Charging overhead - why L1 is so inefficient.

[Tomasz]

Ever wandered how much overhead does the charging process have? Yesterday I've charged my i3 yesterday from 7 to 100% using my 32A EVSE in less than 4h:

Even at 100% it was still drawing about 600W of energy, just to cover it's charging process.

To validate my result - the whole charging consumed 20.2kWh, my battery has max capacity 19.2kWh so from 7 to 100 it needed about 17.8kWh of energy. That leaves about 2.4kWh of overhead, which divided by almost 4h of charging time confirms my findings.

That also shows why L1 charging is so inefficient. Your EVSE will allow the car to draw only 12Amps which is 1.4kW. If you car is wasting 0.6kW just to cover it's overhead your battery is charged at an astounding rate of about 0.8kWh. This also suggests that simply going to L1 5-20 EVSE capable of delivering 1.9kWh should cut your charging time almost in half. Which I'm going to confirm later when I find myself a 5-20 plug.

 

[alohart]

When the i-MiEV reaches an indicated 100%, it continues charging those battery cells whose charge levels are less than those cells that have reached 100%, but at a substantially lower charging rate (i.e., current or power). This is called "top balancing" and can take up to an hour after the battery pack's indicated charge level has reached 100%.

I haven't read that the i3 top balances in a similar fashion, but all series-connected battery packs need to do something like this periodically to keep the charge levels of their individual cells in balance. This could explain the 600 w. that you have measured. If you continued charging for more than an hour after the indicated 100% level, you might observe the charge rate drop to a much lower level after top balancing finishes.

 

[Tomasz]

I haven't read that the i3 top balances in a similar fashion, but all series-connected battery packs need to do something like this periodically to keep the charge levels of their individual cells in balance. This could explain the 600 w. that you have measured. If you continued charging for more than an hour after the indicated 100% level, you might observe the charge rate drop to a much lower level after top balancing finishes.

The graph above shows power measures with the car plugged in until the very end. I did not disconnect the car, the car stopped taking any power from EVSE. Also as I've said - numbers do match, I doubt i3 uses 2.4kWh to balance the pack _every_ single time it's charged, but stable 600W overhead can be easily explained (loses in charger, active battery pack cooling, etc)

BTW: I see similar 'baseline' consumption with our Leaf, in that case it's lower (about 300W) and in line with various internet sources which IMHO even further validates my statement that what I see here is charging overhead.

 

[jadnashuanh]

FWIW, the i3 never allows the EVSE to get the batteries to a real 100%, it uses that buffer at the top (and the bottom) to help preserve them and at the top to have some room if you get some regeneration right after driving away with an indicated full charge...otherwise, it would have to turn off regen or put it through some resistive load, but it couldn't safely try to dump it into the batteries, and the change in driving dynamics would be disconcerting if it didn't maintain the regen (most) of the time. It does adjust the regen depending on wheel slip and steering wheel angle though.

 

[wraithnot]

Ever wandered how much overhead does the charging process have? Yesterday I've charged my i3 yesterday from 7 to 100% using my 32A EVSE in less than 4h:

Even at 100% it was still drawing about 600W of energy, just to cover it's charging process.

To validate my result - the whole charging consumed 20.2kWh, my battery has max capacity 19.2kWh so from 7 to 100 it needed about 17.8kWh of energy. That leaves about 2.4kWh of overhead, which divided by almost 4h of charging time confirms my findings.

That also shows why L1 charging is so inefficient. Your EVSE will allow the car to draw only 12Amps which is 1.4kW. If you car is wasting 0.6kW just to cover it's overhead your battery is charged at an astounding rate of about 0.8kWh. This also suggests that simply going to L1 5-20 EVSE capable of delivering 1.9kWh should cut your charging time almost in half. Which I'm going to confirm later when I find myself a 5-20 plug.

I completely agree that L2 charging is more efficient than L1 charging, but it's more complicated that simply using a constant 600W to cover the charging process. Converting alternating current from the wall outlet to direct current to charge the battery isn't 100% efficient- 90% efficient is probably a good estimate but this will vary a bit based on voltage and amperage. Charging also generates heat and removing this heat will require energy if you have an active cooling system. Finally, you need to have various processors booted up to monitor the whole process- and only this power draw will be independent of charging power.

I was playing with some new gadgets I got for Christmas and measured the following power draws from the wall while charging my wife's i3- 1380W at Max L1 power, 1034W at reduced, and 699W at low. The predicted finish time for the low setting didn't seem dramatically different than twice the time for the max setting so I'm pretty sure there isn't a constant 600W overhead for L1 charging.

I don't have enough information to estimate exactly how much less efficient L1 charging is than L2 for an i3. But if it's similar to my Model S, then L1 charging is something like 80% as efficient as L2 charging (my Model S adds range about 5 times faster with 4x the amount of charging power using our dryer outlet than using a standard wall outlet).

