Level 1 charger efficiency calculations - I ran the math

I was reading posts about how the level 2 charger is supposed to be like 10% more efficient than the level 1. I run my level 1 charger through the Belkin WeMo, which tells me the energy usage. I've confirmed this with the RainForest device that talks to my smart meter, and further confirmed it by logging into my SDG&E account and looking at my usage. The level 1 charger costs 1.2kWh per hour of operation. It takes me 12 hours of charging to gain 52.5% of capacity on the battery. The battery is suppose to hold 24kWh. 52.5% of that is 12.6kWh. So, I supply it with 14.4kWh of juice (12hours*1.2kWh/h) to get 12.6kWh of charge. That's an 87.5% efficiency of energy transfer.

My question is, has anyone run this math for their level 2 charger? It's hard to believe they're getting 97.5% efficiency transfer, but I'll believe it if I see the numbers.

check out this thread

http://www.mynissanleaf.com/viewtopic.php?f=31&t=8583" onclick="window.open(this.href);return false;

Most of that older thread is from 2012. Any chance things have improved in the past two years? Sounds like this OP was pretty thorough.

ElectricMonkey said:

I was reading posts about how the level 2 charger is supposed to be like 10% more efficient than the level 1. I run my level 1 charger through the Belkin WeMo, which tells me the energy usage. I've confirmed this with the RainForest device that talks to my smart meter, and further confirmed it by logging into my SDG&E account and looking at my usage. The level 1 charger costs 1.2kWh per hour of operation. It takes me 12 hours of charging to gain 52.5% of capacity on the battery. The battery is suppose to hold 24kWh. 52.5% of that is 12.6kWh. So, I supply it with 14.4kWh of juice (12hours*1.2kWh/h) to get 12.6kWh of charge. That's an 87.5% efficiency of energy transfer.

My question is, has anyone run this math for their level 2 charger? It's hard to believe they're getting 97.5% efficiency transfer, but I'll believe it if I see the numbers.

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While I think your calculation is close, I believe it is little high for a L1 EVSE. There are a number of sources of error for your calculation.
1. I assume you are using GIDs to get that 52.5% number of how 'full' your battery is. GIDs are the only units that are going to give you the resolution you need to get a efficiency calculation. But depending on cell balancing that GID number may be a little off. LEAFSpy or a GIDmeter are what you need for that.
2. The number of kWh (or GIDs) placed in your battery is a function of it's current capacity which can vary from battery to battery and so it is not necessarily a percentage of 24kWh. Therefore you have to be careful mapping from GIDs to kWh, most use 1 gid = 0.08 kwh.
2. Battery Temperature will change the battery's ability to acquire charge. So one day it might take 12.6kWh from the wall to get 220 GIDs and the next 11.5kWh or 13.5kWh to get the same number of GIDs.

I'll have to find the source, but I believe L1 efficiency is typically < 83% and L2 is around 90% but that depends on your EVSE. This is because there are two sources of loss which will affect the efficiency: (1) the a fixed overhead of the EVSE controller board which varies by EVSE and (2) the conversion efficiency of the AC to DC on the LEAF's on-board inverter. When using a L2 EVSE #1 doesn't change but becomes a lower loss percentage wise when charging @L2 b/c more kW are being pumped into the car. I am unaware of any Solar inverter on the market today >98% effective efficiency (there are weighted efficiencies around 98.5 but that's fuzzy math at best), I believe the LEAF's inverter is around 93%. So while L2 efficiency should always be higher than L1, if the EVSE Controller has a controller board that burns a lot of juice the efficiency may not be as high as another EVSE.

However it's safe to state that you're definitely not going to beat the LEAF's on-board inverter efficiency (93%) no matter what EVSE you use.

And don't forget you have a charging loss of about 333wh the entire time your charging with either the L1 or L2. So in 2 hours charging with L2 you loose about 666wh to overhead, while charging with L1 the same amount would roughly be 8 hours or 2664wh lost. So thats an extra 2kwh turned in to heat by using the L1 vs L2 and that's just in overhead not to mention the greater efficiency you get starting with 240v rather than 120vac.

