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[Killroy]

I wanted to know how efficient the charger was because PG&E rates are high-ish even if they are still a fraction of the cost of gas. I went over my "baseline usage" (where my rates go up) for the first time charging my Zero a couple of months ago.  I have NEVER done that before, so now I am trying to use wisely and have switch to "time of use" rates where off-peak I get a rate discount.  To charge off-peak, I use a simple plug timer, that is set to turn on during off-peak times.  I only need 25% of the battery to get to work.

I used a Kill-A-Watt meter to keep track of my kWhrs.

I commuted home from work and plugged in the next day at 69% state of charge indicated.  Since the website says that the practical capacity of my 2015 Zero SR is 11.0 kWhrs, that puts me at 7.59 kWhr in the battery.

I partial charged from 69% to 97% state of charge and according to my Kill-A-Watt, 3.67 kWhr was used at the 120VAC outlet, which should be, if I am calculating it right, 84% efficiency.

Then, I charged to 100% and the total energy used was 4.62 kWhr, so I calculate that at 74% efficien

In conclusion, the charge efficiency drops down to 74% when the charger is "topping" the pack and balancing. 

So, if you want to be most efficient, don't charge to 100% all the time.  Once in a while it is good to do a balance charge to maintain the back, but I dont think you need to do that each charge.

I would like to do this again at a lower state of charge and take a couple of data points. 

Does that make since?  Did I calculate it right using 11 kWhr for the "nominal capacity"?

[Electric Terry]

The dash indicator percentage is a good general indicator, but by no means a precise or rather I should say accurate measurement.  Better would be to use the kWh from the phone app, but even that isn't exactly accurate, as I've ridden 4 miles after it said there was 0.000 kWh left (the dash had gone to 0% 9 miles before that, the app will still show 4%).

I can say with a high degree of certainty that it is for all practical purposes just as energy efficient to charge to 100% as it is 97%.  A little bit of battery balancing is occurring but the bleed resistors are so small that the energy in kilowatt hours would be not measurable with the significant figures on the kill-a-watt meter.

The energy has to go somewhere.  If the battery isn't getting warm, and the charger isn't getting any hotter than normal, where did the extra energy go?

I know your PG&E rates were why you were thinking about this, but for now consider that the dash % indicator only has 1 significant figure.  92% can sometimes jump to 100 percent after turning off and back on.  There's a different reason that happens, but I'll leave it as my example.

I'm surprised you don't charge at Bell each day.  Surely there is a 110v outlet somewhere on that building.

[Killroy]

OK, cool, I will have to try the same thing with the app state of charge because it is more accurate.  I was going to look at it, but the app was having problems connecting.

What do you think the charge efficiency is?

I have noticed 5% to 10% jumps in the state of charge when turning the bike on and off. 

I do charge at work mostly now even though we have not been given specific permission.  I still was interested in the charge efficiency, so I can know how much it cost to charge.

[Electric Terry]

I am guessing about ~90% from 120v and probably closing in on near 95% from 240 volts as the charger stays much much cooler with a higher input voltage.  The delta Q's and Elcons also run much much cooler at higher input voltage.

I'd be interested in someone doing a test to see what it actually is. 

I've always just measured total miles and divide by energy to recharge (including charger losses since it's something I can't really control)  Although switching to a charger built with silicon carbide mosfets could get you even higher efficiency.  Although electricity would need to go to over a dollar per kWh to ever make financial sense to do this.

[grmarks]

My power is 240V and my power meter goes up 1.4 kw/h when I plug in my onboard charger. So if it is a 1.3 kw/h charger then that is 93% efficient at that instant.

[protomech]

The dash indicator percentage is a good general indicator, but by no means a precise or rather I should say accurate measurement.  Better would be to use the kWh from the phone app, but even that isn't exactly accurate, as I've ridden 4 miles after it said there was 0.000 kWh left (the dash had gone to 0% 9 miles before that, the app will still show 4%).

The phone app appears to simply show kWh using Zero's max capacity rating, i.e. present voltage times present amp hours.

