# Are You Guilty of Charging your Battery?



## Sunking (Aug 10, 2009)

Now that I have your attention are you guilty of charging your battery after every use? 

Well if you are Lead Acid Battery user you should as anything less than 100% SOC starts the sulfate process.

But if you are a Lithium battery user charging after every use can be counter productive. 

I am fortunate, my little LSV, modified 50 mph golf cart has pretty darn good range of 40 miles. Typical day is 5 to 7 mile joy ride around the golf course and lake where I live, I can go a week or more in between charges. I run my battery down to about 10 to 20% SOC before I will charge it up. 

Catching my point yet?

If I were to charge after every time I used the cart I could go through 5 to 6 cycles in a week when only 1 cycle is required. That is 5 or 6 cycles applied to say 1000 cycle life.


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## samwichse (Jan 28, 2012)

Sunking said:


> If I were to charge after every time I used the cart I could go through 5 to 6 cycles in a week when only 1 cycle is required. That is 5 or 6 cycles applied to say 1000 cycle life.


I don't think this is how charge cycles work.


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## EVfun (Mar 14, 2010)

Yes, but I stop at 3.5 volts and 0.05C so it isn't quite a full charge.


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## Duncan (Dec 8, 2008)

The question is
does a full -100% to 20% to 100% cycle cause more or less aging than five cycles 100% - 84% - 100%

And is Evfun correct that a five cycles 90% - 74% - 90% causes even less aging


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## Tesseract (Sep 27, 2008)

Sunking said:


> ...If I were to charge after every time I used the cart I could go through 5 to 6 cycles in a week when only 1 cycle is required. That is 5 or 6 cycles applied to say 1000 cycle life.


Like samwichse said, this isn't how cycles are calculated for lithium. In fact, it's almost exactly opposite from what you have surmised: you get more total usable amp-hours (or watt-hours, if you prefer) from a lithium cell if you limit the depth of discharge each cycle.

For example, the CALB 100Ah SE cell datasheet states life is 2000 cycles at 100% depth of discharge or 3000 cycles at 80% DoD, or 200k total amp-hours at 100% and 240k amp-hours at 80%.

And the trend generally continues as average DoD for each cycle is shallower, which makes sense given that most lithium chemistries have an effectively indefinite "shelf life" (ie - 0% DoD).


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## Carnut1100 (Jan 13, 2015)

Yes I do. I live 40km from town in a hilly area and a return trip in the imiev uses two thirds or so of the pack, so it gets topped up each time. 
When I was commuting 10km to work I would top up every few days.


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## Moltenmetal (Mar 20, 2014)

As long as you have a BMS to limit your top of charge on every cell, topping up should be better than charging less frequently especially if you're running below 70% DOD to do so. Charge cycle life falls steeply with DOD. However, time at high voltage does oxidative damage to the electrolyte, as does time at high temperature, and the degradation depends fairly strongly on cell and electrolyte chemistry- there is big benefit given by certain electrolyte dopants. Setting a safe top of charge threshold and avoiding shunt charging both seem to be good ideas for extending cell life based on the research I've done.


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## piotrsko (Dec 9, 2007)

Uh.... lead acid starts to sulphate the moment the electrolyte is added. It is kind of the process to freeing electrons for use. A FLA will eventually sulphate to death even if it is on trickle charge with a pulse added. It will last longer / shorter if treated differently. FLA that doesn't sulphate to death shorts internally due to either plate failure or sulphate shorting the adjacent plates.


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## Sunking (Aug 10, 2009)

Tesseract said:


> For example, the CALB 100Ah SE cell datasheet states life is 2000 cycles at 100% depth of discharge or 3000 cycles at 80% DoD, or 200k total amp-hours at 100% and 240k amp-hours at 80%.


I only operate between 10 to 90% DOD and quite well aware of that. I would never go to 100%


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## Sunking (Aug 10, 2009)

piotrsko said:


> Uh.... lead acid starts to sulphate the moment the electrolyte is added. It is kind of the process to freeing electrons for use. A FLA will eventually sulphate to death even if it is on trickle charge with a pulse added. It will last longer / shorter if treated differently. FLA that doesn't sulphate to death shorts internally due to either plate failure or sulphate shorting the adjacent plates.


You are not talking about sulphate at 100% float charged and above, you are talking corrosion and flaking plates on that side of the knife edge.


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## piotrsko (Dec 9, 2007)

I have several sulphated examples that have never seen any load and just sat on trickle while waiting as a backup battery. They all fail moments after applying any load, even a few milliamp. Probably why one does a load test.


