Tuesday, June 30, 2009
The long tedious testing of the individual batteries has begun. After talking to our contacts at A123 and other EV teams, we decided that the two most important indicators of a battery's health are voltage and impedance. We plan to first measure the voltage of all the cells that A123 donated to the team and throw out any cell that are not between 3.296 and 3.340 volts. The cells have been sitting for 3 years so if the voltage is within this range, it is a good indicator that the cell is sealed well and that there is very little current leakage through the cell. This range was given to us in the A123 battery pack design manual that is available to the public. This design manual has a lot of very useful information about building a pack and particularly about building a pack with lithium iron phosphate batteries. Today I checked over a 1000 cells. My goal is to get all the cells checked before the car arrives next week. Today, Mike and I also went to the electrical engineering lab and used their impedance tester to check a box full of good cells (cells that passed the voltage test). The average impedance of the cells was 7 milliohms with a very small standard deviation. It seems to me, and the other team members agree, that we only need to measure the voltage of the cells in order to tell if they are good or not. In other words, if a cell passes the voltage test, it should pass the impedance test as well. If we decide that this is a reasonable assumption to make, we will be able to save a lot of time by just testing the voltages of all the cells.
Cell Level Battery Pack Fuses:
Today I ran a couple of tests to determine if the individual cell fuses are sized correctly. Our nickel sheet is .010" thick and the tab fuses are 5.5mm wide. A123 sizes their fuses to be 3.6 mm wide and .007" thick nickel sheet. They rate there fuses to blow at 2100amps in .1 seconds. Our fuses are currently about twice the cross sectional area of A123's. We wanted to investigate this further so we ran a few tests using an A123 26650 cell. We estimated that an A123 cell can output 300 amps in a short. We cut two test strips of nickel. One that was the size of the fuses that we are currently using on the motorcycle packs and one that was the size of the A123 fuses. Both blew when we shorted the battery across them. This was a good sign because the purpose of the fuses is to protect against a short. After talking it over, we decided to stick with the 5.5mm width rather than reducing it to 3.6mm because we do not want to risk the nickel heating up when we rapid recharge these packs. I went ahead and ordered enough nickel to finish the motorcycle packs and also build one of the car modules. We bought our nickel from National Electronic Alloys Inc. They have a very good selection and they also have good prices.
Monday, June 29, 2009
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 and I started mass testing the A123 batteries. We tested the voltage of about 500 batteries. This is enough to do all of the packs for the motorcycle. We are planning to test the impedance of the batteries after testing the voltage because voltage is easier to measure. The good batteries are around 3.3030 +/- .0050 volts.
I finished up the CAD drawing for the motorcycle battery pack in the morning and we had a battery review meeting in the afternoon to critique and improve the design. Many great suggestions came out of the meeting such as ways to improve the battery cooling, ways to simplify the design, and a way to reduce vibration when the battery is on the bike. We decided that we are not going to use FR4 initially and instead are just going to make the battery enclosures out of only Polycarb. We also decided that we are going to have a double enclosure design so that we can put foam in between the boxes to dampen vibration and so that we can use the outer box as a manifold to pull air through the pack. I will integrate these suggestions back into the design later this week. Mike and I are planning to water jet the casing tomorrow or Thursday. We are also going to water jet the tabs for one of the modules this week.
Mike has a friend who offered to help us water jet so we cut the battery tabs today. They look very good. In order to cut the nickel, we sandwiched it between two sheets of Aluminum. We test fit the battery tabs onto the batteries and they fit well. We did not have enough time to water jet the polycarb battery enclosure today.
Today, I helped Matt look for a spline shaft for the electric motor. We have an engineering drawing of the shaft so I thought it would be relatively easy to find. I spent several hours calling up junkyards and transmission rebuild shops and did not have any luck. We do not know what transmission has the spline we are looking for so I tried to look up the spline by the number of teeth and the outer diameter. In the afternoon, I drove out to the junkyard that Matt went to last week and talked to them about the shaft we needed. The junkyard was called Nissenbaum's Auto Parts. They were very helpful and interested in our project. I first looked for the spline shaft in a dumpster they had that was full of busted transmissions. Then I looked for the shaft in their stock of good transmissions. I thought I found the shaft we were looking for so I took down the information for the transmission. The guy in charge at Nissenbaum's had a good suggestion that we look for the shaft at a transmission rebuild shop. He recommended New England Transmissions in Alewife. I called them up and made an appointment to come in around 9am on Friday morning.
