How to measure charge?

[bwilson4web]

I'm not finding the charging stations are consistent in reporting the charge power. The Whole Foods charger does but the free one at a local shopping center and the level 1 charger that came with the car do not. About the only reliable metric is SOC found when I park the car and later when I start it up. Have I missed anything?

The other option is to use the trip meter between charges. The "mi/kWh" divided into the miles traveled will give kWh. With a record of SOC%, at least I'd have something. Right now, I don't have much unless I use the Whole Foods charger.

It strikes me that a smart 'coupling' might measure each charge giving some engineering metrics. Has anyone seen something like that?

It seems so natural to provide charge metrics in engineering units, I'm baffled that there isn't any.

Thanks,
Bob Wilson

 

[brorob]

Are you looking for the charging rate, total KW used, etc.?

If you want to take measurements when using your occasional use cable, you could get a device like the Kill-A-Watt. This measures things like watts, amps, volts, time, KW consumed, etc. I use one of these when I have to use my UAC at home and have (analy) logged stats from all the times I've charged at home. Easy to find out how much I've spent to charge the car.

Don't know of any device for Level 2 but you could calculate the total KW used (roughly) using the SOC. The charging rate would be tougher because the ones I've seen measurements for start high then drop after a while, even at a lower SOC.

 

[gt1]

Bob, you can measure the L1 charging with this
For the home L2 something like this should work. Obviously, not a plug and play device. It is possible to build a short J1772 extension with this meter in the middle for use with the public chargers. Keep in mind that J1772 connectors are expensive. I think you will also need a special crimper.

 

[jadnashuanh]

Don't have any clue how well this works, but this may be your most creative solution https://emotorwerks.com/store-juicebox-ev-charging-stations/1631-juiceplug-a-universal-smart-ev-charging-adapter/category_pathway-23

 

[janner]

You can measure the AC input into your vehicle but then you will have losses for the rectifier (~94%) and the input and output from the battery (~90%). For DC rapid it's similar except the rectifier loss is upstream if the measurement. The losses vary with conditions. What your vehicle battery SOC will show is what the relative SOC is rather than the absolute but what it actually extracts from the battery depends on temperature and driving style. Sometimes it seems rather like Schrödinger's cat.

 

[bwilson4web]

One of the features of the JuiceBox Pro are four, web metrics showing: amps, temperature, volts, and kilowatts. The software does not record these metrics, just a graph showing the kWh. However, I manually read the values and saved them to a spreadsheet:

The first thing is the temperature rise, 35-40C, during this charge event. It appears that when the current drops to the 20-25A range, the temperature increase moderates. One of the control parameters is the house current limit now set to 50A. FYI, I though the car would take 32A so I'm still wondering about that one.

My wife announced at 3:45 AM that she wants a 'hot dog with all the fixin' so I'm about to go out. I'll set the house current limit to 20A and when I get back, see how it measures out. My goal is to trade off current limit and (I**2)*R losses.

Has anyone seen other studies or reports on this technique?

Any thoughts about whether the BMW i3 charger is power or current limited? I am seeing a peak of 7.2kW charge rate.

Thanks,
Bob Wilson

 

[alohart]

The first thing is the temperature rise, 35-40C, during this charge event. It appears that when the current drops to the 20-25A range, the temperature increase moderates.

The temperature increase that you are seeing at 30A can activate the battery pack cooling system. This consumes a fair amount of electricity thus reducing charging efficiency considerably. I charge at 208V 16A with ambient temperatures typically in the high 70's and have heard the battery pack cooling system activate only when I have begun charging immediately after driving. I have always been able to avoid having the battery pack cooling system activate if I delay charging a few hours after driving.

FYI, I though the car would take 32A so I'm still wondering about that one.

I don't recall anyone in the U.S. reporting more than 30A. Some have theorized that maximum current is rated differently in Europe so that 32A is the maximum in Europe.

Any thoughts about whether the BMW i3 charger is power or current limited? I am seeing a peak of 7.2kW charge rate.

Current seems to be limited to 30A because 30A seems to be the maximum regardless of the supply voltage.

 

[bwilson4web]

Thank you!

The first thing is the temperature rise, 35-40C, during this charge event. It appears that when the current drops to the 20-25A range, the temperature increase moderates.

The temperature increase that you are seeing at 30A can activate the battery pack cooling system. This consumes a fair amount of electricity thus reducing charging efficiency considerably. I charge at 208V 16A with ambient temperatures typically in the high 70's and have heard the battery pack cooling system activate only when I have begun charging immediately after driving. I have always been able to avoid having the battery pack cooling system activate if I delay charging a few hours after driving.
. . .

In North Alabama summers, we're often in the 90-100F range when I get home, often +5F measured by the car over our asphalt streets. In one respect, I don't like the idea of 'heat soaking' the batteries when I get home so an early plug-in makes sense with a more modest charge rate. But I can be persuaded either way. ... Hummm, I wonder what BMW thinks should be the best hot climate strategy?

