Joining the 80% Club

[Volusiano]

Frankly, this discussion seems to be divided between people who have time-of-use metering at home, and those who don't, and it appears that neither of these two groups has any understanding of what the other group is trying to say, nor any appreciation of their problems.
Ray

I don't see it that way at all. I just see some people who want to be able to have more options and flexibility on how to charge, but unfortunately the timer options in the Leaf is very limiting and inadequate for their desire. I just see a lot of frustration expressed about this limitation, and maybe a little confusion about the 120V vs 240V operation, and whether the Leaf timer is event-based or interval-based. Also a lot of wish-list type comments.

 

[jwatte]

The Leaf timer is interval-based and not event-based. For example, if your ON time is 1-5pm, but you're not plugged in until 2pm, the Leaf should have recognized that it just got plugged in during the ON interval and turn on the charger. It doesn't care that the ON event is at 1pm because it's not event-based in the first place. If your Leaf timer doesn't work like the example above, then something is wrong and you need to take it in to the dealership to take a look at.

So, given this, I can ALMOST get what I want -- except the "blank" parts of my schedule will be "80%" parts. If "time of use" doesn't matter, then that's probably not so bad. If "time of use" matters, then I still cannot do this using the existing timers.

However, my other, as important, complaint, is that the UI for the timers is from 1979, and just sucks. It's worse than the UI for my first digital watch alarm. The UI I propose is a lot (lot!) simpler to program and understand, and it also gives you a lot more flexibility, while being super simple to implement in firmware (I know, as I've done this kind of thing before).

 

[patrick0101]

What I would like to see is:

• a percent of charge touch-screen menu that comes up automatically when I ask the charge bay door to be opened. It should have a default that I can set via an option.
• a table of times/days when charging at home is allowed. It would be independent of the first point and would apply only when the car is at its home location.

This is a great idea planet!

 

[mdh]

what is the difference between 100% charge and 100% usable charge--in terms of the language not kWh?

that is to say, what are you talking about?
is their some sort of reserve to the battery that we cannot access for driving?

A plain battery - like a car battery or the D cells in a flashlight - is completely unmanaged. If we overcharge them, or leave the lights on, we can completely empty and/or destroy the batteries. Both of these allow us to use -and abuse - 100% of their capacity. We have access to the full capacity of the batteries.

The Leaf's battery has sophisticated computerized management. We do not have full access to the battery. We can only use 80% of the energy the battery can hold.

Ok...this analogy just popped in...let's try it out.

Run to the closest toilet and lift the lid off the tank. The first thing we'll notice is that the water does not come all the way to the top of the tank - the tank can hold more water but we're not using that extra capacity.

Now - flush the toilet. Once "all" the water runs out we can see that there's still water in the tank - we cannot use all the water that's stored there. Also notice as the tank fills that the water will stop again before the tank is completely filled.

We are not allowed to use 100% of the tank capacity - and we are not allowed to drain all the water from the tank.

The Leaf's battery is JUST like that. The car's management system does not let us put too much energy into the battery - we can only fill to about 95% of total (so 100% full on the fuel gauge only means the water has stopped coming in - it doesn't mean the tank is going to overflow. ) At the end of a 100 mile drive, as the last red bar goes dark and the remaining range display changes to --- and the turtle light comes on, we know that we've used all the energy that the Nissan engineers have set aside for us. But just like the toilet tank there is still energy in the battery. We cannot use it, but that energy remaining - that last 15% - is helping our battery last.

100% "consumer" - a full tank on the fuel gauge - is only about 95% of what the battery can hold.
0% "consumer" - an empty tank and a turtle light - leaves about 15% in the battery.
100% consumer capacity is only using about 80% of the energy the battery can hold.

No - there's no reserve. In order to get a long life from the battery we only want to use about 80% of the total capacity. The 5% we cannot access on top, and the 15% we cannot use on the bottom is helping us get an 8-10 year battery life.

(please let me know if that makes sense. I may have to flush another analogy. )

question... there are approx two hidden bars before turtle mode. Will we damage the battery if we use those bars before hitting turtle? Or, does Nissan have enough buffer to protect the battery?

 

[thankyouOB]

good work.
I got it before the analogy, but it was fun, regardless.
So that makes sense, but why is Nissan adding another 20% (charge to only 80% for long life) as suggested to be not used, if it has already protected the battery?
is that extra coverage?

