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Another charging battery life thread

On a regular basis someone posts a question about how much they should charge their battery, does it hurt to charge to xx%?, etc. I've seen the graph below posted several times, usually without much explanation and the graph itself actually raises some questions.
1655349948285.png


Well, I finally found the source an read though the entire article. For reference, the source is here. The general some high points of the article were:
  • The depth of discharge determines the cycle count of the battery. The smaller the discharge, the longer the battery will last.If at all possible, avoid full discharges and charge the battery more often between uses.
  • 80% DOD will degrade 30% after 300 cycles, but 40% DoD will last 1000 cycles.
  • Li-Ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage.
  • At 25ºC, a battery kept at 40% will degrade 4% after one year, a battery kept at 100% will degrade 20%.
  • Most Li ions charge to 4.2V. Every reduction in peak charge voltage of 0.1V doubles the life cycle. Every 70mV reduction lowers the overall capacity by 10% (≈15% for 0.1 mV)
  • In terms of longevity, the optimal charge voltage is 3.92V (=60%)
  • Increasing the cycle depth also raises the internal resistance of the Li-ion cell. The resistance increase is permanent.
  • The cycle count on discharge stress test (the graph above) differs with the battery type, charge time, loading protocol and operating temperature so comparisons are difficult.
 
Ok. So what is a Dynamic Stress Test cycle?
A predefined charge-discharge cycle.
Normally Li battery life is rated in terms of complete discharge cycles (i.e. discharging it from 75% - 25% twice would count as 1 cycle. In a dynamic stress test (my understanding is) they pick a start and stop charge level and a discharge rate to run the test at.
 

Hiline

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A predefined charge-discharge cycle.
Normally Li battery life is rated in terms of complete discharge cycles (i.e. discharging it from 75% - 25% twice would count as 1 cycle. In a dynamic stress test (my understanding is) they pick a start and stop charge level and a discharge rate to run the test at.
Wow this is misleading. Because 2k DST cycles at 40% depth of charge would be the equivalent of 4k DST cycles at 20% depth of charge, in terms of full cycle equivalents. Why didn’t they just translate all of these to full discharge cycles?
 
Wow this is misleading. Because 2k DST cycles at 40% depth of charge would be the equivalent of 4k DST cycles at 20% depth of charge, in terms of full cycle equivalents. Why didn’t they just translate all of these to full discharge cycles?
Agreed and I don’t know. That’s why I took the time to read the entire article. I’ve seen that graphic posted several times and it doesn’t really say what it appears to at first blush.
 
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Ok so 25%-75% seems to be the best discharge cycle for battery longevity?
Well, probably 40-60%, but that only gives you about 60 miles of range. there shouldn't be much more degradation with 25-75% and it's much more useful. In practice, I typically charge it when it gets to 40% or so but it also depends on what my plans are for the next day. Charging in my garage is so stupid convenient that I end up having a pretty low threshold for charging. Unless I forget to plug it in. 🤪
 
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Zoomit

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Wow this is misleading. Because 2k DST cycles at 40% depth of charge would be the equivalent of 4k DST cycles at 20% depth of charge, in terms of full cycle equivalents. Why didn’t they just translate all of these to full discharge cycles?

Agreed and I don’t know. That’s why I took the time to read the entire article. I’ve seen that graphic posted several times and it doesn’t really say what it appears to at first blush.
Awhile ago I digitized the graph, converted each dataset to a polynomial and replotted against equivalent Model 3 miles. Here are the results. Enjoy!

Original graph:
Screen Shot 2022-07-05 at 9.26.54 PM.png


Replotted graph:
Screen Shot 2022-07-05 at 9.24.08 PM.png
 
Or, just keep it at 60% indefinitely and don’t EVER drive the car! Me, I bought it to drive, a lot, and I do, and won’t worry otherwise. Car will be long gone by the time battery life concerns materialize.
Agreed. @zoomit‘s post does a great job of illustrating what all this means in practical terms. Look at the ‘worst case’ of charging to 100% and discharging to 25%. At 200k miles the M3 would still be at about 90% range. If any car has 90% of it’s capacity (or is even on the road) at 200K miles you’re doing pretty dang good!

My big take away from this is that I charge to 80% and try not to let it get too low but I don’t stress about it and if I need to charge it up for a trip then I do, because, like you, I got the car to drive it.
 
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Yep. I take this as reassurance, and changed my habits to charge as often as I can tolerate and live in the 40-70% range. When I have trips, I don't stress charging it to 95% or even 100% for a short period, draw it down to 10% or lower if needed etc, as it's just sporadic.

The difference at 200k miles between being gentle or being harsh is ~3%, it's not like you'll kill your battery. Now this is at 20C which is great for where I live, but it's possible that in very warm climates this becomes a bit more important.
 

Hiline

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Yep. I take this as reassurance, and changed my habits to charge as often as I can tolerate and live in the 40-70% range. When I have trips, I don't stress charging it to 95% or even 100% for a short period, draw it down to 10% or lower if needed etc, as it's just sporadic.

