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Discussion Starter · #1 · (Edited)
Hi folks. I work from home or take commuter rail to work and thus only drive about 20 miles a day average, mostly taking kids to their school/daycare and back. I was wondering whether it makes more sense to discharge while leaving my car unplugged until I go below 30% and charge back up to the 80% limit, e.g. charging every 4 to 8 days or so, or instead setting a lower charge limit (60%?) and leave the car plugged in each night…

I know keeping the battery closest to 50% is best for long-life, but also charging daily means more “cycles” no? Thoughts?
 

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These cars haven't really been around long enough to to do a real world analysis on battery longevity, but likely your first impulse (below 30%, then up to 80%) is best. The most significant battery killers, AFAIK, are frequently discharging to below 20%, and frequently charging to 100%. Anything in between should be optimal.
 

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Charge from 50-x percent to 50+x percent for the smallest x that works for you. There is diminishing return as x gets smaller, but the exact curve is unknown. If you don't mind the increased charging frequency, then go for a smaller x.
 

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Discussion Starter · #5 ·
Also, charging daily does not mean more cycle. Charging from 40% to 60% three times is slightly better than charging from 20% to 80% once.
This above is what I was trying to ask, and I think if what you say is true, that’s what I wanted to go for.

I was hoping of a 40% to 60% swing every day or two — but wanted to make sure that the increased charging frequency doesn’t count negatively as “too many cycles”…

Another benefit for me of a “smaller x” is this allows me to have the car be plugged in each night and thus the pre-conditioning and battery warming/cooling can be done off of AC power instead of the battery. If however the extra charging frequency had an adverse effect, I’d not do it…

That said, of course — If I am ever going on a trip, I’d up the charge limit to 80% (naturally). But that’s easy to plan for and not very frequent… (and 50% SOC on an iX is easily 150-200 miles if you drive efficiently…)
 

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Is it important to be this diligent about charging? I was just going to charge to 80% and plug it in most days, even if I only drop to 70% etc
I think you'll be fine as far as battery degradation, although again nobody has tested it AFAIK in these cars (and you really need to be 8 years out with controlled conditions to measure). The things that nornally affect the life of a Li-Ion battery negatively, including those in cars in more or less descending order are: 1) Allowing the battery to discharge to below 10%, particularly allowing it to get to 0% (conventional appliance Li-Ion batteries may not be recoverable after discharging to 0%, so manufacturers generally build in a "buffer" in the charge indicator that reads 0 when in fact there is a charge left, usually 5-ish percent or less), 2) Partially discharging and charging to 100% frequently (somewhat unique to large car batteries), 3) Frequent rapid charging, 4) Number of charging cycles (fewer is better), and 5) Long term storage in temperature extremes, particularly heat. I suspect these batteries are pretty robust, and other than extreme abuse are likely to last their rated lifespan with the normal degradation of 1-2٪ or less annually with the suggested normal care (charge usually to 80%, don't let it drop below 20% routinely, limit DC charging).
 

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Borrow from Tesla's practice:
1. maintain level between 20-80%; occasionally below 20% and above 80% is fine
2. in winter, charge when battery is warm (drive for a few minutes before charging)
3. slow at home charge will help little bit comparing fast charge
4. leave car charged overnight does not impact battery much

from people who had high mileage, best practice only helped little bit, battery capacity diff is very small. I'd say for most us, go on with your normal business, no need to be sweat about it.
 

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Discussion Starter · #12 ·
For those that charge your car to 80%, is it OK to plug the car back in if you're only down to say, 65-70%? Or when do you plug your car back in the next time?
I actually wondered about that myself — the advantage of plugging it in (presuming you don’t need that 10-15% range) is that you can pre-condition using AC power, no? From what I hear smaller charge discharge ranges are not problematic, but the battery is “healthiest” at 50%, so that’s why I lowered my max charge down to 60% or 55% and change the limit up to 80% only if I know I’m going on a longer trip, but I admit all this would be simplest if I just leave it plugged in each day and set the limit to 80%…
 

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The way I see it, if you need to drive a lot of km today, then go for it and charge to the level you need. But for day to day, let’s say you will need 20% of your battery in your day, then the best charge level is 60%. It will be at 60% in the morning, 40% at the night … the avg must try to be around 50% in the day. This is the very best scenario to keep your battery health optimal, and answer your driving need.
Rectangle Font Plot Parallel Pattern
 

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The way I see it, if you need to drive a lot of km today, then go for it and charge to the level you need. But for day to day, let’s say you will need 20% of your battery in your day, then the best charge level is 60%. It will be at 60% in the morning, 40% at the night … the avg must try to be around 50% in the day. This is the very best scenario to keep your battery health optimal, and answer your driving need.
View attachment 2748
The graph is interesting - the lowest stress region seems to be 20% to 50%, rather than 50%-x to 50%+x. I wonder if this is due to, say, the reserve capacity not being counted as part of the SoC here?

Also, the curve probably depends on the battery chemistry - NMC vs. NCA vs. LFP.
 

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The graph is interesting - the lowest stress region seems to be 20% to 50%, rather than 50%-x to 50%+x. I wonder if this is due to, say, the reserve capacity not being counted as part of the SoC here?
You are probably right, it's also my guess that it is due to the reserved capacity. The real 0% is unreachable, this graph shows the EV SoC that we can see in the car interface.
 

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@jcolinzheng and @Alex L ,
Thank you very much for sharing this information.
This is very similar to what I experienced with the batteries from my previous PHEV. On the other hand, I was testing the "AccuBattery" app on my Android phone.
This App keeps track of how many charging cycles your battery uses each time you charge its battery.
Charging from 0% (Almost Empty) to 100% counts as 1 charging cycle.
But charging from 0% to 80% only 'consumes' 0.26 cycles. So you can charge almost 4 times from 0% to 80% (equivalent to 320%) to take the same damage as one charge from 0% to 100%.
It gets even tougher, because charging from 0% to 60% only consumes 0.10 cycles. So if you charge from 0% to 60% each time, after 10 charges (and therefore 600% of the battery's charging capacity) you will have done as much damage as one charge from 0 to 100%. In the 60% case, the battery life becomes 6 times longer.
Worse yet, always charging your cell phone from 70% to 100% consumes 0.86 charge cycle.
Of course you cannot compare the specified charging cycle and charging percentages of a mobile phone with the batteries of a car, but it does give a clear direction of how you can best handle your battery if you want to enjoy it for a long time and it also explains why BMW itself recommends not to charge above 80% under normal circumstances.
Since I found this app, I only charge my car at home to the percentage that I think I will need the next day. If something unexpected happens, then I go to the fast charger with pre-conditioned batteries and do not charge further than 65% (max 70%).
Hopefully I will be able to drive the longest with my battery this way.
 

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So I did some more research. It turns out that the optimal SoC region for NMC battery longevity is generally around 20%-50% (with a measurable cliff at 60%):
  • Lower SoC generally leads to less capacity fade both when the battery is being stored and when being charged.
  • OTOH, mechanical stress at low SoC will become greater for older batteries. SoC lower than 20% increases internal resistance.

(The optimal SoC region has a small dependency on other factors like temperature.)

The iX has ~6% buffer capacity, possibly 4-5% at the bottom and 1-2% at the top. So, ~17% displayed SoC may be ~20% actual SoC.

So to optimize battery longevity only (not convenience, not daily drivable distance), one can set the daily charging threshold to the lowest level that's enough to ensure it never drops below 20% throughout the day.

This does not contradict BMW's recommendation of 20%-80%, which is a coarse-grain recommendation that also tries to balance the need to have a large drivable range.
 
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