Monthly Archives: July 2014

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What is this Juicebox saved value?

When perusing the Juicebox source in a previous post I also noticed how it was calculating the “saved” value. If you are not familiar with the Juicebox saved value: On the premium Juicebox with a display it shows a $ saved value that increases as you use the JB.

What is this value? How is it calculated?

To begin with the Juicebox makes a few assumptions (constants declared in the code):

  • Price of gas $3.50
  • mpg of car to compare to 25
  • Price of electricity per kWh $0.12
  • Efficiency of the EV 300 Wh/mile

It then uses those values to come up with a “savings per kWh” value:

gas cost per mile = price of gas / mpg (3.50/25 or $0.14 /mile)

gas cost per kWh = gas cost per mile * 1000 / wH/mile ($0.14 * 1000 / 300 or 0.466 per kWh)

savings per kWh = gas cost per kWh – price of electricity per kWh ($0.466 – $0.12 = $0.346)

The total savings then is this savings per kWh value multiplied by the total kWh the JB has put into the car.

The total kWh is calculated by multiplying the input voltage, output current, and time interval (Watts = Volts * Amps, kWh = Watts over standard time interval–hours).

Much like my previous post this all is pretty straight forward math. The number it reports out may or may not be meaningful to you if your situation closely matches the configuration of the Juicebox (25 mpg prior car, EV burning 300 Wh/mile, etc.).

For those of us with the FFE, the Wh/mile value is more like 240-250 (about 15% more efficient) and, in my case since I drove an ICE Focus before the FFE, the mpg figure would be more like 30 mpg (about 15% more efficient as well). Which, for my case, the two differences pretty much cancel out making the $ saved value fairly accurate–if gas prices remain near $3.50.

 

Digging into the Juicebox

Since the Juicebox is open source we can pull back details about the design and poke around. In this post I’m going to give a look-see through the source code running on the Arduino board that runs the Juicebox.

The Juicebox code is a very simple loop running on the Arduino (an ATmega328P microcontroller equivalent). There is no RTOS running on the Arduino–just the Juicebox code. There really isn’t enough room on the chip to have a full RTOS; there is barely enough room there for the code that is running on it.

During setup/startup the code configures the Juicebox:

  • Configure I/O
  • Setup timers
  • Check for and Initialize display
  • Read the last clock value out of EEPROM (there is no real-time clock on the board so it remembers the last time it was powered up)
  • If the “A” button on the remote is pressed all the configuration parameters are reset to default
  • Checks for the presence of the remote (not sure why this is after the above check)
  • Calibrate the pilot signal: It will adjust the pilot signal based on temperature and voltage output
  • Determine the input voltage (120V or 240V)
  • Perform some diagnostics on the GFI circuitry

Now its finally ready to go. You can pretty much see/hear this sequence being performed when the Juicebox is first plugged in as it takes about 10 seconds or so to get through it all.

The main section of code is just a large loop. The loop repeatedly cycles through the following tasks:

  • Check for presence of a car and if it is charging
  • Manage a change in state from the previous iteration through the loop (e.g. car plugged in, car charging, charging complete, etc.)
  • Set the configuration to the new state (if changed)
  • Update the display (real-time status on charging)
  • If there is WiFi send some data out to the Web (EMW)
  • Check if a menu button has been pressed
  • Check if the GFI has tripped
  • Repeat

That is about it. Pretty simple operation but then I really wouldn’t expect a lot from an EVSE since it really has only two purposes: Provide power to the car and provide GFI protection.

One feature that I’d like to see in the Juicebox that it currently doesn’t do: Turn off the display after 5-10 minutes when not plugged into the car. There really is no reason to keep the display lit when not plugged in. (I’m not sure if this is even possible simply by looking at the source code–may have to take a peek at some circuit diagrams to figure that one out.)

 

Quiet around here again…

Haven’t had much EV to post about lately as I’ve been out of town on vacation. If you’ve read some of my other posts about attempting to bring the EV with us camping..this was the trip I was targeting. Unfortunately for the EV it sat at home (it was far cheaper to simply rent a car at the destination than it was to purchase an aluminum car trailer not to mention that the FFE + Camper combo would have been even less efficient than the Camper alone).

It would have been pretty sweet though to post pictures of my Michigan plated FFE in different states!

 

What would it take??

Now that I drive an EV around whenever I’m driving one of our ICE vehicles I like to think about what would it take to make this vehicle an EV?

Lets start with our new car: a 2014 Ford Escape. This exercise should be pretty easy as the Escape is based on the same platform as the Focus and thus shouldn’t require much more than the Focus. Looking at some of the specs for the Escape:

  • 1.6L Ecoboost engine making 178 HP, 184 lb-ft of torque
  • 3500 lbs curb weight

These numbers aren’t too much off from the ICE Focus (2.0L 160 HP, 146 lb-ft, 2950 lbs) thus to electrify the Escape: a slightly larger electric motor (just 20% larger) and more battery for the larger engine (about 30 kWh or so). The challenge with the Escape is that its footprint is about the same as the Focus which wouldn’t leave a lot of room for the Focus battery let alone a 30 kWh battery. It does sit higher being a CUV so there may be some room in the floor which isn’t available in the Focus. Note that using these numbers the converted EV Escape would still only manage about 75 miles on a charge since I’ve only extrapolated the numbers from the existing Focus Electric.

