How long can I keep my LiPos charged - a scientific approach - RC Groups
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May 18, 2017, 02:26 AM
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Julez's Avatar
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How long can I keep my LiPos charged - a scientific approach


Tl; dr:

If you charged up the packs and do not get to use them the very same day, putting them back to storage voltage immediately is better than even leaving the packs fully charged for only a single day.


Hi guys,

it seems that every other thread in this section is about the question how long it is ok to keep packs fully charged, before using them or putting them to storage voltage. Examples:
https://www.rcgroups.com/forums/show...rging-Question
https://www.rcgroups.com/forums/show...or-10-12-hours
While I appreciate that the importance of correct storage voltage can finally be regarded as mainstream knowledge, answers usually range from "a week is ok" to "do it the same day" or "whatever, but not too long".
As long as answers stay that vague, we'll have to endure this topic ad nauseum, so I'll now be trying to find an answer bases on scientific facts.

Basically, both options must be compared in regad to damage done to the battery:
a) discharging to storage and charging back up next time (damage done: Half a cycle)
b) keeping fully charged until next time (damage done: Stored fully charged for length of time)

First of all, we must determine how much damage a "half cycle" does. To put things in perspective, I assume an average lifespan of 100 cycles for our typical RC batteries. If you get more cycles, fine, this would mean that putting a pack to storage voltage soon is even more important.

So, after 100 (or more) cycles the battery only has 70-80% usable capacity left, and can be considered defective (100% damage). Reduced "usable capacity" means that either the actual capacity, the ability to store a specific amount of charge, is reduced, or that the internal resistance has risen to a point where the voltage drop is so severe that the capacity cannot be used at the usual discharge currents which were possible when new.
So, one complete cycle does 1% (or less) damage. This is just an estimate to see how much damage putting a pack to storage would cause.

This chart indicated that, when the depth of discharge is only 50%, only about half the damage is done, as one gets about double the cycles.:



Source.

So, how long does it take to get 0.5% damage by storing a pack at full voltage?

I'll use this chart:



Source.

After 3 months, the battery capacity is down to 80%, so the pack is defective (100% damage). This chart does not refer to self-discharge, what is reduced is the actual ability of the pack to store a charge (mAh value reduced).
3 months is roughly 100 days, so each day of storage at full charge voltage does 1% damage. Half a day would do 0.5% damage.

So, there you have it: If one assumes one's packs last 100 cycles, one should put them to storage charge if one does not plan to use them within the next 12h. Even when one used them the very next day and keeps them fully charged for 24h, one would do double the damage compared to putting them to storage voltage immediately and charging back up the next day.
One also has to keep in mind that putting a pack to storage voltage usually happens at a slower (dis-) charge rate than when the pack is used, so it does (much) less damage than a normal high-current cycle.



Source.

This factor should compensate the fact that a pack is also damaged (but only slightly) when being stored at storage voltage, which was not considered in above calculations, and further move the insight in the direction of "put to storage voltage rather sooner than later".

So, to sum things up: If you charged up the packs and do not get to use them the very same day, putting them back to storage voltage immediately is better than even leaving the packs fully charged for only a single day.

Cheers,

Julian
Last edited by Julez; May 19, 2017 at 02:24 PM.
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May 18, 2017, 06:43 AM
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ggcrandall1's Avatar
Julez,

Thanks for the research. Nice piece of work.

Glen
May 18, 2017, 07:00 AM
Space Coast USA
hoppy's Avatar
Added to the sticky "Lipo Storage Voltage , How to RecoverDead Battery and DIsposal information." https://www.rcgroups.com/forums/show...0#post12583723
Post #1, bottom.
May 18, 2017, 07:27 AM
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Julez's Avatar
Thanks, mate.
May 18, 2017, 07:27 AM
Registered User
Quote:
Originally Posted by Julez
After 3 months, the battery capacity is down to 80%, so the pack is defective (100% damage). 3 months is roughly 100 days, so each day of storage at full charge voltage does 1% damage. Half a day would do 0.5% damage.
Overall you have some interesting information there. But this part is very debatable. Calling a 20% loss of capacity 100% damage is odd. But even stranger is assuming that the loss of capacity is linear with time. Almost nothing else about lipos is like that. It may be that it loses 4% in the first month and the other 16% in the next 2 months. Or do you have any scientific data that shows that the loss is linear ?