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## IamIan (Mar 29, 2009)

Tesseract said:


> Like samwichse said, this isn't how cycles are calculated for lithium. In fact, it's almost exactly opposite from what you have surmised: you get more total usable amp-hours (or watt-hours, if you prefer) from a lithium cell if you limit the depth of discharge each cycle.
> 
> For example, the CALB 100Ah SE cell datasheet states life is 2000 cycles at 100% depth of discharge or 3000 cycles at 80% DoD, or 200k total amp-hours at 100% and 240k amp-hours at 80%.
> 
> And the trend generally continues as average DoD for each cycle is shallower, which makes sense given that most lithium chemistries have an effectively indefinite "shelf life" (ie - 0% DoD).


I wonder .. Wouldn't dSoC from middle be better than dSoC from top ?? ... In terms of the total life time usable cycle AHrs you described ??

ie .. if both experienced the same , say ~20% dSoC .. 
80-100 at top
40 - 60 at middle
etc...
Either way it's the same relative shallowness of any one 'cycle' ~20% dSoC

Which end of the SoC would yield the best overall for the total life of the battery for a given dSoC amount ?

My thought on the middle comes from:
The OEM recommendations for long term storage are usually toward middle SoC .. not upper SoC .. A123 specifically calls for 50% SoC ... supposedly this middle point is better in the long run than either top or bottom for the longest possible battery years of service life.


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## Sunking (Aug 10, 2009)

IamIan said:


> My thought on the middle comes from:
> The OEM recommendations for long term storage are usually toward middle SoC .. not upper SoC .. A123 specifically calls for 50% SoC ... supposedly this middle point is better in the long run than either top or bottom for the longest possible battery years of service life.


I work with a lot of Lithium types. All of them recommend storage at roughly 60%. Anywhere near the top stresses them from everything I read and know. That is the main reason I never charge to 100%.


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## Tesseract (Sep 27, 2008)

IamIan said:


> I wonder .. Wouldn't dSoC from middle be better than dSoC from top ??...


That sounds like a reasonable hypothesis to me, but absent statistically significant data from cycling many cells many times it must remain just that: a hypothesis.


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## major (Apr 4, 2008)

Tesseract said:


> That sounds like a reasonable hypothesis to me, but absent statistically significant data from cycling many cells many times it must remain just that: a hypothesis.


 I believe there is a drift in general consensus to the theory that the primary factor in cell life is time spent at high voltage at high temperature. I haven't stated this very eloquently, but basically full charge like 4.1-4.2V/c (NMC) and upper ambient like 40ºC.

A guy on the EVDL got all over my case because EnerDel hadn't tested using HPC (high precision calorimetry). Geeesss. The technique is only a year or two old. Anyway, some interesting stuff there. https://www.youtube.com/watch?v=pxP0Cu00sZs 

It says basically the side reactions kill the electrolyte and/or electrodes many times faster when at the top of the voltage range and even faster at elevated temperature. This is why you see storage of Li batteries at 50-60% SoC. It also gives rise to altered charging protocols like doing that final hour or two of EV charge just prior to use rather than the conventional method of full charge upon returning home and leaving at 100% SoC ready for the next use.

This does not imply damage is not done at low SoC, extreme DoD, excessive C-rate, etc.


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## tomofreno (Mar 3, 2009)

Hi major, I posted that link on EVDL about the same time I posted it in chit chat here:
http://www.diyelectriccar.com/forums/showthread.php/why-li-ion-batteries-die-136777.html

Very interesting I agree, but I don't think there is much new there as far as battery degredation goes. The video many of us watched several years ago from Jay Whitacre at Carnegie Mellon said similar things. He stated that the electrolyte starts to break down above about 4.3V and said to stay below 4.0V and 50 - 60 C for longer battery life. Probably depends on electrolyte composition, temperature...

Btw, he has founded a storage battery company:
http://www.aquionenergy.com/energy-storage-technology

The guy that jumped on you watched the utube and called Dahn. He is eager to debate all things battery. I now just drive and once in a while check for anything new.


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## GerhardRP (Nov 17, 2009)

major said:


> A guy on the EVDL got all over my case because EnerDel hadn't tested using HPC (high precision calorimetry). Geeesss. The technique is only a year or two old. Anyway, some interesting stuff there. https://www.youtube.com/watch?v=pxP0Cu00sZs


I emailed Jeff Dahn who is giving the lecture and he responded:
Gerhard,

Please take a look at the attached paper which explains why higher voltages are bad. Electrolyte oxidation occurs more strongly at top of charge.