 

[mtd240]

Ever wandered how much overhead does the charging process have? Yesterday I've charged my i3 yesterday from 7 to 100% using my 32A EVSE in less than 4h:

Even at 100% it was still drawing about 600W of energy, just to cover it's charging process.

To validate my result - the whole charging consumed 20.2kWh, my battery has max capacity 19.2kWh so from 7 to 100 it needed about 17.8kWh of energy. That leaves about 2.4kWh of overhead, which divided by almost 4h of charging time confirms my findings.

That also shows why L1 charging is so inefficient. Your EVSE will allow the car to draw only 12Amps which is 1.4kW. If you car is wasting 0.6kW just to cover it's overhead your battery is charged at an astounding rate of about 0.8kWh. This also suggests that simply going to L1 5-20 EVSE capable of delivering 1.9kWh should cut your charging time almost in half. Which I'm going to confirm later when I find myself a 5-20 plug.

Very interesting, thanks for the graph! I have a NEMA 5-20 available - can I just plug in the occasional use cord and charge at 1.9 kW, or do I need a "20 Amp Level 1" EVSE?

 

[alohart]

I have a NEMA 5-20 available - can I just plug in the occasional use cord and charge at 1.9 kW, or do I need a "20 Amp Level 1" EVSE?

Unfortunately not. The maximum current that the occasional use charger can provide is 12 amps.

My JuiceBox EVSE, which is capable of charging at more than 40 amps, is plugged into a 208 v. 20 a. circuit. I have limited its current to 16 a. which is sufficient to charge my i3 fully while I sleep. That's all I need.

 

[janner]

Tomaz - What's happening before 2230?

 

[jadnashuanh]

FWIW, except in a few models, an EVSE is setup for a specific maximum output, and is adjusted at the factory to send out a signal that matches that value to the attached EV. It is then up to the EV to decide how much power it will take from the EVSE, and never trying to exceed what the EVSE's pilot signal says is available. IOW, plugging the EVSE into a bigger supply, unless it has some special internal circuitry (a few do) will not produce a bigger available output. It might mean the difference of telling the car to charge at maximum verses a lower value if that circuit must share loads with some other devices, though (say you have a 20A circuit that is powering the garage door opener and you are using it for the EVSE as well...without dropping the i3's charging rate, you might pop the breaker or fuse on maximum when the door is opened, probably not with a 20A circuit).

 

[janner]

I guess I was trying to establish whether Tomasz started charging at 2240 or at 2150 as I'm not sure what is happening at the beginning of the graph.

 

[imolazhp]

Cool story bro. So you charged your car at L2, recorded the data and poof "L1 is so inefficient."

Where is your L1 data for comparison?

 

[imolazhp]

I couldn't find any other easy way to add this from the FB group, this is not my work.

It shows both L1 and L2 charging at different settings within iDrive (Min/Max), you will note that the difference in overall efficiency between Max L2 and Max L1. 93.4% vs 89.5% respectively; feel free to conduct your own testing and get back with us. A graph showing one charging session on L2 for your car plus some words below it are hardly enough to convince me that L1 is "so inefficient."

Entire document can be found at https://www.facebook.com/download/1...AVT - Idaho National Laboratory - 2014 i3.pdf

 

[jadnashuanh]

There is more energy lost in converting 120vac to 380vdc (approximately) than from 240vac to the same dcv. That is the primary difference in energy use between a level 1 and a level 2 unit. There is some background stuff going on in the car, and that will need to be running longer with a level 1 charge as well, but that isn't huge. Because the level 1 is slower, you will have more of a chance for the heat buildup to be minimized since more of it will be able to radiate away. IOW, you're more likely to need the battery conditioning circuits to run using level2 than with level 1.

Once the car disconnects, the EVSE will still be drawing some current from the mains...it has internal pilot lights (usually), a small logic board and power supply that is constantly generating the pilot signal. My meter only reads to 0.1A, and it never shows other than zero A once the car disconnects. That's less than 24W, and I think, typically probably much less. When it is on and actively charging, the EVSE itself probably draws about 50W or so from the main, and unless you're measuring the in-line output verses the input, you'll have to subtract the EVSE's contribution to the total load. Large contactors and the additional indicator lights during actual use will add to the EVSE's power need - that contactor's coil will use a moderate amount of watts in comparison to its standby use, but is still small in comparison to what is going into the car.

 

[alohart]

I couldn't find any other easy way to add this from the FB group, this is not my work.

We don't need no stinkin' FaceBook. Read the original source instead.

 

[GI3L]

i did charge my i3 and volt when i drove them both for the same distance and same route on the same day
the volt took 3200 watt and the i3 took 3300 watt....

