MechE Battery Party!

MechE Updates

Our primary concern today was designing structures to hold and support our battery packs in the back of the Opel. By the afternoon, literally everyone on the MechE team was engrossed in this task. We were even honored to have Sir Nicholas Arango from the EE team come and help us generate ideas.

Being a team with a small car and big ambitions, one of our biggest design constraints in our Opel project is that we have 464 individual battery packs to distribute between what used to be the engine compartment and the space which used to house the back subframe and gas tank. Each group of cells has to be housed such that none of the cells are at risk of being ruptured in the case of a collision. At the same time, the front and back compartments each require a cooling system for these cells, and in addition to our subframe, motor, and transmission in the rear compartment, each compartment will eventually house one or more modules such as our motor controller or our battery controller. Our design constraints, therefore, are difficult to work with, and it took hours narrowing down what path we should take.

Alex helping lead discussion. Alex and Joey, who had done a great deal of work designing our battery supports in weeks past, proved invaluable.

Leave it to Rango to bring out the best in people.

After hours of work, first in small subteams and then as a whole, we were able to come up with a modular design for our supports which could be implemented universally across virtually any permutation of battery placement. We designated two teams—one to generate plausible battery placements with batteries oriented towards the back of the vehicle, another with cells toward the passenger side—to create a library of battery placements to choose from. This coming week, we will select the design that seems most feasible, stable, and space-conscious so we can start building the support frame itself.

A set of one of many sizes of battery module CADded by Joey

In addition to this, Jacob, Ryan, and Olivia worked with Jimmy from the EE team to decide how and where to mount potentiometers to the accelerator and brake pedals in the Opel, and Joey created comprehensive CAD of the structural members of the Opel frame.

Joey’s CAD of the structural components of the Opel frame

A New Cell Cycler

Recharge your electric car in less than 10 minutes! This has been the dream of many current and future electric vehicle owners. But imagine the batteries in your car could only handle this rapid recharging maybe 400 times - about the lifetime of a laptop battery, charging at a normal pace. Let's do some math: 100 miles (the range of the Porsche 914 BEV) x 400 cycles = 40,000 miles per car. Who would want a car with a battery pack that only lasted three years? [1] Not EVT! We wanted top of the line batteries that could handle our abusive rapid recharging and put up a fight for an acceptable number of cycles. Just in time, we came across A123, one of the industry's leaders in battery technology.

A hundred battery boxes later, we were ready to put our A123 cells to the rapid recharge challenge. In order to test our new batteries, Lennon Rodgers set up a station to rapidly cycle - charge and discharge - an A123 cylindrical 26650 battery, pictured here. After cycling the cell repeatedly, he hoped that even with the deteriorating effect of rapid charging on any type of batteries, the cell wouldn't lose much of its original capacity. His results are shown in the graph below.

Excitingly, after almost 1500 cycles, the cell barely degraded. So, 1500 cycles corresponds to 150,000 miles which is nearly the average lifetime of a car. [2] This is much more reasonable for a commercially marketable battery pack. Though this result is exciting for the possibility of rapid recharge technology, only one cell was tested which does not constitute a statistically significant sample size. Therefore I have spent the last few weeks setting up a new cell cycler. This cycler will rapidly cycle the same type of battery, a cylindrical 26650 battery and hopefully support the results from the first test.

Here are some technical details of the circuit. The cycler has two parts connected

to the battery through a set of relay switches: 1) a power source used for constant current charging at 10Amps and 2) a .25Ω resistor through which the battery can discharge. During the entire cycle, the battery temperature, voltage and current as well as the temperature of the resistor are monitored and constantly checked for dangerously high or low values. The data is sent through an Arduino-controlled circuit to a wireless ioBridge module which allows us to remotely upload data to monitor the status of the battery and keep a running chart of its capacity.

After a brief setback in which I killed a cell (thank you to Shane Colton for his assistance in soldering a new replacement cell), the cycler is up and running. In the time since it was turned on last Friday, July 30th, it has undergone over 300 cycles. It will take about a month to complete the goal of 1500 cycles, but the cycler is well underway and showing promising results. Stay tuned for the results on the second cycler test as a step on our way to rapid recharge technology.

[1] http://www.epa.gov/oms/consumer/f00013.htm

[2] http://www.arb.ca.gov/regact/grnhsgas/vmt.pdf

The Battery Pack

The past week or so has been spent designing a prototype battery pack for Lennon's electric motorcycle, eMoto (http://www.electricmotion.org). We'll be replacing his 240 pounds of old-school lead acid batteries with a 30 pound pack of A123 LiFe cells. Of course, this pack will only have about 75% the capacity of the lead acid stack, but the power and weight improvements will be significant.

This pack isn't a favor for Lennon though. It'll serve as a test platform for future pack design for the elEVen, as well as demonstrate rapid-recharge capabilities on a smaller scale. Consequently, the pack has a very similar architecture to that of the proposed elEVen pack. We've run into a lot of problems during the design process which we never anticipated. It's very likely we'll run into the same problems on the full sized elEVen pack, so hopefully we'll be able to get a lot of bugs out with this small scale version.