At the very bottom of discharge (say 5% and below), any imbalances between cells in series strings may lead to overdischarge of one ot more cells (bad).

I believe the most important things to extend Li-ion battery life are:
1. Avoid exposure to elevated temperatures (>35C)
2. Limit charging voltage per cell to less than 4.05 V (Less than 4.0 V is better) (Note: SAFT Li-ion cells for satellite use have a DEMONSTRATED actual test life of 20 years and still going at about 80% initial capacity. The upper cutoff for these (NCA - like Tesla cells) was 3.9V in the SAFT tests).
3. Avoid highly rapid charge (less than 1 hour) which may cause Li plating in an aged pack.

I do not think running 80% to 10% would be any worse than 80% to 30%, for example.

Hope this is helpful.

Jeff


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## GerhardRP (Nov 17, 2009)

GerhardRP said:


> I emailed Jeff Dahn who is giving the lecture and he responded:


And this in response to a question of LFP batteries:
"You can post my reply if you like. Please note that LiFePO4 cells will not give extra longevity if the cells are exposed to T > 40C. They are truly terrible at high T. They also have terrible energy density. No serious EVs use LFP anymore (i.e. Telsa, Leaf, Volt, plug-in Prius, etc.)."


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## Sunking (Aug 10, 2009)

GerhardRP said:


> I emailed Jeff Dahn who is giving the lecture and he responded:
> Gerhard,
> 
> Please take a look at the attached paper which explains why higher voltages are bad. Electrolyte oxidation occurs more strongly at top of charge.
> ...


Jeff thanks for the link. Unfortunately not a breath about LiFeP04 cells.


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## Sunking (Aug 10, 2009)

GerhardRP said:


> And this in response to a question of LFP batteries:
> "You can post my reply if you like. Please note that LiFePO4 cells will not give extra longevity if the cells are exposed to T > 40C. They are truly terrible at high T. They also have terrible energy density. No serious EVs use LFP anymore (i.e. Telsa, Leaf, Volt, plug-in Prius, etc.)."


While I agree LFP is a poor choice for commercial uses, DIY normally cannot afford anything other than LFP. At least LFP has longer cycle life than cobalt and not so sure Lithium Manganese use in Volts or Leafs is any better than LFP in temp performance.


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## Hollie Maea (Dec 9, 2009)

Sunking said:


> While I agree LFP is a poor choice for commercial uses, DIY normally cannot afford anything other than LFP. At least LFP has longer cycle life than cobalt and not so sure Lithium Manganese use in Volts or Leafs is any better than LFP in temp performance.


Cobalt Oxide has very poor cycle life. Manganese oxide, as is in the first generation Leaf, is also very poor. NMC, like in the Volt and now in the new Leaf batteries, rivals LFP. NCA, if done properly, also rivals LFP and has the benefit of being nearly as good as Cobalt Oxide in terms of energy density.

LFP is a good hobbyist solution though.


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## Moltenmetal (Mar 20, 2014)

I'd like to know what the mechanism behind the battery vendors' claim that cycle life decreases with increased DOD. Sinopoly lists 1000 cycles at 90% DOD, 2000 at 80% and 3000 at 70%, and other vendors list similar tables, but I've never seen an explanation for that. Some feel that this is related only to reducing the risk of cell reversal if cells go below the LVC, but I'd like to know if somebody understands the failure mechanism. Somehow, that explanation runs afoul of my commonsense a bit, but that's just a gut feel and not based on any data.

As I said before, from what we know about electrochemical damage to the electrolyte, shunt charging to stuff an Ah or two more into a few cells at top of charge would seem to be a bad idea for cycle life. Setting a conservative HVC is also a good idea for that reason. That said, I doubt the voltage of a resting cell post charge is high enough, except at very high temperature, to do any significant damage to the electrolyte or electrode materials. The degradation reactions likely have an activation energy that needs a certain threshold voltage before substantial reaction occurs, and it's doubtful that this would happen quickly enough to worry about except during charging. 

As to long term storage, I suspect that 50% just reduces the risk that cells trickle down to the LVC during storage due to parasitic losses and self discharge- as low as that is with the Li-ion chemistries it is non-zero.


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## dcb (Dec 5, 2009)

Well it is a chemical process, so it probably isn't real binary. The dramatic effects of overcharging (plating) are probably happening to a degree even before full charge happens, and the breakdown of materials on discharge too. The less you push it to the extremes, the better.