 

[surfingslovak]

good work.
I got it before the analogy, but it was fun, regardless.
So that makes sense, but why is Nissan adding another 20% (charge to only 80% for long life) as suggested to be not used, if it has already protected the battery?
is that extra coverage?

Yes, very nice write-up. I'm wondering if it would be better to have a separate thread on battery care and best practices. There is also some interest to set up a wiki that would be connected to the forum. Some of the questions come up often and as we see more new owners, having an FAQ and a consensus would be helpful.

On a related note, I've been crunching all the numbers I would come across on this forum. I don't want to be a stickler for detail, but I believe that the minimum charge stored in the battery after it stops dead in turtle mode is closer to 7% than to 15%. Nissan gives us access to 85% of the rated battery capacity, possibly even 87.5%.

 

[jwatte]

good work.
So that makes sense, but why is Nissan adding another 20% (charge to only 80% for long life) as suggested to be not used, if it has already protected the battery?
is that extra coverage?

I believe when the battery meter shows "100%" (as far as you're allowed to charge), you get 95% of the theoretical max capacity of the battery (when new).
I believe when the battery meter goes to turtle (as far as you're allowed to go down), you're at 15% of the theoretical bottom of the battery.

The reason you're not allowed to go to the extremes is that going to the extreme *significantly* reduces battery life. A laptop li-ion that doesn't have these precautions, when run through full cycles, will last for only a year. Also, if you go down to turtle, and then don't charge for a while, internal drainage will further reduce the charge; I imagine that the "15%" at the bottom give you some protection against that, too.

Another way of looking at it: Given the same "10 Ah nominal" li-ion stack, you can deliver one of these two products:
- a battery with short lifetime, and 10 Ah of rated charge
- a battery with a long lifetime, and 8 Ah of rated charge

Once you buy a product, you can then choose to prolong the life of the product by not cycling it fully, but instead staying in the middle range of its capacity -- in the latter case, using it as a 6.5 Ah battery (80% of "full" charge) will prolong the life further.

 

[surfingslovak]

I believe when the battery meter goes to turtle (as far as you're allowed to go down), you're at 15% of the theoretical bottom of the battery.

I hope you don't mind me being direct. Would you have any numbers to back up the 15% claim or is it just something you believe to be true?

 

[jwatte]

I have no hard numbers. I have previously had battery discussions with engineers involved with Tesla, and other engineers involved with laptops, and have read/listened to what little information is available about what Nissan is doing. My explanation is the closest I can come within the given uncertainty limits. It's quite possible that the "unused" range is smaller on the bottom -- 10%? 7.5%? but I don't think it's smaller than 5% at the top -- possibly bigger -- as going to the full top (or full bottom) will significantly limit battery life.

If you can get Nissan to spit up the real numbers, that would be fantastic, but I haven't seen that happening yet, so I have to go by what I know of the technology. Given that, I can say with very high confidence that the range we "see" (turtle to full) is probably in the range from (5%-99%) through (15%-90%) of battery maximum limits. Pick your own numbers in between

That being said, the question asked was: why recommend 80% when "100%" is not full? Answer: because backing off to 80% of "full" means even more battery life than whatever upper limit Nissan has chosen.

 

[surfingslovak]

So the quoted 24 kWh capacity of LEAF's pack is the accessible capacity (80%), and its raw capacity is ~30?

Yes. We think the full capacity is 27-28kWh but that's not fully nailed down.

Nissan has stated that the energy available for us to use is 24kWh while the total pack capacity is proprietary.

No, this is very likely incorrect. The rated full capacity is 24kWh. Assuming 85% charger efficiency, this is supported by meter numbers reported when charging from turtle to full. There were three instances where one owner reported a higher number than expected, but that was less than 2kWh difference, not 4kWh.

The available energy is about 21kWh and likely less, something like 20.3kWh. This is supported by the range and energy economy numbers many owners are getting. All things being equal, it's very likely that the rated capacity is 24, and the available capacity is ~21 or ~87% of the total.

EPA supposedly used wall-to-wheels numbers, which is similar to measuring the amount of gasoline that goes into a car and then measuring how far it can go. Assuming 85% charger efficiency and 5% other losses (battery + drivetrain), the energy economy they saw during their cycle was 3.7 or 3.8 MPK. And again, the range numbers the EPA reported would support ~21kWh of available capacity, not 24.