Here's an overview of the basic design, both for the elEVen and the eMoto:

Modularity: It is rather difficult to make an electric vehicle without using high voltage, which can be very dangerous to work with if proper precautions are not taken (just like gasoline in ICE cars. It's just a property of high energy systems). The obvious solution for dealing with the dangers of high voltage is to eliminate the high voltage itself. We are accomplishing this by building several independent low voltage modules, which will be wired in series to create the high voltage required only at the very end.
The eMoto pack will consist of two 19.8 volt (6 cells in series) modules and two 16.5 volt (5 cells in series) modules wired in series to reach the nominal voltage of roughly 72 volts (required for the motor controller). Each module will have 10 cells in parallel to achieve a capacity of 1.6 kWh for the entire pack. We originally intended to use three 24 volt modules, since the elEVen will be using 24 volt modules, however, we decided to use the existing battery mounting on the eMoto. This made arranging seven of the ten cell batteries in series very difficult. Thus, we arrived at the much simpler pack design we currently have.
We anticipate that the elEVen pack will consist of 24 volt modules with 72 cells in series. However, the issues we ran into with the geometric constraints of the motorcycle could arise when designing around the existing chassis of the elEVen. Hopefully the experience with the eMoto will help us keep our future designs a little more flexible. Additionally, the eMoto will have the four modules wired directly to each other. We plan to connect the elEVen modules with contactors, allowing the packs to disconnect and drop the maximum voltage to 24 volts. This will provide a safer working environment as well as making the vehicle safer if it is ever involved in an accident. Finally, each module will have a built in Battery Management System (BMS), which will then communicate to a master module to balance all of the modules together.

Inter-cell Connections:The individual cells of the modules have to be connected some how. We took a lot of advise from the likes of Bob Simpson (http://www.evdrive.com) and Bill Dube (http://www.killacycle.com/) and have decided to use nickel plates spot welded to all of the cell terminals as the interconnects. These nickel plates will be bent to help arrange the cells in each module. They also have tabs for individual cells with integrated fuses to protect the pack should a cell internally short.

Cooling:As we rapidly recharge the cells and continuously discharge them, they will heat up. Though warm cells have a lower internal impedance and are therefore more efficient, running the cells too hot will damage the internal structure and reduce the life of the pack. We'll be air-cooling the cells across the electrical contacts, since they are both electrically and thermally coupled to the inside of the cell. We don't anticipate that a lot of cooling will be required (see the section on battery data which will be posted soon), but we want our design to be flexible in case more cooling is required. We accomplished this by spacing the cells vertically to allow airflow. However, we have not finalized the method of spacing (the leading idea can be seen below). We're also looking into various methods of cooling, including passive cooling and using various arrangements of computer cooling fans for active cooling. We'll be testing the effects of various cooling systems once we finish the prototype eMoto pack.

Cell enclosure: This has surprisingly been one of the more difficult aspects of the pack design. The cells need to be restrained somehow, and in the case of the eMoto, the enclosure also needs to be able to take a compressive load. We were originally looking at using FR4 as the primary cell enclosure for its insulating and fire retardant properties, and an aluminum secondary enclosure to provide rigidity. Since then, we've looked at a single FR4 enclosure design, an FR4 primary and polycarbonate secondary, polycarbonate primary and secondary, and a single polycarbonate enclosure. Currently, we've decided on a 1/8” polycarbonate primary which will likely be encased in a polycarbonate secondary with possible rubber shock-mounts in between the two layers. The shock-mounting will reduce the effects of vibration on the cells, hopefully preventing any of the connecting welds from breaking.
After figuring out the enclosure materials, we had to design a good way of creating a box. This sounds trivial, but there is a lot more that goes into it than meets the eye. In our case, we were limited to making the box out of sheets of polycarbonate. This was the only cost effective way of obtaining all of the necessary features, such as cooling slots and cell dividers. Machining a solid piece of polycarbonate into the shape we wanted would take too much time, cost too much money, and waste too much material. Therefore, we had to come up with a good way of joining the polycarbonate sheets together into a box. We looked into drilling and tapping screw holes into the thin face of the polycarbonate, but in order to do so, we would need to use #0-80 machine screws. That method of fastening seemed impractical and labor intensive while possibly being insufficiently strong. We also looked into chemically bonding the polycarbonate together. We've decided to do so on the prototype eMoto pack, with the addition of interlocking tabs for added mechanical reinforcement. We are considering using polycarbonate angle stock to reinforce the corners. As of now, the box seems pretty rigid.



As of now, we've assembled the primary enclosure with dead cells, and we are waiting on the battery welder. We've received the weld samples, so the welder should come in sometime this week. Once we have that, we will begin matching cells and electrically assembling the prototype pack. To prepare for this, Paul and I have been testing boxes of cells to weed out the obvious bad ones. We're looking to manufacture the secondary enclosure this week, as well as experimenting with various cooling designs. Additionally, Shane and Lennon have been working on our first rapid-recharge charger for the motorcycle. If we're lucky, we'll be rapid-recharging our first pack by the end of the week.



Look for test data and more images soon.

-Mike