Lipos have no power

[Dave Bran]

Posted by Paris Skoutaridis on 16/11/2016 22:26:50:

Some packs/brands are more sensitive to storage voltage and general abuse than others. For example Turnigy nano techs are not very tolerant of either long term (more than a couple of days) of full charge storage or over-discharging.

And this is where it all gets extremely subjective. I use lots of Nano-Tech. I have never ever done anything other than leave them in full charge storage, sometimes for months............to NO ill effect AT ALL.

I have three times now had differing Nano-Tech cell packs discharged so low that they measured only 0.0 to 0.2V with multi-meter. One 1600mAh 3S was exposed in a tree top for a rainy month after Tx RF failure, still connected to ESC. It was charged on NIMH to 3v per cell then switched to a normal LiPo 1C 3S charge. It charged, balanced, and has been flown and recharged now at least ten times since, with all normal duration, power, etc. (and its in a fast wing that sucks current).

NOT saying what you say is wrong, but my experience is very different. As they say, "Your Mileage May Vary".

[Steve Colman]

I found this on another forum. Whether reliable or not I can't say.

 

Quote:

"At what point should I be concerned about differences in internal resistances between cells?
I've searched around and here's all that I could find on "ideal" internal resistance measurements."

Quote:

Based on my online research, combined with my own experience and findings, I would say, as a general rule, a per cell rating of between 0-6 mΩ is as good as it gets. Between 7 and 12 mΩ is reasonable. 12 to 20 mΩ is where you start to see the signs of aging on a battery, and beyond 20mΩ per cell, you'll want to start thinking about retiring the battery pack.

Edited By Pete B - Moderator on 17/11/2016 22:18:46

[Steve Colman]

Also found this on the painless360 channel which seems to confirm the figures in my previous post.

Relevant section of video starts at 12:15

A

Edited By Steve Colman on 17/11/2016 09:27:17

Edited By Steve Colman on 17/11/2016 09:27:37

[Paris Skoutaridis]

Chris, what is acceptable in terms of IR depends on a pack's discharge current rating or, in other words on the combination of its capacity and its C rating. The higher the discharge current a pack is supposed to support the lower the IR will be. The worse performing cell in a pack will be the limiting factor.

There is a tool that was created empirically where you enter the highest IR of a pack as well as capacity and it returns an effective C rating (which you can use to derive a safe discharge current). If you punch in your readings there you should get an idea of whether a pack is doing well or is on its way out. You can find it here: http://jj604.com/LiPoTool/

Dave, I completely agree with the YMMV mantra. In fact you get differences not only between brands but also between batches of the same brand. However, I don't see how you managed to recover a pack showing 0.2V since a fully discharge Lipo cell only drops to 3.2v (resting, not under load) at which point it's already getting damaged. Did you check the actual capacity and steady state current draw from those packs after you recovered them?

One final general note. The C rating on all lipos ranges being an exaggeration of the truth to outright bs. E.g. a 4000mAh pack rated at 50C constant is supposed to be able to safely (I.e. no damage to the pack no excessive heat, and minimal voltage drop resulting from the gradual discharge as opposed to sag etc) support a discharge current of 200A.I can guarantee that almost none of the 4Ah packs rated at 50C can actually support 200A constant current without overheating and possibly puffing, both of which are signs of damage.The take away from this is that when buying lipos for a model it is generally a good idea to buy packs that are rated for about 10% to 20% higher constant current than the model requires if you want your packs to last long.

[Martin Harris - Moderator]

Just a thought - where were the batteries stored during the lay off? I understand that cold is good for storage and they deteriorate faster if stored in hot conditions (although perform better if warm!) Given your location, unless they were in an air conditioned house then it's quite possible that they may have been affected by the heat - especially if they were fully charged in cooler conditions just prior to storage.

Edited By Martin Harris on 17/11/2016 11:30:31

[Chris North 3]

I found the time to test all of the batteries this evening so here are the updated results. Note that the IR was measured for each pack using the IR mode on the charger. Each battery was checked a couple of times because I noted that the results were not always constant. That said the cells with the lowest rating were within 1mOhm each time while the highest IR cell could vary by 7 or 8. As can be seen there appears to be a duff cell in each pack.

Batteries 1 - 4 are all Gens Eco 2200 3S 20C , not Gens Ace as I said previously

Batteries 5 &6 are Mega+ EonX25 2200 3S

said

#1 => 74 / 40 / 27

#2 => 75 / 42 / 62

#3 => 18 / 26 / 41

#4 => 26 / 27 / 16

#5 => 28 / 7 / 9

#6 => 45 / 11 / 9

Chris

[Charles Smitheman]

Martin you may have hit the nail on the head.

[Simon Chaddock]

The IR of a LIPo cell should be less than 9.

Above 20 shows the cell has started to deteriorate.

Above 40 on any cell and the battery can still be used quite successfully but only in light duty applications.

Martin, you need some new batteries.

[Martin Harris - Moderator]

Mine are OK! Tested one this afternoon and I needed to use full throttle for the entire flight!