The next contemplation is a bit more, um, serious! Our RV:

  • 6.8L V-10 engine cranking out 305 HP and 420 lb-ft of torque
  • Weighing in at a hefty 12,000lbs

Now we’re talking large multiples (at least 2X engine size and 4X weight). Just taking that into account we’d need a 2X electric motor producing roughly 200 kW–or would the implementation be easier by just using two 100 kW motors: one driving each rear wheel. The real trick to electrifying the RV, though, is battery: This is due to the fact that for an RV to be useful you’ll want a ton of range (our RV has a 55 gallon gas tank giving it an effective range of about 600 miles). Simply doubling the battery size from the FFE won’t be enough, we’ll need something more like 10 times the battery size (due to the range requirements, and the additional weight of the RV). So now we’re talking about a battery around 250 kWh. How big would that be? Is it practical?

The best EV batteries today are about 240 Wh/kg (Tesla Model S). The translates our monster 250 kWh battery to be around 2300 lbs. That is a heavy battery enough so that our RV would have to bump up the chassis from the E-350 its based on to the E-450–it may also necessitate an increase in electric motor size simply to compensate for the additional weight (ok so lets put the motor at 250 kW from 200 kW).

Is there enough room in the RV for such a large battery? My initial thought would be yes: The electric motor can simply be bolted to the rear axle freeing up the engine bay, drive shaft tunnel, and exhaust pipe routing for battery usage. Again using the numbers from the Model S (about 700 Wh/L of volume) results in a battery that is: about 12 cubic feet in size. If we flatten that to a 1 foot high slab we get a battery that is roughly 3 feet by 4 feet by 1 foot high–easily tolerable in the RV.

If this appears all feasible how come we aren’t seeing ERV’s? Well something I haven’t mentioned, but also needs to be calculated, how much would a 250 kWh battery cost? A reasonable estimate for battery costs today is around $250/kWh thus a 250 kWh would cost $62,500 just about doubling the price of the RV–not including all the R&D that would be required for building something entirely new. Would someone pay that? If you were looking at two brand new RVs sitting on the dealer’s lot both identical to each other on the outside with identical floorplans inside but the left one had an electrical powertrain with a price 2X the one on the right with a gas engine would you purchase the electric one? (It should be noted that if there was a 3rd one with a Diesel engine its price premium over the gas engine one would be about 10% – 15%.)

Note that the numbers I came up with here are really just guesses (battery size and motor size) from scaling up the FFE’s motor–doing the real math to figure it out may come to significantly different values.

 

Drop by any time you like and stay for the charge.

Locally in Southeastern Michigan I’m noticing more and more hotels add charge stations. In the area here are but a few:

A charge station at a hotel makes perfect sense: You’ll be there more than long enough to charge up any EV available today with a Level 2 charge (e.g. overnight). It works out for the hotel since they will get the extra customers who are looking for such an amenity.

That last one on the list is what prompted this posting: I only just noticed that they had a charge station and are advertising it. Even prominently on their website:

Weber’s Boutique Hotel and Restaurant in Ann Arbor, Michigan has complimentary EV charging stations for their guests. We offer the PEP STATIONS PS1500. It is a commercial 208 volt, 30 amp EV charging station with SAEJ1772 connectors that can accommodate two cars at once. Owners of Volt, Leaf, Tesla will be pleased with this complimentary service while in Ann Arbor. Please notify Weber’s when reserving your room so we can block off a protected carport space exclusively for you. Weber’s is among Michigan’s first hotels and restaurants to offer EV charging stations.

This is great, while we’ve never stayed at the hotel we have dined there a few times. Looks like its time again to go visit for a meal and perhaps a charge (if they also allow patrons of the restaurant access to the charge stations as well…).

 

 

Over range drive…

So what would be a good term to use when you go for a long drive that is longer than a typical charge (in the FFE’s case that would be 70+ miles)? Over range? Simply “long drive”? On a car with such a short range this is a novelty, on ICE cars its a daily occurrence.

Nevertheless these summer weeks have provided plenty of opportunities to take our FFE on trips longer than my usual daily drive (30, sometimes 45 miles). Two recent instances of this include:

  • Going to a graduation party about 30 miles from home
  • Going to a Detroit Tigers game after coming home from my commute to work

With the graduation party being on a weekend it was easy to start out with a full charge and make the round trip on a single charge. I was even able to drive conservatively enough to have 30% of the battery remaining when I completed the trip (including taking the highway most of the way).

The Tigers game, however, was a bit more of a challenge: Comerica Park is also 30 miles away from my house but those 60 miles would be in addition to my daily commute of 30 miles (totaling 90 miles if you’re paying attention). I didn’t want to attempt all those 90 miles on a single charge as the last few miles would involve returning home late at night–didn’t want to have to stop somewhere and charge since I did have to go to work the next day. The solution, in this case, is to use a public charger near my work to top off at work and then topping off again at home before departing for the game. The 15 miles in to work typically requires just under an hour refill, and thus the 15 miles to home also took just under an hour at home. If I was not able to charge up at work I don’t think I would have been able to make this trip–at least not within the allotted time.

As you can see, driving an EV sometimes requires a little bit more planning than driving your average ICE vehicle simply due to the limited range. If my FFE had an operational range just double what it does now (increasing the range to 150 miles or so) then this additional charging would not have been needed.

In the coming weeks I’ll have a few more opportunities to try these long drives..more grad parties across town, more Tiger games, more….