If charge tapers near the end, why doesn't watts/amps draw drop?

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tomcon

Active member
Joined
Jan 7, 2019
Messages
27
Location
Seattle, WA
I am charging my new 2018 Leaf at 120V. have a plug-in meter that plugs into the wall plug and then the charger is plugged into the meter. When i start charging, it reads about 12A and about 1150 Watts draw. Right near the end of charging to 100%, like the hour or two before the charger goes off, it is still reading the same Amps and Watts. I had expected those to drop near the end, since it seems like you are not charging the battery as much near the end (tapering) and so why is it drawing the same power? Like in the old days when you charged a 12V battery you could see the Amps drop to near 0 at the end, and then you knew charging was done. Whey doesn't charging the Leaf battery have the same behavior? Thanks, if you understand this issue and can reply!
 
When you charge at 120 volts (L-1) you are already charging at a low rate. There is no need to taper this charge down because it is already using a slow charge rate. At the very end, when all three charge lights are lit solid blue, you might see fluctuations as the pack equalizes.
 
LeftieBiker said:
When you charge at 120 volts (L-1) you are already charging at a low rate. There is no need to taper this charge down because it is already using a slow charge rate. At the very end, when all three charge lights are lit solid blue, you might see fluctuations as the pack equalizes.

+1. For a 2018, Level 1 is a trickle charge to begin with.

Not sure about the readings. 12A @ 120V should be closer to 1440W 1150W would indicate a voltage sag to 95V. That is extreme and would indicate some serious problems with the wiring. Or hopefully it's just the meter that is faulty.
 
To add to what LeftieBiker said, and relating to your other post, your 2018 LEAF would charge at up to a 6.6 kw (240v x 27.5A) rate (obviously, that doesn't account for losses, which are less for L2 charging). If charging on an L2 EVSE, you would see that at around 85% SOC, the rate would steadily taper between 85% SOC and 100% SOC.

Of course, if you follow what seems to be best practices then you would only occasionally charge to 100% (when you need it) and most of the time charge to 80% SOC. In that case, you should see very little tapering when charging with L2 (ie you should see a 6 kW or more charging rate for the entire charge cycle)
 
Thanks Nubo, yes, i erred, it was about 1350 watts, and more exact on the Amps i think is about 11.5 (I had rounded to 12).

I keep hearing not to charge to 100%...from everybody here, but not Nissan! But, i think i shall try it, though they make it inconvenient. What do people do, take the cars estimate for time to full charge and subtract a few hours, and then set the timer? And the purpose of that is just battery longevity, that is improved if not fully charged? Does anybody know of an estimate for effect on longevity, say, if you charge only to 80% all the time instead of 100% all the time? An extra year? (If so, not insignificant!). I currently charge to 100% (for convenience/simplicity), but not every night, only if i've used enough to drop below about 70% or so. So, i don't charge to 100% every night, but i do charge to that if i charge at all.

Had the car just 2 months. Trying to get my rhythm/technique down!

Thanks!
 
The "evidence" is mostly anecdotal, but it's actually more important to not leave the pack sitting at 100% SOC for hours, particularly when ambient temperature is high. As long as you are driving shortly after the pack reaches 100% SOC, and only charge to 100% SOC when it's not hot outside, then you are very likely not going to see any worse degradation.

The combination of high temps, a high state of charge for long periods of time, and in particular rapid charging under those conditions will definitely impact the life of the battery pack and cause premature degradation.
 
alozzy said:
The "evidence" is mostly anecdotal, but it's actually more important to not leave the pack sitting at 100% SOC for hours, particularly when ambient temperature is high. As long as you are driving shortly after the pack reaches 100% SOC, and only charge to 100% SOC when it's not hot outside, then you are very likely not going to see any worse degradation.

The combination of high temps, a high state of charge for long periods of time, and in particular rapid charging under those conditions will definitely impact the life of the battery pack and cause premature degradation.

1+
The only thing I add to that is to avoid charging a hot battery if possible. My routine is to start charging around 3am in the summer. This gives the battery some hours of cool air at night to first cool down and keeps the time the battery is at high SoC down. It is also simple and convenient, and I do not have to fiddle with calculated stop charge times. My routine is with an L2 EVSE and a 6.6 kW max rated OBC
 
Thanks for all the great thoughts. I'd still like to actually observe the taper at the end of a charge if i could. Just for the fun of it.

As for charge habits. I rarely have the fully charged car sit very long, even if i charge to full, i'm still going to use it sometime that day. So, i think i'll go with, maybe 9 months a year (except summer), i'll do what i'm doing now, charge overnight to full, if i'm below 70% and not at all if i'm still above 70%. At 110V if i'm below 70% we're still talking at least 10-11 hours to full charge anyway. In the summer i like the idea of not starting the charge until later to give the battery some time to cool, and maybe i'll try to cut it off a little before 100% if i can estimate it. Unless of course i specifically need longer range the next day.

Overall, though, its a really fun machine and i really like it. Thanks for all the getting-started help.
 