But even ignoring that, you are claiming e.g. after 1 day a battery has 1% damage and after 7 days 7% damage. But what you are calling 1% damage is a loss of capacity of around 0.20%....so a 5000mAh battery might have lost 10mAh and after a week at full charge maybe 70mAh.

And then I wonder if there is any evidence, scientific or otherwise, to show that the losses are actually cumulative. E.g. is leaving a battery charged for 1 day once a week for a year with charges and discharges in between really equivalent to leaving it fully charged for 52 days in one go ? Has anyone done the tests ?

It's no wonder we keep discussing this...scientific data simply doesn't exist (or at least I can't find it, and I have looked).

Steve
May 18, 2017, 08:45 AM
Electric only
rkopka's Avatar
Quote:
Originally Posted by slipstick
But even ignoring that, you are claiming e.g. after 1 day a battery has 1% damage and after 7 days 7% damage. But what you are calling 1% damage is a loss of capacity of around 0.20%....so a 5000mAh battery might have lost 10mAh and after a week at full charge maybe 70mAh.
My heli batteries are 5000mAh and are always stored fully charged with longer periods between flights. With this calculation they will be damaged completely after 1,5years. I can affirm that they still work. They might have lost a bit of power, but I fly the same program (about 60% discharge) as always.

RK
May 18, 2017, 08:47 AM
Proud to eat Kraut ;-)
Julez's Avatar
Hi Steve,

thanks for your interest in this topic.

Quote:
But this part is very debatable. Calling a 20% loss of capacity 100% damage is odd.
Well, we have to start somewhere. We could also say, 50% capacity loss is 100% damage. Or 27.8%. This would, however, not change anything if we assume that the losses are linear. I took 80% because that's the End-Of-Life criterion used by most. From what I have read here, they are not linear, but accelerating with age, and cumulative. Cumulation makes sense, especially considering the variety of damages a cell can have. Loss of capacity and increase of internal resistance are only some.
So, if a cell gets discharged with the same current during it's life, but the IR rises, this means that the C-Rate the cell gets discharged at rises as well, making the added damage more severe every cycle.

Considering that one mode of damage is growth of the Solid-Electrolyte-Interface which is irreversible, I'd say it is safe to assume that storage damage is cumulative. At least the figures in this presentation and paper point in that direction:
http://www.batteries2020.eu/publicat...E15/Ageing.pdf
http://jes.ecsdl.org/content/163/9/A1872.full

Concering the damage obtained from cycling, this it seems to be cumulative as well:
Quote:
The scientists investigated charged-discharged samples of Li2MnO3 during the first and 33rd cycles by X-ray absorption spectroscopy (XAS) using synchrotron facilities of BESSY II at HZB and DORIS at DESY. “The element selectivity of XAS provides a unique opportunity to probe electronic, chemical and structural changes occurring at and around individual atom types in a material. Such a combination of element-specific information is rather difficult to obtain from X-ray diffraction which provides average changes in long-range structure of a material”, says Rana. They observed oxygen removal from the material during the first charge and shearing of oxygen layers as a result of the Li+-H+ exchange. These phenomena were previously proposed by various research groups. “The cumulative effect of such repetitive shearing of atomic layers during cycling is that the material gradually loses periodicity in atomic arrangements and, as a result, the electrochemical performance of a battery degrades upon cycling”
Source.

So, all the sources I found suggest that the damage is at least linear, if not accelerating, and cumulative in all aspects. This means that the "rule of thumb" not to leave a pack fully charged for even a day will make sense throuout the whole lifespan of a pack.

RK, keep in mind that that the "100% damage" I use in my calculations does not mean "100% capacity lost". If 80% capacity loss is ok for your application, the pack still works of course, but one cannot conclude from that that a pack which still kinda works has not been damaged whatsoever. It would be interesting to know though how your current IR and capacity compares to the IR and capacity when the pack had when it was new.
Last edited by Julez; May 18, 2017 at 09:01 AM.
May 18, 2017, 12:05 PM
Space Coast USA
hoppy's Avatar
Lot's of backup to Julian's conclusion.
https://www.google.com/search?q=Agei...hrome&ie=UTF-8