The difference at 200k miles between being gentle or being harsh is ~3%, it's not like you'll kill your battery. Now this is at 20C which is great for where I live, but it's possible that in very warm climates this becomes a bit more important.
Yeah that's the thing. 20C is not a very practical temperature to keep the battery at, and the study doesn't build in calendar aging. I assume degradation in reality is way worse that the chart would suggest - I have often heard that degradation in the first year is typically 5%-10%.

We can easily see the trends separating into two clusters. In the top cluster, 25%-75% seems to be the most balanced between longevity and practicality, let alone easiest to remember, so I'm going to call this the rule of thumb. So, generally, try to not go too much over 75 or too much under 25, and charge as frequently as possible to minimize the depth of charge, and we're good to go. I highlight generally, because there will definitely be times when I need to expand the range on a temporary basis. On a daily basis, 25-75 is the safety zone for me.
 
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Agreed. @zoomit‘s post does a great job of illustrating what all this means in practical terms. Look at the ‘worst case’ of charging to 100% and discharging to 25%. At 200k miles the M3 would still be at about 90% range. If any car has 90% of it’s capacity (or is even on the road) at 200K miles you’re doing pretty dang good!

My big take away from this is that I charge to 80% and try not to let it get too low but I don’t stress about it and if I need to charge it up for a trip then I do, because, like you, I got the car to drive it.
There will be calendar degradation on top of cyclic degradation. But the optimal for calendar degradation---keep state of charge low and keep battery cool---isn't much different than for minimizing cyclic degradation. For calendar in particular, low SOC is better universally it seems.

I now put my maximum charge limit at 50% (there is a long thread on the Model 3 Battery & Charging board) and recharge frequently when I am able to do that. When I need to charge higher, I do and don't stress.

The converted x-axis in equivalent miles shows that most people with recently purchased cars which aren't driven heavily in fleets are seeing more calendar than cyclic degradation. Cyclic degradation gets most of the attention in the research literature (it's critically important for utility energy storage which needs to full cycle daily) because it's much faster to get results in the lab.
 
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There will be calendar degradation on top of cyclic degradation. But the optimal for calendar degradation---keep state of charge low and keep battery cool---isn't much different than for minimizing cyclic degradation. For calendar in particular, low SOC is better universally it seems.

I now put my maximum charge limit at 50% (there is a long thread on the Model 3 Battery & Charging board) and recharge frequently when I am able to do that. When I need to charge higher, I do and don't stress.

The converted x-axis in equivalent miles shows that most people with recently purchased cars which aren't driven heavily in fleets are seeing more calendar than cyclic degradation. Cyclic degradation gets most of the attention in the research literature (it's critically important for utility energy storage which needs to full cycle daily) because it's much faster to get results in the lab.
True, but you can’t stop time, and most people can’t do much about the temperature, either.

From what I recall from the article, 60% was the optimal charge that minimized charge stress on the battery. The article isn’t loading right now so I can’t double check that. Based on that I try to charge around 40-80% so it averages 60%, for whatever that’s worth, but like I said, I don’t stress about it that much. Like you said, time plays as much of a roll as my charging habits do.
 
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True, but you can’t stop time, and most people can’t do much about the temperature, either.

From what I recall from the article, 60% was the optimal charge that minimized charge stress on the battery. The article isn’t loading right now so I can’t double check that. Based on that I try to charge around 40-80% so it averages 60%, for whatever that’s worth, but like I said, I don’t stress about it that much. Like you said, time plays as much of a roll as my charging habits do.

The thread, and primary author AKKEE has unearthed some recent research results. For calendar aging, lower SOC is always better. And there seems to be a step improvement for SOC under 55% in one paper.

Keil et al, Calendar Aging of Lithium Ion Batteries

The result is in Figure 2. The NCA are Panasonic 18650 and probably commercial relevant to Tesla.


To reduce calendar aging in practical applications, lithium-ion cells should preferably be stored at low temperature and at a low or medium SoC. The SoC should remain below the SoC location of the central graphite peak, which represents the transition from the plateau of medium to low anode potential. This avoids storage periods at lowest graphite potential, which causes fastest capacity fade owing to increasing anodic side reactions. Furthermore, avoiding high SoCs also minimizes cathodic side reactions.

40-80% might be talking about cyclic aging effects. I would personally use even lower SOC as calendar aging is going to dominate most people's degradation mechanisms if they aren't very heavy drivers.
 
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Is Depth of Charge more important or State of Charge more important?

If I charge to 75% and drive to 50%, should I recharge back to 75%? (Less Depth of Charge)

Or should I wait until 25% to recharge? (Less State of Charge)

This is an advanced question to which I don’t think there is an answer. Generally the more you drive, the less state of charge matters since the car won’t be staying at a high state of charge for too long. And the more you drive, the more frequently you charge anyway. So in a way, just let nature take its course.
 

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