 

[mdh]

I have no hard numbers. I have previously had battery discussions with engineers involved with Tesla, and other engineers involved with laptops, and have read/listened to what little information is available about what Nissan is doing. My explanation is the closest I can come within the given uncertainty limits. It's quite possible that the "unused" range is smaller on the bottom -- 10%? 7.5%? but I don't think it's smaller than 5% at the top -- possibly bigger -- as going to the full top (or full bottom) will significantly limit battery life.

If you can get Nissan to spit up the real numbers, that would be fantastic, but I haven't seen that happening yet, so I have to go by what I know of the technology. Given that, I can say with very high confidence that the range we "see" (turtle to full) is probably in the range from (5%-99%) through (15%-90%) of battery maximum limits. Pick your own numbers in between

That being said, the question asked was: why recommend 80% when "100%" is not full? Answer: because backing off to 80% of "full" means even more battery life than whatever upper limit Nissan has chosen.

I wonder... if Nissan does something similar to what is happening in solid state disks (SSD) in the IT World... there is a certain amount of over subscription in the MLC/SLC flash to protect against SSD cell degradation. Maybe there are more battery cells then we know?

Nissan needs to be very smart and practical on how they do partial and complete battery upgrades several years from now. I have a feeling they will be smart and practical to protect this early/vocal group of buyers. We are the pump primers.

I also have a funny feeling that the Nissan Leaf will turn out to be a well designed over engineered car. Look at the pending Focus EV.... the battery pack is much smaller... ditto for the pending plug in prius.

Fingers crossed.

 

[waidy]

Let me say this: I was totally out of juice 30' in front of my garage or 40' in reach of my charger. I refilled it with 23.897 AC kWh (reading from the charger). The battery pack is 24 DC(?) kWh so you may now do your calculation. Thanks for accepting my data.

 

[surfingslovak]

If you can get Nissan to spit up the real numbers, that would be fantastic, but I haven't seen that happening yet, so I have to go by what I know of the technology. Given that, I can say with very high confidence that the range we "see" (turtle to full) is probably in the range from (5%-99%) through (15%-90%) of battery maximum limits. Pick your own numbers in between

Absolutely, I'm with you on that and thanks for your answer. I've been diligently crunching all the numbers I could get my hands on and threw in a little bit of speculation for good measure as well. Right now, it looks like we have 5 or 6% reserve at the top and 7-8% reserve at the bottom of the capacity rage. Based on what I know, I would say that this is fairly aggressive and the driver has a high degree of autonomy. We can chose to fully utilize the battery to support our daily needs or cut back a bit and be more conservative in exchange for improved battery life. And no matter what we do, we cannot ruin the battery, even if we are pretty reckless.

I looked at number of application and research reports, and the lowest capacity fade I saw was for a 30% depth of discharge (DoD) cycle. It's about 0.001% per cycle, which was seven times smaller than the fade seen in a 60% DoD cycle. Yes, it's a different application and different cells, but shallow cycling and keeping a lower state of charge seems to be a universal principle used to keep battery degradation from cycling to a minimum. To quantify it a bit, I believe that cycling losses when fully utilizing battery capacity available to us could approach or even exceed 10% at the 10 year mark. Cutting back a bit and using only half of the pack capacity on a regular basis would reduce the fade to about 5% and cutting back further to about 3%.

I would expect degradation due to limited calender life and lack of active cooling to be the major issue we'll have to contend with. However, if you can avoid 5% of capacity fade from cycling over the lifetime of the battery, it might be worth doing. It depends what your priorities are however. If you need to drive your Leaf constantly, you might want to charge to full and not worry about the battery. Just don't let it sit fully charged for weeks on end.

 

[surfingslovak]

Let me say this: I was totally out of juice 30' in front of my garage or 40' in reach of my charger. I refilled it with 23.897 AC kWh (reading from the charger). The battery pack is 24 DC(?) kWh so you may now do your calculation. Thanks for accepting my data.

Waidy, thank you for reporting this! Assuming 85% charger efficiency, this would imply 20.31kWh of usable battery capacity (23.89 x 0.85). This would be in line with the 21kW some of us started to use for range predictions.

I crunched additional numbers provided by folks with an SOC meter, which is quite a bit more accurate than anything else we have, and their trip reports suggested 72Wh per SOC tick. The battery reportedly has 281 SOC ticks to represent available charge when full, which then translates to 20.23kWh (281 x 72). Very close to the number you reported.

Keep in mind that there is always a degree of uncertainty and we rely on estimates in most cases, but it's good if things are consistent. That means that we have a better understanding of the car and its design.