I did notice the display shows "immediate charge" suggesting I can probably delay it. I'll try later this morning to see the options.

I am perfectly happy when at a 'free' charger to let the car have no limits and the ability to cool the pack off as soon as possible. It is just when I get home, my electric bill, I would like an optimum solution for both pack life and utility bill.

For our non-technical friends:

• 5VAC = 243-238 VAC - the voltage change during charge
• 5VAC * 30A = 150W - minutes 0->10
• ~75W - minutes 10->22 # could use minutes 32 as the heating interval
• 1VAC * ~15A = 15W - minutes 22->29, temperature flat lines

These are measured by the JuiceBox Pro (40) and hopefully represents losses at the box. Certainly the temperature rise suggests that is what is going on.

Bob Wilson

 

[alohart]

In North Alabama summers, we're often in the 90-100F range when I get home, often +5F measured by the car over our asphalt streets. In one respect, I don't like the idea of 'heat soaking' the batteries when I get home so an early plug-in makes sense with a more modest charge rate.

Good point! Our climate isn't so hot, so I don't have to worry as much about our battery pack heat-soaking. Maybe the best strategy would be to check the battery temperature when you arrive at home using the hidden service menu. If the temperature is higher than a certain temperature, begin charging immediately to cool the battery pack. Otherwise, delay charging. But what would be the appropriate temperature to begin charging immediately? Maybe BMW has published an operating temperature range for the battery pack.

5VAC = 243-238 VAC - the voltage change during charge

Are you suggesting that charging is responsible for this voltage change? I could understand the voltage increasing as the current decreases after minute 16 (less IR loss), but your voltage began increasing before the current began decreasing. The voltage at my EVSE varies due to external loads unrelated to charging. Is your no-load supply voltage pretty constant?

 

[gt1]

The first thing is the temperature rise, 35-40C, during this charge event. It appears that when the current drops to the 20-25A range, the temperature increase moderates.

The temperature increase that you are seeing at 30A can activate the battery pack cooling system. This consumes a fair amount of electricity thus reducing charging efficiency considerably. I charge at 208V 16A with ambient temperatures typically in the high 70's and have heard the battery pack cooling system activate only when I have begun charging immediately after driving. I have always been able to avoid having the battery pack cooling system activate if I delay charging a few hours after driving.

I think that JuiceBox shows its own internal temperature, an it has very little to do with the temperature of the battery. It is a purely safety measure- high internal temperature indicates bad connections around the contactor with risks of the wire burning and arching. When the temperature is above the threshold, EVSE would reduce the current or stop the charging. Anyway, this is how my OpenEVSE box works.

 

[alohart]

I think that JuiceBox shows its own internal temperature, an it has very little to do with the temperature of the battery.

Good catch! I should have noticed this myself.

My early JuiceBox includes a temperature sensor on its main circuit board and makes this temperature available for display. I assume current JuiceBoxes, including those with WiFi transmission of data, are similar. The J1772 protocol, the only means of transmitting data between an EVSE and an EV, does not include the EV's battery pack temperature, so no EVSE could display battery pack temperature.

That said, the battery pack temperature increases with increasing charging current, so there's some ideal charging current that would maximize charging speed without increasing the battery pack temperature enough to activate battery pack cooling.

 

[gt1]

If that the cooler is not activated, the high current charging is more efficient.

 

[bwilson4web]

We are seeing metrics in the JuiceBox feeding the J1772 cable. So the voltage drop is likely to occur in the JuiceBox. Unfortunately it leads to heat from the change in voltage times the current. What causes this is not clear but exists.

Bob Wilson

 

[engineear]

I'd like to know if there's something I can plug in to find out how much power I'm really using...if there is such a thing.
I have a Siemens basic unit. They do have an upgraded model, that's a few hundred more, with access to an app that shows what it's using.
I'm curious because my kW fluctuate quite a bit every night. All that's running at night is the A/C, refrigerator, pool pump. When I come home I usually have about 40
miles left so it shouldn't be charging more that 2.5 hours.

 

[bwilson4web]

I noticed eMotorWerks have an instrumented J1772 connector so that might provide the data.

Bob Wilson

 

[RealB]

Search for "AC 80-260V LCD Digital 100A Volt Watt Power Meter Ammeter Voltmeter 110V 220V" in ebay.
You can install this device before your charging station. Voltage, current, power.

 

[jadnashuanh]

FWIW, I installed the meter above just now and note that the combination of its power requirement and the Clipper Creek 30A EVSE has a reading of 2.7W, and I'd guess maybe half of that is from the meter itself, mostly from its backlighting. My car is fully charged, and I just finished installing it, so do not have any further info to add. On the EVSE, the power LED is lit and I'd guess the pilot signal is going out along with the other interlock signals...IOW, almost no standby power is required, at least with this EVSE (if you had WiFi and other stuff going on, it might be higher).