It would be helpful for you to post your location with your information to the left of your post (i.e. in your profile). In a discussion like this, charging practices are strongly influenced by the seasonal climate where your vehicle is located. The issue of capacity loss related to charging practices mostly seems to be linked to battery pack temperatures.
 
Dooglas said:
It would be helpful for you to post your location with your information to the left of your post (i.e. in your profile). In a discussion like this, charging practices are strongly influenced by the seasonal climate where your vehicle is located. The issue of capacity loss related to charging practices mostly seems to be linked to battery pack temperatures.
If asked that the location field be mandatory many times. It hasn't happened. :(
 
My location is Seattle. I just updated it, but it does not show up in this thread. Perhaps it will only show up in future posts.
 
My location is Seattle. I just updated it, but it does not show up in this thread. Perhaps it will only show up in future posts.
 
tomcon said:
I am charging my new 2018 Leaf at 120V. have a plug-in meter that plugs into the wall plug and then the charger is plugged into the meter. When i start charging, it reads about 12A and about 1150 Watts draw. Right near the end of charging to 100%, like the hour or two before the charger goes off, it is still reading the same Amps and Watts. I had expected those to drop near the end, since it seems like you are not charging the battery as much near the end (tapering) and so why is it drawing the same power? Like in the old days when you charged a 12V battery you could see the Amps drop to near 0 at the end, and then you knew charging was done. Whey doesn't charging the Leaf battery have the same behavior? Thanks, if you understand this issue and can reply!

It actually does. Look at any declining charge curve and then take the last 2%. That rate you see there is where your 120 volt 12 amp feed runs all the time. What that means is you still have the ramp down but its like the last 2% of the charge curve unlike a DC Fast charge where its the last 40% of the curve or L2 charge where it the last 5%.
 
I'll see if i can actually see it. Sounds like it is going to be difficult. I was thinking if the plug keeps reading 1350 watts but the charge is tapering, maybe that is where alot of waste is, near the end. But, its looking like that is not the case. But, i'd like to actually see it if i can, just for personal satisfaction! Thanks again for all the comments!
 
tomcon said:
I'll see if i can actually see it. Sounds like it is going to be difficult. I was thinking if the plug keeps reading 1350 watts but the charge is tapering, maybe that is where alot of waste is, near the end. But, its looking like that is not the case. But, i'd like to actually see it if i can, just for personal satisfaction! Thanks again for all the comments!

The reading will not stay at 1350 watts or whatever your rate is. It will drop. The issue you have is that the rate is so slow. On a fast charge, the rate is constant (like yours) until it hits the knee which is when the current starts to drop. (voltage ALWAYS stays the same)

Its this curve that illustrates the maximum rate at which the batteries will take a charge under that given condition. As the SOC rises, the amperage continues to drop and will eventually drop to the rate of your level one charge. the slower the rate of charge, the higher the SOC when the knee occurs. This is quite literally near full in your case. You are talking the difference between taking 40 hours on level one to 90 minutes on DCFC. If you are part of the Seattle LEAF FB group, keep an eye out. I will be posting a pix of a curve that goes pretty low but still does not get down to level one speeds.
 
tomcon said:
I'll see if i can actually see it. Sounds like it is going to be difficult. I was thinking if the plug keeps reading 1350 watts but the charge is tapering, maybe that is where alot of waste is, near the end. But, its looking like that is not the case. But, i'd like to actually see it if i can, just for personal satisfaction! Thanks again for all the comments!

LeafSpy will let you graph the charging session, assuming it still works with 2018 model. I personally would not recommend unattended long-duration use of "kill a watt" type meters in conjunction with vehicle charging.
 
tomcon said:
I am charging my new 2018 Leaf at 120V. .... When i start charging, it reads about 12A and about 1150 Watts draw. Right near the end of charging to 100%, like the hour or two before the charger goes off, it is still reading the same Amps and Watts.... why is it drawing the same power?

My 2017 30kwhr Leaf does taper a bit, even when charging at 120v 12A. Below is a picture of my whole house power use. The car is sucking 1440 watts all night. The little 100 to 300 watt steps up and down all night are refrigerator & freezer. The big 1kw jumps from 6:30am-9am are the furnace fan heating the house up. The 1200 watt spike at 9:30 is a toaster. You can see that the Leaf tapers from 10am to 11:15am, as the pack is balancing. Then the Leaf has three bounces at the very end of the charge cycle (I'm not sure what that is, but I see it often). Then it's done at 11:45am. Then the house returns to its base load (about 150 watts, plus 100 to 300 as the fridge & deep freeze cycle).

(ignore the green stuff, that's the solar panels)

leaf-three-bounces.jpg
 
I have the 10kw sectec home CHAdeMO it doesn't even start to taper below 8kw until above 90% SOC. Then it slows down to 3 to 4kw. Then it usually shuts off between 90 and 96%.
So you may be able to charge at 1.4kw until it cuts off.
 
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