Does charging to 100% waste electricity?

Hi,

I just bought a 2015 Leaf earlier this month and I can't seem to find the answer to this question so forgive me if this has been asked already.

I noticed when charging from say 30% to 90% it's pretty fast. But it seems like, unless I'm imagining things, that the last 10% is really slow. I wonder if charging it from 90% to 100% is taking the same amount of electricity as charging from 30% to 40% or does somehow charging efficiency decrease as you get closer to 100%? Or put it another way, "would you be paying for 2kwh but your battery may be only getting 1 kw?"

The thing is - usually I use my free nrg Evgo to get me up to 90% then when I get home sometimes I'll plug in the 120v and bring me up to 100%. That final 10% should only be 2.4kwh (in theory) but I wonder if I'm pulling (and being billed for) much more than 2.4kw and maybe I should just leave it at 90%.

BTW...I've read the comments about how it could degrade your battery when charging past 80%. So I don't always charge to 100% unless I'm going to do a lot of driving that day.

The car draws less power when the charge rate tapers. There is a slight decrease in charging efficiency when the charge rate tapers because there are some fixed loads like the small coolant pumps, but the difference is very minor. Charge to 100% if you need the range, otherwise you can stop charging sooner. I always charge to 100% overnight because I routinely use the complete charge driving during the day.

Gerry

rebeccaloo in my experience I would say yes. I charge to 80% all summer and in winter most of the time it is to 100%. Pretty much every time I charge to 80% I get a better wall kWh to miles range than the 100% wall kWh to miles range. I attribute this to the extra time the pumps run and other normal losses when the car is doing it's top off or balancing charge at the top end.

Having said that I do charge to 100% at least once a week, even in summer, to make sure the car does do that balance charge.

It would be interesting to see the true percentage of efficiency charging up to 90% and then from 90% to full although I would imagine it would be different each time depending on the pack and other factors as well.

Welcome to the future rebeccaloo.

Charging slows down as the battery pack nears 100%, it does use a little more power, but it's also balancing the cells, so it's good to do occasionally, but using your NCTC card to 90% most times, when you don't need 100% is fine.

I, too have wondered about this also. I can easily figure it out if someone can tell me what percent of full charge the battery is at when it starts to taper (see graph link below). Is it exactly 80%? I have all the numbers in a spreadsheet for every charge since 2012.

cwerdna said:

Rauv said:

Here is my typical charging load profile that clearly shows tapering and bumps, presumably when balancing is occuring.

http://www.tc.umn.edu/~rauvo001/2013 Leaf Charging.pdf

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One item not mentioned yet is REGEN. Above 93 - 95% the regen is limited. If you have some amount of downhill from your home then you will not get that regen and will hurt your efficiency.

In my case it is a big hit. We almost always charge to 80% and by the time we reach the freeway we are at or around 10 miles/kwh. We usually get back home at around 4.3 miles/kwh. If we charge to 100% (we do this every other week or so for balancing) we will only be at around 4 miles/kwh when we get to the freeway and by the time we finally get back home we will be at or below 4 miles/kwh for the trip. We limit our freeway speed to 55 to 60 mph so as to improve our efficiency, but the hills we have to climb getting home really eats up our gains.

If your initial drive to get you down to 95% charge is fairly flat and you are not losing regen, then the only efficiency loss is what was stated above, the fixed current draw that continues when the charging rate tapers off above 95% charge until it reaches 100%. The last hour or so is just battery balancing, but this only needs to be done a couple of time a month IMHO.

I hope this helps.

Rauv said:

I, too have wondered about this also. I can easily figure it out if someone can tell me what percent of full charge the battery is at when it starts to taper (see graph link below). Is it exactly 80%? I have all the numbers in a spreadsheet for every charge since 2012.

cwerdna said:

Rauv said:

Here is my typical charging load profile that clearly shows tapering and bumps, presumably when balancing is occuring.

http://www.tc.umn.edu/~rauvo001/2013 Leaf Charging.pdf

Click to expand...

Click to expand...

Click to expand...

For the '13, it's definitely not 80%. It's somewhere well past 90% on the dash display, possibly as high as 98% and possibly as low as ~95% or so. I hardly charge at home (and don't have L2 at home anyway) and don't usually sit in my car/not near it when charging to 100% (am at work, where car is parked in a structure).

If the dash display reads 98%, it's almost certainly already in the taper/bounce phase. If I unplug at that point, the dash display will often jump to 100%.

About the only times I go to 100% (or almost 100%) and am sitting in my car are when I'm using free L2 public charging, which isn't on any network, so I can't see a graph nor anything indicating EVSE output levels. However, I do have Leaf Spy Pro now and can make a note of when those (x.x/x.x kW) numbers fall below ~5.4 or 5.5 kW in the charging graph screen.

Is that free NRG EVgo a quick charger by any chance? There's a major decrease of power delivered to the battery during quick charging past 90%, if someone is waiting it's best to just unplug and top off on a regular charger (smaller plug), it'll be same charging rate, or just drive if you have enough.

Valdemar said:

Is that free NRG EVgo a quick charger by any chance? There's a major decrease of power delivered to the battery during quick charging past 90%, if someone is waiting it's best to just unplug and top off on a regular charger (smaller plug), it'll be same charging rate, or just drive if you have enough.

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Yes, it's a DC/Quick charger. The EZ-Charge/NCTC card only gives me 30 minutes free so usually when my time's up I'm right around 90% and I always leave after that and open up the spot for someone else.

GerryAZ said:

There is a slight decrease in charging efficiency when the charge rate tapers because there are some fixed loads like the small coolant pumps, but the difference is very minor.
Gerry

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Actually it is quite large when you consider how low the charge level can go at the end of the cycle and how the parasitic loads can almost be on par with the charge rate at times. Of course this is not a long time but the curve along the way is pretty bad at the cycle end.

EVDRIVER said:

GerryAZ said:

There is a slight decrease in charging efficiency when the charge rate tapers because there are some fixed loads like the small coolant pumps, but the difference is very minor.
Gerry

Click to expand...

Actually it is quite large when you consider how low the charge level can go at the end of the cycle and how the parasitic loads can almost be on par with the charge rate at times. Of course this is not a long time but the curve along the way is pretty bad at the cycle end.

Click to expand...

EVDRIVER,

You are correct that the losses from the coolant pumps and other fixed loads can be large compared to the energy going into the battery near the very end of the charge cycle, but the overall impact on charging efficiency for the complete charging cycle is minimal. Estimating the fixed loads at 400 watts and tapering time at 1 hour yields 0.4 kWh of wasted energy to get a full charge vs. stopping the charge early. The bottom line is charge to 100% if you need the range because the extra cost is trivial.

Yes I am aware of that however is is still technically very inefficient when you map them tother at a certain point you can be almost at a 1:1 ration. Considering no cooling is needed on the charger at the end of the charge cycle in many cases.

For myself I was investigating the charging draw using the EVSE that comes with the Leaf. I plugged it into a kill-a-watt meter when charging my vehicle. The charge seemed to take a very long time 2+ hours to from 98%-100% and during this time the EVSE was still trying drawing 12 amp and 1300 watts of power. I will have to investigate if further but it seems to take a excessively long time to go the last couple of % points and it is drawing full power through the unit. Just my observation on the matter.

I only use public chargers and can track on Chargepoint. The drop off in amps after 90% is significant. I expect the EVSE works same away. Charging above is not a waste of electricity as much a waste of time. If someone else awaits my station, I need to move.