Fully charge battery or charge as needed for kWh savings

Am wondering whether I am "wasting" electricity by routinely fully charging my 2013 Model S - we have two 2013 Model cars and one 220v station in my home. Now on that I am on PV at home- I am watching my electricity usage more. One car has to charge daily due to daily travel distance. I can drive mine to work/back charging only once every 3-4 days. Does routinely charging to full "waste" kWhs? Likening this to always charging my phone when it isn't needed. I know the internal charger will stop once full - but recall hearing about the slow charging from 80% or so to full - wondering if just a habit I should break. thanks in advance

maui1 said:

- but recall hearing about the slow charging from 80% or so to full

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On L2, that above is definitely not true, it somewhere above 94% (as indicated on that car's % SoC display), possibly as high as 96 to 98%.

I quoted a '13 Leaf charging group at http://www.mynissanleaf.com/viewtopic.php?p=338038#p338038" onclick="window.open(this.href);return false;. In the last hour or so on L2, very little energy makes it into the battery due to the ramp down and bounces.

As I've reported at http://www.mynissanleaf.com/viewtopic.php?p=339977#p339977" onclick="window.open(this.href);return false; and elsewhere, if you pull the plug when the % display says 98%, it will often "jump" to 100%. I'm pretty sure that when indicating 98%, it's ramp down phase, if not the bounce phase.

To reduce electricity use, you should definitely charge on L2. Don't charge at 120 volts, unless you want to throw away more electricity, due to higher charging losses on L1.

Unlike your cellphone, the EVSE disconnects itself when fully charged, so there is no wasted energy. Once that contactor in the EVSE opens, the electricity flow stops. I believe that the slow charging from 80% to full you have heard about is on Quick Charge stations, which is a MUCH higher rate of charge and heat issues to deal with, which don't apply to L2 charging.

Especially if you live on a hill, charging to full will lead to some wasted energy and more wear on the brake pads due to the lack of regenerative braking. Depending on how much braking you have to do near the beginning of your drive, this may or may not be consequential.

+1 to this. I recomend daily 80% charging for the one that drives less miles so you do not loose the regen from 100% down to 95%. It should take less than one hour daily, then you can plug in the other one.

abasile said:

Especially if you live on a hill, charging to full will lead to some wasted energy and more wear on the brake pads due to the lack of regenerative braking. Depending on how much braking you have to do near the beginning of your drive, this may or may not be consequential.

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maui1 said:

...Does routinely charging to full "waste" kWhs?...

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Yes.

In addition to the regen energy loss mentioned when driving from "100%", the battery's overall trip efficiency is also clearly lower, but without results from the lab, the efficiency losses are difficult to quantify.

We do have OTE data (albeit at unknown temperatures, which is an additional variable) from a "completely depleted to fully charged":

P.21

A) Wall Plug energy 21.722 AC kWh
C) DC energy to pack 18.529 DC kWh
E) DC Test energy * 17.957 DC kWh

Overall Trip Efficiency E/A 82.7%

Analysis Note:
Values are based on
13 level 2 battery
charge events from
completely depleted
to fully charged

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http://www.transportation.anl.gov/D3/data/2012_nissan_leaf/AVTALeaftestinganalysis_Major%20summary101212.pdf" onclick="window.open(this.href);return false;

But how much higher OTE is for using less of the pack, not accessing the capacity either at the top or the bottom, has not been reported by the AVTA, nor any other reliable source, AFAIK.

I do 90%+ of my driving in 50-55 mile trips, starting with ~2,000 ft descent (~1500 ft net) and ending with the same ascent, beginning at "80%", and ending anywhere from just before the LBW (in Summer) to just before the VLBW (in Winter).

Charging to "100%" costs me only about ~.6 kWh of regen from the initial descent on each of these trips (this varies by season, and remember, most of my descent energy is still recovered directly, even when I have a "100%" battery pack) but I find it really annoying to have to use my brake pedal so much.

I can report that I seem to get an additional ~ 2% improved OTE (~85%) on average year round on these trips as compared to the different route (from "100%" to ~VLBW, and only once, all the way to ~turtle) I use to test my maximum available capacity.

Why don't you want to set your charge to "80%"?

Since you have a 2013, the answer is simple. Charge to 80% every day. There's no downside, and you never know when something will come up and you need to run somewhere. No sense running the battery down farther than needed. Charge to 100% if/when you expect you might need it.

I have also noticed that if I stop charging just shy of 100%, say at 98% and then unplug the car, the car will show 100% capacity. Also I've taken battery readings at this state and the Gids are nearly identical to a full 100% charge. My guess is that most of what occurs in the last few percent is simply cell balancing, so you're still at roughly 100%.

I charge to 100% most of the time, but that's just because we charge up at work and someone else is paying the bill so I don't care about efficiency. I think charging to 100% depends on the year of your car. The 2015 models seem to not care at all about charging to full and this doesn't seem to affect capacity loss. Even so, I'm charging to full after lunch so it never sits for more than an hour at full capacity. Also consider your driving habits at full capacity. if its a usual commute where there is no regen capabilities it shouldn't matter. For example, my co-worker literally leaves his house and drives straight up the Santa Cruz mountains, so for him, there's no sense in only charging to 80% since he won't be able to take advantage of regen.

Maui1, what rate plan are you on? Have you considered a time of use(TOU) plan?

What about the energy lost due to carrying around all of those extra electrons? That must really kill your range. I'm just joking.

But seriously, there is an energy loss if you always top off the battery. I don't know if it is meaningful, but it is real. When you charge above 97%, the car goes through a cell-balancing algorithm. The way that it balances cells is that it stops charging, measures each cell, finds the high ones, and puts a load on the high ones until they discharge to the same level as the others. In other words, it wastes energy when it balances. It may not be enough energy to matter, but it is waste.

On the other hand, there is a good reason to charge often, and it is surely measurable. You may want to take an unexpected side trip. That extra charge will allow you to go farther without stress.

Bob

Thanks everyone for the detailed and quick replies - I know where to turn for answers - Mahalo!