All EV in 2014 BMW i3-REx

[bwilson4web]

Introduction

Having found the ShorePower, truck electrical stations midway between Huntsville AL and Nashville TN, I decided to do an all electric, round trip to measure times, distances, and costs.

Huntsville To Nashville And Back

• Arrive Tennessean Truck Stop - 6:37 to 7:39, started with 70 miles EV indicated at 100% SOC. Tripmeter reports 1:02 hr, 55 mi, average speed 54.0 mph. Connected to ShorePower getting 212VAC @30A, 6.36 kW but though power was on, took about two minutes to contact activation service for 2 hour charge, $3. Had breakfast and used excellent WiFi to plan Nashville destination, Green Hill YMCA, in Nashville. Found a data plate: Model TSE-4, Part: SP-0585-R02, 120/240 VAC, 102 Am NEMA 3Rm Wire Size: 4awg to 3/0, Intertek.
• Arrive Green Hills YMCA EVgo - 9:43 to 10:48, left with 81%, 62 mi indicated. Arrived 13.5% with 11 mi indicated. Total distance 115 mi, 2:08 hr, 55.1 mph, 4.3 mi/kWh. Connected to EVgo which reported: 45 min and 8 seconds, 17.07 kWh, cost $15.75 at 11:36. I returned to charging station at 12:10 AM not realizing the charging session had ended. The e-mail notification was OK but an iPhone SMS text would have been more useful. Had a Lox cream cheese, toasted bagel for lunch.
• Arrive Tennessean Truck Stop - 11:58 to 13:04, left with 96.5%, 69 mi indicated. 1:05 h duration, 60 mi, 3.9 mi/kWh, 56.8 mph. Arrived 12.5%, 9 mi indicated. Connected to ShorePower which was still hot without calling the enable service. Had a bowl of soup and set destination for the HSV fast DC charger. Left with 71% and 59 mi indicated to map distance of 49 mi.
• Arrive HSV Fast DC charger - 14:55 to 15:57, ICE started 30 yards from charger, 6% and 6 mi indicated. Started charger and paid $0.25 parking meter fee.
• Arrived home 17:14.

Lessons Learned

• The phantom charge is misleading. The last 91-100% charge does not have the same kWh of energy per percent as the lower percentages. This makes sense as the current falls off above 88% so it makes sense we are not storing the same kWh as before. So even though I left with an indicated 10 mile pad, I arrived at the HSV Fast DC charger at the threshold that just started the ICE. I stopped it in about ~10 seconds as I parked.
• It appears ShorePower activation is 'honor system' at the Tennessean Truck Stop. I had to turn on the NEMA 14-50 circuit breaker but it was never turned off by their activation service.
• Every hour of driving is ~2 hours of L2 charging at 208-210 VAC at 30 A. In contrast, a fast DC charger takes about 45 minutes. However, both suffer from a higher indicated charge in the 91-100% SOC level that does not have the same kWh energy level as the 81-90%. What this means is a detour to a fast DC charger needs to be less than an hour or you might as well use a full-power, L2 charger.
• EV costs were $3, $15.75, and $0.25, for a total of $19.00. Had I driven on gas, it would have cost ~$12 and saved nearly 5 hours of charging. The total trip time was ~11 hours for both driving and charging.

Bob Wilson

 

[bwilson4web]

Now I can compare and contrast two routes, driving EV only, between Huntsville and Nashville:

Via direct route, 30A, 208VAC, L2: (GOOD, CHEAP)

• Shortest distance, -30 mi., 114 mi
• Cheapest charging, $3 (or honor system)
• Longest time, +20 min, total 4hr 5 min

Via detour to fast charger: (GOOD, FAST)

• Longest distance, +30 mi., 143 mi
• Most expensive, $12
• Shortest time, -20 min, total 3hr 46 min

Via direct route, REx: (FAST, CHEAP)

• Shortest distance, 114 mi
• Medium cost, $6, 2.3 gal gas
• Shortest time, -1hr 40 min, total 2hr 5 min

Bob Wilson

ps. GOOD, FAST, CHEAP - pick two

 

[bwilson4web]

So Monday I drove from Huntsville to Nashville via "Tennessee Truck Stop" and was able to look at the 'false' SOC %. I came to realize the SOC is a voltage metric, not a power, kWh, metric. What happens is in the 90-92% SOC range, below it, there are real kWh available. Above this threshold, the SOC is misleading in a bad way.

I had noticed that SOC in the low 90% and above, is nonlinear, very shallow. The available power to move the car is thin. But once you get below that threshold, approaching 80-85%, the battery has enough strength and power to sustain the car on a trip.

The exact relationship is probably vehicle specific but I have some speculations that SOC may be voltage, not an actual power metric. But I also realized there is a way to mitigate the rapidly shallowing, SOC challenge, especially during charging.

The trick is to enable "pre-conditioning" when the charging power rate first falls off. The grid charger is power limited but as the battery charging falls off, I recommend enable "pre-conditioning" to fill the gap which is parallel to the battery load. The heat added to the cabin and battery remains even after the tapered, power to the battery, falls off, that keeps the grid charger in "a good place."