Friday, June 26, 2009
Friday, June 19, 2009
Thursday, June 18, 2009
After Arya and Matt picked up the lift on tuesday, the team has been working hard to set it up. Most of Wednesday was spent getting all the necessary parts for the lift together and assembling it. Though the lift has a few bugs to be worked out, it is operational and can lift the Porsche.
I spent today mostly focused on finishing the installation of the lift. In order to ensure that the lift does not buckle under load, Radu and I braced the overhead crossbar with two pieces of U-shaped steel. After the overhead beam was reinstalled, we ran the cables and hydraulic line across it and attached them to their respective insertion points. Then, the two of us installed the safety cable (which stops the lift if the car gets too high), and we tested the lift. In testing, we noticed that the latches on the lift arms were not engaging the safety catches at the same time, so I tightened one of the overhead cables in order to make the latches engage simultaneously. The installation was complete by the day's end, we even managed to raise the Porsche 914 BEV. Even so, we encountered two problems. First, the hydraulic fluid tends to bubble and foam, which is troublesome because it indicates the presence of air in the system. Second, the fluid tank is precariously attached to the pump by nothing more than a horizontal hose clamp, which is troublesome because the tank could fall and spill if knocked a little too hard. We are looking for an additive to solve the first problem, and I fixed the second by putting a support bracket underneath the fluid tank.
Mike has been working on getting sponsor decals and trying to find a place for the Porsche. He is also working with Paul on a battery pack design for a motorcycle. On Wednesday he continued cell testing and helped out with the lift.
On Wednesday, Radu had a meeting with MIT lawyers. He was also part of a meeting during lunch about battery management systems and setting up for a motorcycle pack that will be configured to be similar to the automotive pack
The Porsche on the lift
Wednesday, June 17, 2009
Tuesday, June 16, 2009
Highlight of the day: The lift was picked up and is in the process of being assembled.
- Kevin has finished securing the motor for testing. He is now focusing on ordering parts necessary for wiring the motor and the motor controller.
- Paul is working on a solidworks model for the battery pack geometric design and spatial arrangement.
- Mike is doing more cell testing at 15 amps. The discharge at pass was rated 4. He will be doing more testing on the bad cells to see what they do. He will be working on a set up to test all the cells and help Paul on the battery pack design
- Matt went with Arya to pick up the lift. He has also been emailing battery manufacturers in order to get lead acid batteries donated for the charging array.
- Arya met Gary Bloon and picked up lift from him. He went to hardware stores to get items necessary to assemble the lift.
- Radu picked up the components to begin assembling the lift, and began putting it together.
The team arranged the space in the shop for the lift and began drilling the anchors into the concrete (4” deep, ¾” holes).
- The team had a big meeting after hours for BMS system design, and will be building a smaller battery pack – very similar in configuration for the battery management electronics with what they are doing with the elEVen project – for Lennon’s lead-acid electric bike. This will be a smaller-scale version, and will be a large step in the development for the full-size battery pack.
Monday, June 15, 2009
- Mike is testing cells for temperature rise.
- Arya is working on updating the team proposal and sponsor package.
- Matt is contacting battery manufacturers for information about making a lead acid battery pack for charging. He is also working on couplings.
- Kevin is still working on securing the motor to a table for testing.
Saturday, June 13, 2009
Friday, June 12, 2009
The elEVen team had a meeting with Bill Dube from Killa cycle. Bill Dube builds a123 drag bikes and has extensive EV experience.
Radu Talked to Bill Dube from killa cycle and gave advice on general EV design. It was an informative meeting, and they received two weeks worth of worth of research in one hr from Bill Dube.
Kevin is currently working on mounting the motor to a sturdy table in order to test it.
Matt discussed plans with Bill Dube and the rest of the team. He began solid modeling a coupler for drive train and finished specing out drive train components. The team will be using a ford 9 differential for the drive train.
Mike was also present for the talk with Bill Dube. Mike received a lot of information about battery packs, charging, transmission, and the motor. He has begun working on testing voltage on the cells. He has tested about 30 cells out of a total 10,400 that we received.
Arya focused on getting information about insuring the Fusion and contacting people at insurance offices. He is also working on setting down a timeline for the project.
The Porsche team is still working on the transmission.