 

[GaslessInSeattle]

...the question asked was: why recommend 80% when "100%" is not full? Answer: because backing off to 80% of "full" means even more battery life than whatever upper limit Nissan has chosen.

Nice, that is probably the shortest answer that can be given to an otherwise complicated topic.

and I should add, I've been doing almost exclusive 80% charges for almost 4,500 miles without being inconvenienced, it really is a very well thought system, IMHO.
g

 

[AndyH]

If you can get Nissan to spit up the real numbers, that would be fantastic, but I haven't seen that happening yet, so I have to go by what I know of the technology. Given that, I can say with very high confidence that the range we "see" (turtle to full) is probably in the range from (5%-99%) through (15%-90%) of battery maximum limits. Pick your own numbers in between

Absolutely, I'm with you on that and thanks for your answer. I've been diligently crunching all the numbers I could get my hands on and threw in a little bit of speculation for good measure as well. Right now, it looks like we have 5 or 6% reserve at the top and 7-8% reserve at the bottom of the capacity rage. Based on what I know, I would say that this is fairly aggressive and the driver has a high degree of autonomy. We can chose to fully utilize the battery to support our daily needs or cut back a bit and be more conservative in exchange for improved battery life. And no matter what we do, we cannot ruin the battery, even if we are pretty reckless.

I looked at number of application and research reports, and the lowest capacity fade I saw was for a 30% depth of discharge (DoD) cycle. It's about 0.001% per cycle, which was seven times smaller than the fade seen in a 60% DoD cycle. Yes, it's a different application and different cells, but shallow cycling and keeping a lower state of charge seems to be a universal principle used to keep battery degradation from cycling to a minimum. To quantify it a bit, I believe that cycling losses when fully utilizing battery capacity available to us could approach or even exceed 15% at the 10 year mark. Cutting back a bit and using only half of the pack capacity on a regular basis would reduce the fade to about 8% and cutting back further to about 4%.

I would expect degradation due to limited calender life and lack of active cooling to be the major issue we'll have to contend with. However, if you can avoid 5 or 10% of capacity fade from cycling over the lifetime of the battery, it's worth doing IMHO.

What type of cell/battery reports did you find, what specific chemistry, and in what applications did you use to arrive at this management plan?

 

[surfingslovak]

What type of cell/battery reports did you find, what specific chemistry, and in what applications did you use to arrive at this management plan?

Andy, I wouldn't call it a management plan. I simply stuck my neck out and tried to put a number or two in the sand. There is so much angst out there and I thought that it would be helpful.

I don't expect Nissan to come out with a battery care guide anymore. They consider so many aspects of the Leaf to be proprietary and confidential, that they won't say anything, even if it translated to poor customer service. In the light of this, and until someone thoroughly examined a few Leaf battery modules, everything we say is speculation and a matter of opinion.

I would hope that the smart and knowledgeable people on this forum could agree on a consensus for battery care, something we would feel comfortable recommending to other owners, even though it was just an educated guess.

But back to your question. The numbers I used in the post you quoted were based on a JPL report for the Mars rover mission. They found the capacity fade from cycling to be about six times higher at 60% DOD when compared to 30% DOD. They used SAFT LiNiO2 cells with graphite anode and cylindrical stainless steel hardware. The cells were tested in 30% DOD regime (5000 cycles) with average energy fade rate at 4.0V at 0.000704% per cycle and 60% DOD regime (500 cycles) with average energy fade rate at 4.0V at 0.00430% per cycle.

I also found the Tesla battery care guide from Dan Myggen to be of interest. I believe that the Roadster is using 18650 form-factor LiCoO2 cells. I don't claim to be the world's foremost expert on lithium ion cells, but this is an important topic and I would love to continue the discussion. I'm not sure if this is the right venue though, this thread is already 24 pages long.

 

[mdh]

If you can get Nissan to spit up the real numbers, that would be fantastic, but I haven't seen that happening yet, so I have to go by what I know of the technology. Given that, I can say with very high confidence that the range we "see" (turtle to full) is probably in the range from (5%-99%) through (15%-90%) of battery maximum limits. Pick your own numbers in between

Absolutely, I'm with you on that and thanks for your answer. I've been diligently crunching all the numbers I could get my hands on and threw in a little bit of speculation for good measure as well. Right now, it looks like we have 5 or 6% reserve at the top and 7-8% reserve at the bottom of the capacity rage. Based on what I know, I would say that this is fairly aggressive and the driver has a high degree of autonomy. We can chose to fully utilize the battery to support our daily needs or cut back a bit and be more conservative in exchange for improved battery life. And no matter what we do, we cannot ruin the battery, even if we are pretty reckless.

