Am I not "topping up" when I let it charge to 100%, it did go through 95% on then way to 100%

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ACStark

Member
Joined
Jan 26, 2020
Messages
5
I don't understand why it is good to charge up to 100% occasionally to "balance the cells" but bad to top up if it was already at 95%. When charging to 100% there is a time when it is at 95% then it continues to charge to 100%. Why is that last 5% OK in that charging cycle, but take a car at 95% and plug the charger in again to "tops up" to 100 % is bad.
Doesn't seem to make sense. Is charging to 100% as bad as topping up to 100 from 95%? If not why not?

Thanks.
 
Having it at a high SOC for a long time is what you generally want to avoid. When you are at 95%, you were already at a high SOC.
 
Much of the very last part of charging a lithium PACK (many cells) is balancing. The charger puts energy into low cells and may actually remove some energy from cells already full. In actual usable capacity put in, there isn't much. You would use that little bit in a couple miles.

If you plugged it in, charged to 95%, unplugged.....then topped it off > that is not really any different than charging to 100%.

If charged to 95%, then it sits around, and you decide later to top it off, that's when you are doing as @jiv indicates.
 
ACStark said:
I don't understand why it is good to charge up to 100% occasionally to "balance the cells" but bad to top up if it was already at 95%. When charging to 100% there is a time when it is at 95% then it continues to charge to 100%. Why is that last 5% OK in that charging cycle, but take a car at 95% and plug the charger in again to "tops up" to 100 % is bad.
Doesn't seem to make sense. Is charging to 100% as bad as topping up to 100 from 95%? If not why not?

Thanks.

That's a good question. There must be some reason that the onboard computer tallies these events. I can only speculate, but maybe it goes like this:
During a normal charge, before the time SOC reaches 95%, the charge algorithm has detected the need to switch between constant-current and constant-voltage charge modes and the charge is well into taper mode. It's possible that by beginning a charge session with a 95% state, it takes some time for the computer to determine the need to switch to taper mode, thus exposing the battery to a period of time where the charge rate is inappropriately high. Again, just speculation.
 
I don't know how the Leaf charger does it, but normal lithium battery chargers (RC, 18650/26650/etc) all have to 'investigate' the battery charge status before they engage in charging. For RC chargers part of the final charge algorithm is dependent on the state of charge it finds initially.
If very depleted it will charge as aggressively as the system is set or allowed, The taper is a percentage function of that charge rate.
If mostly charged the initial charge rate would be much lower, and the taper would be smaller but likely longer.
For recreational batteries I'm not sure it makes a whole lot of difference as long as the current is not excessive for the cell,
It's a complete extrapolation to do that to the Leaf but not unreasonable.

FWIW, someone could actually check this out on Leafspy. I've only watched on quick charge. It started pretty aggressively and started tapering immediately. Past 80% it is much slower, probably not much different than L2, but I have not compared or checked.
 
flydiver said:
I don't know how the Leaf charger does it, but normal lithium battery chargers (RC, 18650/26650/etc) all have to 'investigate' the battery charge status before they engage in charging. For RC chargers part of the final charge algorithm is dependent on the state of charge it finds initially.
If very depleted it will charge as aggressively as the system is set or allowed, The taper is a percentage function of that charge rate.
If mostly charged the initial charge rate would be much lower, and the taper would be smaller but likely longer.
For recreational batteries I'm not sure it makes a whole lot of difference as long as the current is not excessive for the cell,
It's a complete extrapolation to do that to the Leaf but not unreasonable.
...

It's also true that once you take the battery off the charge, the measured voltage falls. So an initial determination of SOC may be less accurate at high SOC. As I say, I'm just speculating and I'm certainly open to better explanations.
 
Li-ion cells are a lot more complex than simple models of them. I'll ask the battery expert I used to share an office with years ago the next time I see him.

What I do know is that early laptop batteries didn't last as expected. The people that put their computers on a charger and used them (charging to 100% then discharging to 99.x%, repeat) had the batteries die far faster than either a 100% charge life test or people actually using the batteries (discharge from 100% to ~20%, then charge). Don't ask me to explain why, I don't know.
 
ACStark said:
I don't understand why it is good to charge up to 100% occasionally to "balance the cells" but bad to top up if it was already at 95%. ...
Is charging to 100% as bad as topping up to 100 from 95%?

Yes both are Equally "bad", if you don't really need 100% everyday for your commute then don't do it, especially in hot weather.

If you are a number cruncher who constantly needs to collect lots of numbers from your car, then you might drive it down to a low "fuel" gauge level and then charge to 100% Full in order to get an updated calibration value for the pack capacity and read this using an OBDII phone app. Then you can make files and lists of these useless numbers and post them in meaningless and endless threads here on this forum, where nobody will read them.

Or just enjoy your amazing electric car as you drive your happy ass around within the 30-mile radius shown on the fabulous International GPS Navigation System.
 
I don't have a very solid grounding in chemistry (I took the 'reader's digest' version in school) but my understanding is that much of the actual degradation occurs as the battery nears 100% charge, with more occurring when/if it nears 0% charge. To put it another way: charging from 90-95% to 100% incurs most, or at least disproportionately more, of the normal full charge/discharge degradation, so with so little range gain involved you are cycling the pack faster over far fewer miles when you regularly charge from 80-90%, where most of the range is, to 100%. This results in the pack "aging" faster and possibly failing early, which is why Nissan wants Leaf drivers to avoid doing it on a regular basis.
 
WetEV said:
What I do know is that early laptop batteries didn't last as expected. The people that put their computers on a charger and used them (charging to 100% then discharging to 99.x%, repeat) had the batteries die far faster than either a 100% charge life test or people actually using the batteries (discharge from 100% to ~20%, then charge). Don't ask me to explain why, I don't know.

100% charge all the time + heat while operating. Some laptops can get pretty warm. Almost the worst thing you can do for a lithium battery.
Think AZ Leaf, a black one, fully charged, parked in the hot sun all day. Not too good.

If you are going to operate a laptop this way you are better off pulling the pack at 50% and storing it unless actually needed.
 
ACStark said:
I don't understand why it is good to charge up to 100% occasionally to "balance the cells" but bad to top up if it was already at 95%. When charging to 100% there is a time when it is at 95% then it continues to charge to 100%. Why is that last 5% OK in that charging cycle, but take a car at 95% and plug the charger in again to "tops up" to 100 % is bad.
Doesn't seem to make sense. Is charging to 100% as bad as topping up to 100 from 95%? If not why not?

Thanks.

1) It is NOT good to charge to 100% for any reason other than driving need.

Time at high SOC is what you need to avoid so doing it to comply with some sort of wive's tale is not recommended.

A quick and easy tutorial on battery management can be had from your phone. Install a battery management app. In it will be tutorials that show the estimated "hit" on health based on charging scenarios you select. These scenarios are general info only so you need to use the app several months before your actual usage can be evaluated against your effect on degradation.

You will notice that shallower cycling provides the best chance for longevity. Be it 40-60% or 20-80%, the shallower the better. So you need to decide what you need in your pack and charge accordingly.
 
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