really simple rule of thumb is that a 'good' pack should have an IR less than 12,000 divided by its mAh.

But, since the significance of IR depends partly on what use the pack is being put to, I generally go by the 'if it gets hot ...' rule.

Chili Pants26/03/2019 21:13:33
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I have been looking at IR and C rating recently, I have quite modest requirements for C but its nice to know these things, I just picked up a cheap charger that can read IR, though it will mostly be used monitor the lipo`s health occasionally.

To get IR or C rate I used this on line tool, quite simple to use, just input your battery details, dont know how accurate it is as I am new to all this electric stuff but it saves me from making my head hurt.

Here`s the link https://www.soko-heli-tools.com/calculators

PeterF26/03/2019 21:23:47
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Posted by Chili Pants on 26/03/2019 21:13:33:

To get IR or C rate I used this on line tool, quite simple to use, just input your battery details, dont know how accurate it is as I am new to all this electric stuff but it saves me from making my head hurt.

Here`s the link https://www.soko-heli-tools.com/calculators

That is the same calculation method as the one I referenced above on RCGroups and gave the equation for, just coded up by someone else, although they have extended it to calculate the voltage drop that occurs at maximum current.

Chili Pants26/03/2019 21:48:39
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I did wonder if they were based on the same calculation method, I have tried them both, its just I found this one more user friendly also I like that you can input your batteries C rate and get what the IR is supposed to be and that it gives a C rate based on our IR without doing any headwork, the V drop is a nice touch as well.

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