I looked at number of application and research reports, and the lowest capacity fade I saw was for a 30% depth of discharge (DoD) cycle. It's about 0.001% per cycle, which was seven times smaller than the fade seen in a 60% DoD cycle. Yes, it's a different application and different cells, but shallow cycling and keeping a lower state of charge seems to be a universal principle used to keep battery degradation from cycling to a minimum. To quantify it a bit, I believe that cycling losses when fully utilizing battery capacity available to us could approach or even exceed 15% at the 10 year mark. Cutting back a bit and using only half of the pack capacity on a regular basis would reduce the fade to about 8% and cutting back further to about 4%.

I would expect degradation due to limited calender life and lack of active cooling to be the major issue we'll have to contend with. However, if you can avoid 5 or 10% of capacity fade from cycling over the lifetime of the battery, it's worth doing IMHO.

surfingslovak, I really appreciate the contribution that you are making to the community. For simpleminded people like me, I am trying to figure out what are the best practices for charging (based on your work in progress). For example, if i charge to 80% and drive about ten miles for a quick trip... should i do a top-off charge to bring myself back up to 80% upon my return home or should i finish my day and maybe bring the battery down to a few remaining bars, as an example, as I use the car for other tasks and then charge back to 80%?

 

[surfingslovak]

For example, if i charge to 80% and drive about ten miles for a quick trip... should i do a top-off charge to bring myself back up to 80% upon my return home or should i finish my day and maybe bring the battery down to a few remaining bars, as an example, as I use the car for other tasks and then charge back to 80%?

Other people might disagree and I don't have any hard data, but if it was up to me, I wouldn't charge the Leaf until it was at five bars or less. I know this is tough, but if I was able to charge to 8 bars via a timer on a daily basis, I would. I only consume 4 or 5 bars, and a shallow daily cycle would suit me well. This is based on the recommendation from Dan Myggen I quoted below. All things being equal, if you can keep the Leaf between 2 and 10 bars and minimize the amount of time that it spends sitting fully charged, you are likely doing well.

If you prefer to keep a lower average state of charge in an attempt to maximize the life of your battery, I would instead suggest that you utilize the Roadster’s built in charge timer and current limiting options to find an average state of charge that works for you.

 

[DaveEV]

All things being equal, if you can keep the Leaf between 2 and 10 bars and minimize the amount of time that it spends sitting fully charged, you are likely doing well.

That's probably the simplest best advice right there.

If that means occasionally going down to 1 bar rather than charging to 100% - I would opt for that as well - unless you time the 100% charge such that you'll be leaving as soon as the charge is done or just want to have the extra range cushion.

TOU rates can make this a bit more difficult - If I'm driving less than 50 miles - I never bother with charging more than 80% unless I know it will be done at high rates of speed (over 65 mph).

If more than 50 miles and on the highway, I will typically charge an extra 45-60 min after 80% to give me a bit of range cushion. For example, I drove just over 50 miles both Sat and Sun - both trips would be mostly highway at 65 mph with the A/C on. For both I charged about 45-60 min before on-peak rates kicked in - just used a stopwatch timer to stop charging before 100%.

Using Tony's handy range/speed/chart I estimated about 60 miles at 65 mph before draining it on an 80% charge. 5-7 miles is cutting a bit close unless I want to drive below the speed limit, so 45-60 min more charge should get me about 8-10 miles more according to the chart. This charged the car to 11 bars and had lost 2-3 regen bubbles (I really hate charging to a full 100% unless I really need to especially as I live on a bit of a hill and having regen means getting 7-8 mi / kWh vs 4 mi / kWh the first couple miles). Got home with 17 miles and 2 bars (4.0 mi / kWh) on Sat and 13 miles and 2 bars (3.8 mi / kWh) on Sun - so everything went right according to plan.

The previous weekend I drove 78 miles mostly highway at 65 mph - that was with a 100% charge and 4 hours of trickle - got home with about 15 miles and 2 bars remaining. I wish the car had about 25% more capacity. This would let me do nearly all my normal trips on a 80% charge and a few more on a 100% charge without having to worry about range. Of course - a couple of strategically placed DC quick charge stations would also go a long way.