Edited By Martin Harris on 17/11/2016 17:56:45

[Paris Skoutaridis]

Hi Chris, those are some mighty high IR values! Based on that tool I linked earlier, the max safe discharge current for an IR of 75, 40 and 27 is 13A,18A and 22A respectively. Safe discharge current means they don't overheat and damage themselves even more.

Compared to the 44A and 55A theoretical rating you can see these batteries need replacing. According to the tool once more, a safe discharge at 44A and 55A from a 22000mAh pack translates to an IR of Around 7 or 5 respectively.

If it were me I would retire those batteries.

[Andy48]

The next question is why these have gone in just a couple of years. I note you don't have a wattmeter, and I think this is essential to ensure you are not drawing too much current from them.

[Chris North 3]

Well it looks like a trip to the model shop and the tip!

Thanks for all of the feedback and information. I think I will invest in a watt meter to monitor what is going on and see if I can keep some kind of log on IR and battery usage for the next set.

Chris

[Martin Harris - Moderator]

Posted by Steve Colman on 17/11/2016 09:16:59:

I found this on another forum. Whether reliable or not I can't say.

Quote:

"At what point should I be concerned about differences in internal resistances between cells?

As nothing can be done to recover an aged LiPo I simply apply the "if I don't get sufficient performance and duration for the model being flown I'll throw it away" principle.

I also had a fully discharged (virtually 0v per cell) pack which had spent a couple of weeks nesting in a tree and recovered it by a similar method to Dave's - although maybe a little down on ultimate performance, it certainly passed the above test.

[Cuban8]

Worth a watch when you have a moment. **LINK**

[rcaddict]

how do you test IR

[Charles Smitheman]

On the subject of why these packs may have deteriorated, I understand that as the temperature of a lipo increases the voltage increases too, so a pack charged to 4.2v per cell could over voltage if left in a hot car or in the sun.

After all we all know that dogs die in hot cars, Cars can easily get very hot, even here in the UK. So where Chris lives this would be even more of a potential problem.

One of the steps that I took after having so many packs fail was to keep them in lipo sacks, in a cool bag and keep them out of direct sunlight at the field. In winter I put a hot water bottle in the coolbag with them.

Perhaps Nanos fail because of their dark blue wrappers allowing them to overheat in the sun faster than their pale blue brothers?

Could this be a reason why some of us have had these inexplicable failures.

Paris I am interested to hear if you think this could be relevant?

[Paris Skoutaridis]

Gents, I'll repeat my original "disclaimer" just in case it was missed for avoidance of doubt. I am by no means a LiPo expert and what I state here is either paraphrasing things I have read elsewhere (that were proven one way or another to be correct), or things I have deduced myself from a combination of experience and the online knowledge base available.

Chris, before taking that trip to the tip you need to make sure that the batteries are fully discharged. I wouldn't suggest the "salt water" or "pierce them and let em burn" approaches as the first isn't guaranteed to fully discharge them and the second is dangerous, even the fumes coming out of a burning lipo are very very toxic. All lipos need to be fully (as in 0 voltage or thereabouts) discharged before disposal as only in that state are they incapable of igniting.

Oldgit to test IR you need a LiPo charger that has that facility or a purpose built device such as Wayne Giles' ESR meter. The latter has been discussed in other forums extensively and shown to be the most accurate IR measurement device.

Charles, I haven't heard of or noticed LiPo voltage varying with temperature. What has been shown to vary with temperature is the ease with and speed at which a LiPo's internal chemical processes take place. So, a LiPo at full charge deteriorate faster than a LiPo at storage charge, but both deteriorate. Both will deteriorate faster yet at high temperature than at a lower temperature.

As far as nanotechs are concerned, from what I seem to remember reading elsewhere it's their internal design that makes them more sensitive to any abuse. Basically something internally (I forget what) is designed thinner than in non-nanotech packs in order to reduce internal resistance and improve discharge rating. It is this design feature that actually makes them less resilient to abuse apparently. If I can find where I originally read this I will post the link here for reference.

Other than that, I think your approach of keeping the packs cool in summer and warm in winter until you're ready to use them is a good idea. Especially in low temperatures, LiPo internal resistance increases quite a bit (Turnigy Graphenes have been shown in another forum to be more resistant to this, and brands other than TUrnigy have now started producing their versions of Graphene batteries) which means that when you use them they are at their least efficient at the flight phase that is most likely to require the highest current (full throttle) - take off. So what has been shown to happen is that the combination of high IR and high draw causes their temperature to increase rapidly, which in turn then causes the IR to reduce to more reasonable levels until some sort of equilibrium is reached. The problem is that this rapid increase in temperature at the beginning of the flight damages the battery to some extent.

[extra slim]

As oldgit requested, my charger measures IR, but by looking online I could not find any form or equation to bring meaningful practical decisions to a reading. I recently was given an aircraft from the old man to give it some air, has a 5 cell nimh rx pack in it which hasnt been used for a year or so, 2600mah... what is OK, what isnt?.. On this thread people mention IR must be below 20milli ohmns per cell, but is that the same for 1000mah and a 8000mah pack???.. It appears to be a black art... but I'm sure it cant be that difficult..