ElectricMonkey said:

The level 1 charger costs 1.2kWh per hour of operation.

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Really? It ought to draw something closer to 1.44 kW (120 V x 12 A). I'll measure mine sometime — can't do it now — to see what number I get (haven't used L1 in ages).

It takes me 12 hours of charging to gain 52.5% of capacity on the battery. The battery is suppose to hold 24kWh. 52.5% of that is 12.6kWh.

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No. The usable portion of the battery is around 22 kWh. If you are using SOC % as your gauge you should be aware that it is a very rough calculation, not completely linear, and that "100% is actually in the 93-95% range (on older cars, might be a bit higher on the new ones).

So, I supply it with 14.4kWh of juice (12hours*1.2kWh/h) to get 12.6kWh of charge. That's an 87.5% efficiency of energy transfer.

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When you add up the small errors I think you will get a very different figure.

My question is, has anyone run this math for their level 2 charger? It's hard to believe they're getting 97.5% efficiency transfer, but I'll believe it if I see the numbers.

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I generally see L2 efficiency in the 88-90% range with a 3.8 kW OBC (it ought to be somewhat higher with the 6.6 kW OBC, due to reduced overhead). That's using a wall meter and the dash mileage efficiency meter over a month (to get more usable data). But the dash meter is coarse (only one decimal) and, perhaps, of questionable accuracy.

Also, when checking efficiency of charging, be aware that charging to "100%" is less efficient than charging to a lower SOC level. Why? Because the charge tapers as it approaches "100%" but the charging overhead — cooling system, electronics, and the like — remains the same. So, the efficiency numbers will vary depending on whether or not you routinely charge to "100%".

FWIW.

padamson1 said:

the conversion efficiency of the AC to DC on the LEAF's on-board inverter.

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Rectifier

ElectricMonkey said:

It takes me 12 hours of charging to gain 52.5% of capacity on the battery. The battery is suppose to hold 24kWh

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As dgpcolorado pointed out, this is your big error. Forget whatever you're using to determine that "52.5%", and forget the nominal 24kwh capacity. Instead measure how much energy you used driving after a full charge by resetting your efficiency meter and trip meter before you start driving. Before you charge again, divide miles by miles/kwh to get kwh used. Then divide that number by the kwh consumed from the wall to determine charging efficiency. Carwings could also give you that number but at least with my 2011 it's terribly inaccurate. You should do this over the course of several days and average out the results to account for the rounding errors in your data sources. Also you might want to compare the efficiency using both 80% and 100% charges (just make sure you use the same type of charge before and after you get the kwh used data from the car.)

I just performed some similar calculations on my Rav4-EV and observed about 80% efficiency on L2 (strangely the same for both 24 and 40 amps) and 66% efficiency on L1 (16 amps.) I guess that's the price we pay for thermal protection.

In short any energy calculations on a battery is hard.. like quantum mechanics

pkulak said:

padamson1 said:

the conversion efficiency of the AC to DC on the LEAF's on-board inverter.

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Rectifier

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GP. Somehow I my mind got hooked in on Inverter (DC->AC which the motor uses) and not the rectifier (AC->DC) which is used for charging. But the efficiency numbers are still similar. While low-voltage rectifiers are low efficiency (<85%), efficiency does improve with voltage so it is still safe to say 92-93% is the high end at 240v.

So, I finally broke down and ordered a Level 2 EVSE from EVSE Upgrade. It should come in the next few days. I had an electrician pull the wire and wire up a plug for it. He tapped into the electric wire for the dryer and extended it, because our dryer is gas, so we're not using the 240V circuit. That saved money. The plug install was less than $200, which surprised me. He went from the laundry room on the house-side of the garage, through that attic, all the way to the street side of the garage, so that's got to be like 30 feet of wire.

Anyway, my reason still wasn't economic. My reason was purely convenience. We're getting into that busy time of year at work, and I don't think I'll be able to get a full charge overnight when I start putting in 10 to 14 hour days again. I also doubt that I'll have time to visit a charge station.