Bob Wilson

 

[jadnashuanh]

FWIW, power = volts * amps, so those 208V units won't provide the same amount of power as say a 240vac unit at the same amperage (About 13% less). In the USA, lots of commercial acv is 208, but 240vac is much more common in homes.

Love my i3 for running around, the extra time to deal with longer trips is a major detriment and I switch to my ICE. I don't consider the i3 a good substitute for either an ICE or a hybrid with enough battery capacity to deal with my normal day before it turns on its ICE.

 

[alohart]

The exact relationship is probably vehicle specific but I have some speculations that SOC may be voltage, not an actual power metric.

I don't believe we know how the i3's BMS calculates the SOC of its battery pack. However, voltage has a particularly poor relationship to SOC, especially when current is flowing in or out of the battery pack. Even when no current is flowing, the voltage of a Li-ion battery cell doesn't change much over most of the usable SOC range. Only at very low or very high SOC's does the voltage begin changing more rapidly as the SOC changes.

A common way of calculating SOC is to measure and keep track of current in and out of a battery pack. The movement of electrons, current, determines the charge directly, not the voltage. However, it's not possible to measure self-discharge current because it flows only within each battery cell and not through an external circuit where it could be measured. Fortunately, the self-discharge rate of a Li-ion cell is quite low, so the SOC calculation can remain fairly accurate over an extended time period. However, inaccuracies do accumulate, so recalibration could occur when the voltage rises above or drops below certain levels which indicate certain SOC's near full and empty, respectively. Such recalibrations could also help calculate the total battery pack capacity.

It's too bad that BMW or the supplier of the BMS doesn't make such information public.

 

[bwilson4web]

I've noticed that when plugged in to a charger, the heater no longer blows warm air. However, enabling pre-conditioning does blow warm air while on charger. But then comes the question of when, especially if traveling.

As an experiment, I waited until the L2 charging current reached ~16A, about 50% of the normal 32A rate:

The preconditioning load draws more current to heat the cabin. But this only brings the load up to the maximum the car can take. After a brief return to peak power, it ramps down but shares the load with battery charging. In effect, we are thermally charging the cabin while the battery charging ramps down.

Bob Wilson

 

[bwilson4web]

The exact relationship is probably vehicle specific but I have some speculations that SOC may be voltage, not an actual power metric.

I don't believe we know how the i3's BMS calculates the SOC of its battery pack. However, voltage has a particularly poor relationship to SOC, especially when current is flowing in or out of the battery pack. Even when no current is flowing, the voltage of a Li-ion battery cell doesn't change much over most of the usable SOC range. Only at very low or very high SOC's does the voltage begin changing more rapidly as the SOC changes.

Although we agree about the relatively flat voltage curve, it still does not explain the phantom loss of SOC in the ~91-100% range.

A common way of calculating SOC is to measure and keep track of current in and out of a battery pack. The movement of electrons, current, determines the charge directly, not the voltage. However, it's not possible to measure self-discharge current because it flows only within each battery cell and not through an external circuit where it could be measured. Fortunately, the self-discharge rate of a Li-ion cell is quite low, so the SOC calculation can remain fairly accurate over an extended time period. However, inaccuracies do accumulate, so recalibration could occur when the voltage rises above or drops below certain levels which indicate certain SOC's near full and empty, respectively. Such recalibrations could also help calculate the total battery pack capacity.

It's too bad that BMW or the supplier of the BMS doesn't make such information public.

Those are typically called, coulomb counters. Regardless, there are some simple tests that could document the SOC % vs actual battery energy. My proposed protocol:

• Configure display to show trip display in front of a time lapse video camera (aka., iPhone)
• Drive long enough to bring the SOC down to ~75% SOC.
• At every ~5% decrease, reset the trip meter.
• Enter the data into a spreadsheet to calculate the kWh per SOC % ranges.

Bob Wilson

 

[jadnashuanh]

Unless you live somewhere out in the great plains, finding a long, flat stretch of highway you could just cruise on far enough to get reliable data without lots of repetition. Things like wind speed, road slope, heating/cooling requirements, exact road speed, etc, the energy use could vary . It isn't just miles traveled as mile to mile, things change. You could run the test on the same path but in reverse, assuming you could recharge at one end. That would help eliminate at least some of the variables. That's one reason why the government tests are done inside on a dynamometer so they can control the variables and get repeatable results.

 

[bwilson4web]

The trip meter has two metrics:

• mi/kWh
• miles (mi)
• kWh = mi / (mi/kWh)

The only thing missing is the SOC % but I think my dash cam has enough resolution to get both. Two legs behind the driver seat and the third stretched back and held with gaffer table.

I appreciate the challenge of trying to use a reproducible route and happy to say, I have local opportunities. Usually I run highly precise benchmarks after midnight and weekends to avoid traffic.

Bob Wilson