:shock:

 

[bwilson4web]

i did charge my i3 and volt when i drove them both for the same distance and same route on the same day
the volt took 3200 watt and the i3 took 3300 watt....
. . .

Speed is the determinant. The lower, aerodynamic Cd, of the Volt and cross-section, it should do better at speeds over 45 mph. But at urban speeds, the BMW i3-REx should do better. Regardless, we'll assume these are 3.2 kWh and 3.3 kWh.

In my BMW i3-REx, I would expect 3.3 / .220 ~= 15 miles.

I've not ridden in a Volt but noticed they are much lower than our BMW i3-REx. Did you find the lower stance a problem or just different?

The reason I ask is my REx maintenance light came on Tuesday so I had to run a long enough trip to run the REx. Then I realized I hadn't driven our 2010 Prius for the same period. So I drove it into work this morning. Lower than the BMW i3-REx, it was not as bad as trying to get in a BMW Series 2 that felt like getting into a bathtub.

Bob Wilson

 

[GI3L]

i did charge my i3 and volt when i drove them both for the same distance and same route on the same day
the volt took 3200 watt and the i3 took 3300 watt....
. . .

Speed is the determinant. The lower, aerodynamic Cd, of the Volt and cross-section, it should do better at speeds over 45 mph. But at urban speeds, the BMW i3-REx should do better. Regardless, we'll assume these are 3.2 kWh and 3.3 kWh.

In my BMW i3-REx, I would expect 3.3 / .220 ~= 15 miles.

I've not ridden in a Volt but noticed they are much lower than our BMW i3-REx. Did you find the lower stance a problem or just different?

The reason I ask is my REx maintenance light came on Tuesday so I had to run a long enough trip to run the REx. Then I realized I hadn't driven our 2010 Prius for the same period. So I drove it into work this morning. Lower than the BMW i3-REx, it was not as bad as trying to get in a BMW Series 2 that felt like getting into a bathtub.

Bob Wilson

well the volt is a lot lower.... not good when you have a bad back.

the trip was 24,8 and a average speed in km/h of 37.8 the trip lasted round trip 40min

 

[jadnashuanh]

I like the height of the i3. Not only is the seat height on the Volt a lot lower, the seat to roof opening of the door is shorter, and for someone who is stiff and tall, it hurts every time I try to get into one, so it was a no-go from the start. Then, it's a Chevy...need I say more?

 

[GI3L]

i did charge my i3 and volt when i drove them both for the same distance and same route on the same day
the volt took 3200 watt and the i3 took 3300 watt....
. . .

Speed is the determinant. The lower, aerodynamic Cd, of the Volt and cross-section, it should do better at speeds over 45 mph. But at urban speeds, the BMW i3-REx should do better. Regardless, we'll assume these are 3.2 kWh and 3.3 kWh.

In my BMW i3-REx, I would expect 3.3 / .220 ~= 15 miles.

I've not ridden in a Volt but noticed they are much lower than our BMW i3-REx. Did you find the lower stance a problem or just different?

The reason I ask is my REx maintenance light came on Tuesday so I had to run a long enough trip to run the REx. Then I realized I hadn't driven our 2010 Prius for the same period. So I drove it into work this morning. Lower than the BMW i3-REx, it was not as bad as trying to get in a BMW Series 2 that felt like getting into a bathtub.

Bob Wilson

well the volt is a lot lower.... not good when you have a bad back.

the trip was 24,8 and a average speed in km/h of 37.8 the trip lasted round trip 40min

o i forgot

The volt reported 24,8km and 2700-2800watt/h its only displays 2.7 so it can be almost 2.8
thats a calculated 2700 to 2800 / 24,8 = 108,87 to 112,9 watt per km.

the i3 reported 24,8km and 11.2kwh/100km or 112 watt per km.

and at the socket after charging 3300 for the I3 en 3200 for the volt.

again 37,8kmph average speed.

 

[bwilson4web]

Excellent!

. . .
The volt reported 24,8km and 2700-2800watt/h its only displays 2.7 so it can be almost 2.8
thats a calculated 2700 to 2800 / 24,8 = 108,87 to 112,9 watt per km.

the i3 reported 24,8km and 11.2kwh/100km or 112 watt per km.

and at the socket after charging 3300 for the I3 en 3200 for the volt.

again 37,8kmph average speed.

• 24,8km = 15.5 mi
• 112,9 watt(1) per km = 180.6 W/mi
• 112 watt per km = 179.2 W/mi
• 37,8kmph = 23.6 mph

1) I assume this is Whr.

I will try to replicate this with our BMW i3-REx. There is a rain-delay in the baseball game and "she who must be obeyed" announced I have to get some beer. It is night and moderate temperatures.

Bob Wilson