Just guessing.


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## Moltenmetal (Mar 20, 2014)

Sounds logical. The process isn't "binary", but there is a threshold below which reactions are so kinetically limited that they happen too slowly to be of concern. As an example, all forms of carbon including diamond spontaneously reacts with oxygen to form CO or CO2- but at room temperature it happens so slowly that it can be completely ignored.

It sounds like there's more to this low SOC effect on cycle life than merely the risk of bottoming out and reversing an individual cell. Somebody out there knows what it is! C'mon, spit it out!


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## dcb (Dec 5, 2009)

Rickard made a video with some diagrams as he understands it, which is probably pretty good.

http://evtv.me/2009/11/get-rid-of-those-shunt-balancing-circuits/


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## wb9k (Apr 9, 2015)

In LFP anyway (and I'm speaking about A123 cells specifically but this applies to others as well), the dominant mechanism you are trying to avoid at high SOC is growth of the SEI layer (Solid/Electrolyte Interface), a film that grows on the surface of the electrodes, slowly trapping more and more cycleable Li, reducing the capacity of the cell. This phenomenon is accelerated when the cell is at higher SOC's. This is the reason for the 50% SOC storage recommendation--the SEI layer growth will be mitigated while also leaving lots of room for self-discharge during storage so damage via overdischarge is unlikely as well. Even better during storage is to keep the cells below 50 degrees F (10 degrees C). The low temperature arrests SEI layer growth completely, basically holding the cell in a state of "suspended animation", where there is virtually zero loss of capacity while on the shelf, regardless of SOC.

In plug-in applications, there isn't a whole lot you can do to mitigate this mechanism. Having your cells charged and ready to use stands in direct opposition to maximizing calendar life in this regard. You could lightly chill cells at all times, but this will hurt performance a bit (cell impedance goes down with temperature rise) and would also cost a good deal of energy. For now, SEI layer growth is a fact of life we must deal with. There is a lot of research these days on the SEI layer, and there have been recent innovations that allow real-time observation of the SEI layer in working cells. Perhaps this will yield some improvements in the next few years. 

In HEV applications, you do have some recourse against this mechanism if you can oversize the pack enough to have a "target SOC" of 25-35% or so. There is a (series) hybrid bus program A123 has that has shown this quite clearly. Having a low target SOC yields significantly longer working calendar life than having a target SOC of 70-80%. It's been a while since I've looked at the data, but I want to say the difference is on the order of 20% or so. 

Keep in mind that this is just one factor in a very complex scenario. If you want your EV/PHEV ready for use at all times, you are pretty much stuck with the effects this has on the SEI layer. There are other things you can do to extend cell life, however. Using level 1 charging whenever possible helps. Cell should always be charged as slowly as possible while still giving the service needed in the time allotted. C/5 is a good target charge rate when feasible. Fast charging SHOULD NOT be in the daily routine for anyone unless they absolutely must have it. Avoiding very high temperatures in a pack is also critical--use active cooling if you must. If cell temps are regularly reaching 40 degrees C, it's too hot. Heat is a life-killer--paradoxical when you consider that cells perform the best at high temps. Finally, don't overcharge. For LFP, I try never to have a terminal voltage higher than 3.650 Volts at any time, and a fully charged cell should come to rest at a maximum of 3.600 Volts. You are plating Li onto the cathodes (permanently) above this level. The higher you go, the worse it gets. 

Those are the dominant mechanisms, and the best ways I know to deal with them. Hope that helps clarify things a bit.


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## wb9k (Apr 9, 2015)

Moltenmetal said:


> I'd like to know what the mechanism behind the battery vendors' claim that cycle life decreases with increased DOD. Sinopoly lists 1000 cycles at 90% DOD, 2000 at 80% and 3000 at 70%, and other vendors list similar tables, but I've never seen an explanation for that. Some feel that this is related only to reducing the risk of cell reversal if cells go below the LVC, but I'd like to know if somebody understands the failure mechanism. Somehow, that explanation runs afoul of my commonsense a bit, but that's just a gut feel and not based on any data.
> ...