The voltage and amp-hours are likely to be correct, but the kWh readout is useless (or worse, given the confusion it's generated on the forums).

Although switching to a charger built with silicon carbide mosfets could get you even higher efficiency.  Although electricity would need to go to over a dollar per kWh to ever make financial sense to do this.

Little reason to do it for reduced electricity usage. A Zero over 100k miles will use around $1000 in electricity. Improving from (for example) 92% to 94% efficiency will save about $20 over the 100k miles, but more importantly it reduces waste power - mostly heat - by 25%! This allows for more power dense and compact chargers to be built.

[MrDude_1]

...more importantly it reduces waste power - mostly heat - by 25%! This allows for more power dense and compact chargers to be built.

bingo.
Until we have 1C or less on-board chargers that limit themselves by their input connection, there is major room for improvement.
First step to that is having a reliable compact charger... preferably one that does not need a fan.

[Killroy]

Little reason to do it for reduced electricity usage. A Zero over 100k miles will use around $1000 in electricity. Improving from (for example) 92% to 94% efficiency will save about $20 over the 100k miles, but more importantly it reduces waste power - mostly heat - by 25%! This allows for more power dense and compact chargers to be built.

So, I should be charging on 220 VAC? ha ha

Looks like you have good rates are ride efficiently, because I calculate that it would cost me $2000 to ride 100,000 miles.  I get the point. 

Yet, I would like to charge on 220 VAC anyway if I could find a plug adapter for $20-40.   

[Killroy]

The phone app appears to simply show kWh using Zero's max capacity rating, i.e. present voltage times present amp hours.

The voltage and amp-hours are likely to be correct, but the kWh readout is useless (or worse, given the confusion it's generated on the forums). .

What is a better way to do calculate state of charge/ kWhr capacity?

[Killroy]

I redid the test using the phone app even though it was suspect.

I started at 73% state of charge indicated,  and apparently the charge efficiency is 107%.

One thing I noticed is that the spec on the 2015 SR is 11.0 kWhr practical battery capacity, but at 100% state of charge the phone app showed 11.6 kWhr. 

If it is really 11.0 kWhr, then the efficiency calculated to 90% - if you trust that.

[Electric Terry]

92% from 115v, 94% from 220v according to specs I found here:

http://www.greenwattpower.com/pdf/evc1200.pdf

So to calculate your real battery capacity used, use the kill-a-watt meter to record kWh during the charging session and then multiply by .92

[Richard230]

My 14.2 2014 S currently shows 13.34 kWh on the Zero app when fully charged to 100%.  The one and only time I ran my battery pack down to 0% and recharged with a Kill-A-Watt attached to the wall (which was showing 122v at the time) my total power consumption was 13.75 kWh according to the meter (which I don't necessarily trust as I have the cheap one    ).

Given Terry's specification of 92% efficiency for the charger, that would have repacked the pack with 12.65 kWh of electricity.  At the time, when it was fully charged, the Zero app read 13.22 kWh.  I have no idea what all this means, but I just thought I would throw the numbers out there in case they are of interest to someone.   

[Electric Terry]

Thanks Richard! Those are some good real world numbers and pretty darn close.  Surely there are some energy losses going into the battery, some in the cord and other places which are hard to account for so this about makes perfect sense!

[xmjsilverx]

Aren't the SOC percentages just a calculation of the energy used by the bike and therefore aren't even truly accurate because the bike doesn't actually measure the battery SOC?  Maybe I am confused.  If that's the case then there really isn’t a way to determine charging efficiency.

[Doug S]

I'm an EE, I've spent a fair amount of time designing with batteries, and although it would be really nice if batteries would precisely behave according to some fixed model, they just don't. SoC is a highly situational thing, and trying to represent it with a single number just really isn't adequate. It depends quite a bit on rate of discharge, ambient temperature, cell self-heating, the nature of the load (constant vs. pulsed), etc., etc.

Believe me, the user ALWAYS wants to know to the second how long his phone battery is going to last, and it's just not possible to provide that information in a scientifically precise way. The best you can do is provide a good estimate, and from what I've seen Zero does a pretty good job of estimating SoC.