Here's one of interest. http://jes.ecsdl.org/content/163/9/A1872.full
Last edited by hoppy; May 18, 2017 at 12:11 PM.
May 18, 2017, 12:22 PM
Registered User
I agree that 20% loss is a good point to say they are no longer good enough to use much. In my experience, the degradation is more related to IR increase than capacity loss though. It would be interesting to see how IR changes with storage and number of cycles. I have some very old Lipos that I stored fully charged for years and they still have almost full capacity but their IR has increased to the point that I can't use them for flight.
May 18, 2017, 01:53 PM
Registered User
So lemme toss this in to muddy the water... What about batteries that aren't used in high power applications? A rise in IR wouldn't be as much of a problem. Capacity doesn't seem to take as much of a hit as IR based on what's been said above. So how about lipos reserved for receiver power and flying surface control (say 1000-2200mah at 5-10 amps max) rather than direct motor power? Do any of these studies cover this or are all the studies related to high energy usage?
May 18, 2017, 02:53 PM
Wallop!
Martin7182's Avatar
Interesting stuff. But in practise the internal resistance is far more important than capacity. At least if you draw high currents, which for 99% of us is the case. Lipos build up internal resistance when they're charged, so keeping them charged is bad.

A 50% capacity lipo with almost zero internal resistance flies your plane half the time.
A 100% capacity lipo with too high internal resistance doesn't fly your plane at all.

Your flying style depends a lot. A 'dead' lipo can be perfectly fine for an electric glider while it is totally worthless for a ducted fan jet.

Martin
May 18, 2017, 03:06 PM
Senior to who? Member
crossup's Avatar
While this is interesting and points to the basic mechanisms involved, the chart on capacity loss is labeled "estimated" when you go to the source. Also Lithium chemistrys have both shared traits and dissimilar ones, the information presented is for various batteries and perhaps none of it is accurate for LiPo let alone HV and Graphene varients. Drawing conclusions as have presented are hardly any more scientific than what gets post here anecdotally.
The real wonder to me is that manufacturers dont give us this info which most certainly exists. Draw you own conclusion.
May 18, 2017, 03:09 PM
Registered User
Quote:
Originally Posted by Martin7182
Interesting stuff. But in practise the internal resistance is far more important than capacity. At least if you draw high currents, which for 99% of us is the case. Lipos build up internal resistance when they're charged, so keeping them charged is bad.

A 50% capacity lipo with almost zero internal resistance flies your plane half the time.
A 100% capacity lipo with too high internal resistance doesn't fly your plane at all.

Your flying style depends a lot. A 'dead' lipo can be perfectly fine for an electric glider while it is totally worthless for a ducted fan jet.

Martin
I agree far to many assumptions and variables to be of much use and then there is the cost vs convenience factor. Take quad batteries vs a 6s 5000, a quad battery being cheap and the other expensive, a quad battery can be destroyed in a single crash but 5000's are more protected and people tend to crash these less. So cost vs convenience is a factor for people as there is not much point spending time and effort on a cheap battery that can be destroyed in a single crash.
May 18, 2017, 03:32 PM
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crossup's Avatar
I agree with the theory and now, I have stopped storing at full charge. What I am here to say is full charge storage is NOT necessarily a quick path to capacity death. As shown, like many here, I have a LOT of lipos, from 10 mah 1S to 4S 2200 and have owned Lipos since their introduction to the hobby. Shown is a old 3s 2200 HK pack, which puffed on the first flight in 2008 and has been stored(and used) for 9 years at full charge. Yesterday it was flown for 6 minutes using 1600mah and came out of the plane with no detectable temp raise on a 90 deg day.
Until this year, I always flew packs until they noticably sagged, frequent to the point where landing was the only option. I have had plenty of packs die but to me its obvious, good packs take all manner of abuse and poor ones die for almost no reason. I see very little correlation with price or brand although I do not have anything like a representative cross section of brands. And yes, I do buy into the Turnigy Graphene hype....until I see a change in reports they are getting my business when they have a product that meet my needs- too bad they dont have small cells, that what I mostly buy.
May 18, 2017, 05:04 PM
tic
tic
thunderscreech
tic's Avatar
My uncles friend leaves his packs fully charged and has never had any problems so that's what everyone else should do.. ... Sorry I couldn't resist.. I think this article is a great starting point for lipo storage parameters in regards to time.. Personally, I always leave my packs at storage voltage and only charge at the field. I try and plan to not have any fully charged packs at the end of the day. If I do, I discharge when I get home.


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