Good question. I believe the answer lies in the fact that the lower 20% or so of a cell's capacity is the portion that takes the most work from the cell to source. There are several things at work here. Chiefly, cell impedance is rising at a significant rate because the electrochemical scenario is changing when SOC is this low and falling. This causes temperature to rise, which helps keep impedance somewhat in check, but imposes stress on the cell in other ways. Falling SOC will raise impedance faster than the heat will keep it down, so the heat rise tends to continue to get worse as 0% SOC is approached. At the same time, your controller (or you) may be trying to keep power constant under a load that hasn't changed. With falling voltage, this means pulling more current from the battery. This amplifies the losses from rising impedance, causes more heat rise, more almost-abusive conditions that only worsen until you just run out of power. Doing this regularly takes a toll on cells. Occasionally doing this might not make much difference, but if you make a habit of if, expect to pay a noticeable penalty in calendar life. 

That's not a great explanation from an electrochemical viewpoint, but it should be pretty good for the typical end user.


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## Sunking (Aug 10, 2009)

dcb said:


> Well it is a chemical process,


Not for lithium as it is ion exchange from anode to cathode and back again.


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## dcb (Dec 5, 2009)

good point. Some of the degradation, reduction and oxidization, would be chemical, but pushing ions back and forth not so much.


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## wb9k (Apr 9, 2015)

There is tons of chemical stuff going on in a cell, and the Li plays a part in that, even if that means it's "extracurricular" to the role of ion exchange. If this were not the case, our batteries would last forever. Chemical processes that permanently trap previously cyclable Li are the leading causes of cell degradation absent abusive conditions. 

This is one reason that solid-state electrolytes are such a holy grail---no liquid electrolyte=no SEI layer=immortal cells.


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## Moltenmetal (Mar 20, 2014)

wb9k has given what I think is a very good explanation of what happens at low SOC, and hence why avoiding it is necessary to keep cycle life reasonably high.

The SEI layer seems to be partially composed of reaction products between oxidized electrolyte and various other components of the cell. It may tie up some lithium, but it also is a diffusional barrier which lithium ions have to move through- generating more effective internal resistance and more localized heat, which is of course autoaccelerating. It can also block off macropores in the electrode surface eventually, reducing capacity by capping off tiny sections of the cell.

Fortunately I have 180 Ah cells and a comparatively small charger, so charging at 120 VAC is less than C/10 and at 240 V is less than C/5. But while I understand that reducing the self-heating during charging by keeping charge currents low is a good idea, it would also tend to keep the cells at a higher voltage for longer. My BMS is set to approximately 3.68 V high voltage cutout and I'm going to stop charging after the first cell reaches HVC which should keep the shunt charging to a minimum.


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## wb9k (Apr 9, 2015)

Moltenmetal said:


> wb9k has given what I think is a very good explanation of what happens at low SOC, and hence why avoiding it is necessary to keep cycle life reasonably high.
> 
> The SEI layer seems to be partially composed of reaction products between oxidized electrolyte and various other components of the cell. It may tie up some lithium, but it also is a diffusional barrier which lithium ions have to move through- generating more effective internal resistance and more localized heat, which is of course autoaccelerating. It can also block off macropores in the electrode surface eventually, reducing capacity by capping off tiny sections of the cell.
> 
> Fortunately I have 180 Ah cells and a comparatively small charger, so charging at 120 VAC is less than C/10 and at 240 V is less than C/5. But while I understand that reducing the self-heating during charging by keeping charge currents low is a good idea, it would also tend to keep the cells at a higher voltage for longer. My BMS is set to approximately 3.68 V high voltage cutout and I'm going to stop charging after the first cell reaches HVC which should keep the shunt charging to a minimum.


Thanks for clarifying some of what I expressed more....crudely. 

Your balancing strategy should work well as long as SOH remains even across the pack. Over time, you may wish you had set up a means to make sure your delta V was limited across the pack before charging/balancing is stopped. What you describe here is blind to cells that lag behind the others over time due to variations in impedance and self-discharge characteristics. This can be disastrous in time, especially if there is no cell-level supervision for LVC. I suppose you could do LVC the same way you're doing HVC...that should alert you to an imbalance in plenty of time to correct the problem before it gets too gross.


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## Moltenmetal (Mar 20, 2014)

I have cell level LV warning rather than a cell level LVC interlock cutoff, but since my car is really designed for my commute (eventually!) I shouldn't really be challenging the bottom of the pack. My target is to always stay above 70% DOD on average. With the capacities of the cells not varying by more than about +\- 5% I should be at low risk of getting into trouble without any kind of extended top balancing. Several cells- whole groups of them actually, go into BMS shunting mode before any one cell hits the HVC and trips the charger- I just have no intention of topping the whole pack to an average of 3.65 V per cell which is where my charger is currently set to shut off.


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