[Martin Harris - Moderator]

In simple terms, internal resistance relates to the amount of current that a cell can deliver without getting excessively hot. The lower the resistance the more current can/will flow.

NiMH technology is different to LiPos and the internal resistance of a good pack will be different from the figures given in this thread. However, old neglected NiMH packs are not a great prospect. While good quality NiCds could last for many years, NiMHs - especially high capacity ones - rarely last more than a couple of years before showing signs of decline. As an example, I recently tried to use an old 800mAh AAA pack which I cycled several times - the capacity was significantly lower than the nominal value but discharging at 200mA gave me a consistent 500mAh discharge over several cycles and it took a 300mA rate without excessive voltage drop. I was going to fly a small model with it (3 servos) but as soon as I connected it my telemetry reported low signal quality and the servos refused to move consistently. Testing with a different battery gave normal results.

Initially, I put the behaviour down to using miniature digital servos but trying the battery on a model with analogue servos gave the same result. The reported voltage was stable at 5.something volts but I believe the figure displayed was probably the last measurement before signal/telemetry loss so it seems that transient demands were pulling the voltage down unacceptably.

The bottom line is to treat any older or unmaintained NiMH with great caution - i.e. bin it!

P.S. A quick google suggests cell internal resistances in the order of 50 milliohms for typical freshly charged NiMHs - but the IR increases as the cell is discharged, particularly towards the end of the cycle. As there are not normally any facilities to monitor individual cells, any reading from a charger would be for the pack as a whole.

Edited By Martin Harris on 18/11/2016 11:38:12

[extra slim]

Thanks Martin, so if I put my pack on my charger and measure IR, as the Nimh pack is 5 cell, if the IR is 250 milliohms of less (250/5 = 50) then it should be fine?, OK... what if it is 70milliohms, or 100.... where do I draw the line??... I suspect many people are happily flying with Nimh packs unaware of IR rating, and suspect, that the IR's are probably higher than the 50... I'm looking looking for a cutoff figure, since I can measure it..something basic, like, IF IR >65 milliohms per cell, but < 80, dont use of him drain applications, i.e. multiple digi servos, but OK for analogue 4 servos, but IF > 80milliohns..=. bin

[Martin Harris - Moderator]

I'd think that you need to talk to the manufacturer (not necessarily the assembler) of the cells in your pack to get an authoritive answer on that one!

[Charles Smitheman]

https://dl.dropboxusercontent.com/u/2590535/PMA/OptiPower/Charging%20and%20keeping%20safe%20with%20Lithium%20Cell.pdf

Hello Paris,

From Optipower's website this is the link to their Charging and keeping lipos safe page. (under downloads)

Under the last heading "Storage" it says as follows:

"If you are not using a lithium cell it should be stored at what is known as the „Storage‟ charge this is 3.84v/cell or around 42% of full capacity, good brands of charger have a „storage‟ charge function. This will allow the cell to cope with temperature change whilst stored. Always try to keep cells in a fairly stable temperature environment"...

So this infers that a lipo may not be able to cope with a temperature change when fully charged.

Also My Cellpro charger does a "cold weather" charge and stops before full, indicating a percentage of full charge at termination.

I found the page to be well written and informative for any user of any brand of lipo.

Charles

[MattyB]

It is interesting that reputable RC battery manufacturers themselves believe in the damaging effect of storing Lithium Ion packs at elevated states of charge, as do electric car makers (Nissan found this out to their cost with the Leaf; more technical info here) and consumer electronics providers (chargers in laptops and mobiles are often configured not to charge their batteries to the full 4.2V in order to extend their cycle life). If you go over the Batteries and Charging forum on RCGroups there is also a wealth of users who have very demanding applications for their packs (high amp discharge in EDFs and RC boat racing for instance), and all of them seem to believe in reducing unused packs back to storage too to maintain their performance for as long as possible.

All of the science backs this up - dendrite growth at the anode occurs more quickly at higher cell voltages, and there are studies available that show capacity degradation can be pretty severe in the medium term if stored at full voltage (as capacity reduces IR increases too, reducing the batteries ability to provide the current requested by the powertrain):

Clearly the real world effect of degradation through storage at full charge on battery life will also depend a lot on patterns of usage - if you fly regularly (every 2-3 days) in a cool climate like Britain it probably is pointless returning your packs to storage, but if you only fly every other weekend it becomes much more important. Others in this thread have a different view, but in my opinion the OPs packs were held at elevated voltages for a significant enough percentage of the 1-2 years that they have been used in a pretty hot climate, and the combination of those factors has aged them prematurely.

[MattyB]

PS - As noted in the table above, not all Li Ion packs are the same. There is a fair variety of construction types amongst those we use, but it does seem in general that the higher C rated packs are more vulnerable to degradation due to high voltage storage, presumably because they have more of the gel layer in them through which dendrites can grow. Perhaps this is the reason some people do not see any significant degradation over time, though with so many other variables in play in addition to the storage voltage (cell construction, variable QC, discharge and charge rates, number of cycles, time between discharge/charges, ambient temperatures etc etc) it is exceptionally difficult to prove anything through comparison between different peoples results in this area without taking a truly scientific approach and controlling most of these variables.