Battery Boxes, part 5 - Finishing the Front Rack

After the base frame was completed, I made tabs that fit the gas tank mounting locations and tack welded them to the battery rack. Then I made the upper frame of the battery rack.

The left mounting tab is in place and ready to be tack welded.

The right mounting tab is at a compound angle and many cuts must be made to fit the many curves of the car's frame.

The frame is complete.  The right, front corner of the upper frame bolts into place and is designed to clamp the batteries securely in place. 

Here is the rack populated with 28 cells.  There will be a sheet metal base to support the cells, and open sides, as the cells in the trunk are fully protected from road splash.  The top will be covered with an acrylic sheet to protect against voltage hazard, but still keep the cells visible - especially important if the car is ever displayed.

First Charge Complete: Balancing the Cells, part 2

 A few months back I started charging the cells for the first time. Details of the equipment and process are recorded here in a previous post: http://eporsche911.blogspot.com/2012/10/balancing-cells.html

The charge took 20 days. The charger I will use in regular service will charge an empty pack in 8-10 hours, but the first charge is done more carefully, with the goal of getting all of the cells fully charged and to within a few millivolts of each other.

I measured each cell quite frequently, and immediately noticed that the cells connected closer to the charger where rising in voltage faster than cells farther away. So I added a few extra wire runs from the charger to the cells in the pack.
You can see new spikes in the chart when the new connections were made. When the cells reached about 3.38 volts, I noticed that the voltage would increase faster than cells below 3.38 volts. This is a know pattern called a “knee” that occurs when the battery is nearing full charge and cannot as readily accept additional charge. The voltage for the knee is not a fixed value, but depends on how rapidly the cells are being charged.

This graph shows the average, minimum, and maximum voltage of the cells. The range of cell voltage was about 30 millivolts while under charge, but settles to within 0.2 millivolts within 24 hours of switching the charger off.

Once the charge was complete, I observed that the cell voltage decayed exponentially. In fact, I was able to curve fit the voltage fairly well. The so called resting voltage of my cells, at near full charge, is 3.3735 volts.

Lithium cells are said to be Coulomb efficient. Almost all of the electrons that are pumped into the cell are stored and available for use at a later time. The electrons do not leak internally across the terminals. The practical implication is that the cell does not self discharge while on the shelf. It also means that I shouldn’t have to repeat the balancing procedure again, because the cells tend not to drift out of balance over time. Being charge efficient does not mean that the cell is 100% energy efficient. To charge the battery, the voltage must be held higher than the resting voltage, and during discharge the cell will sag to a lower voltage. The difference in voltage is how the cell can lose energy, but not electrons during charge/discharge cycling. This variable voltage, and more specifically the slow relaxation time after a charge or discharge event, makes it difficult to accurately determine the battery state-of-charge (how full it is) based on voltage. A compromise to an accurate multiday charging process is to charge at a fixed current until the cells reach a specified voltage, and then hold the set voltage until the current has dropped to a specified current. This charge profile is called, “constant current, constant voltage” and gives a more predictable indication of where the settled voltage may end up. Under driving conditions, where the discharge current is wildly variable, I will be using an amp-hour meter to count how many electrons I have used, and this gives an accurate measure for state-of-charge, once I’ve determined where full is during the initial charge.

Battery Boxes, part 4

28 of the 60 cells I purchased will be mounted up front where the gas tank used to live. After designing a layout pattern, I made a plywood template to test the fit. It looks like there would be space to mount more batteries on the right, where the 12 volt starting battery was mounted, but there is a platform that raises the batteries up too much, and the cell terminal would touch the hood (bonnet).
I used the plywood to layout the base frame. Once the pieces of angle iron are cut to length and clamped down with square corners, the frame is welded together. I oversized the frame by 3/16” (4.75 mm) so that the frame can be lined with a rubber mat.

Testing the fit of the cells in the frame.

There is not much wasted space.

To connect adjacent cells I will use braided jumpers. The jumpers are flexible to minimize stress of the terminals. I want to minimize the number of cell connections I will have to make out of welding wire and crimp ring terminals. This layout requires only two additional jumpers (the curved connections in the drawing) and maximizes use of the space in the front of the car. As a bonus, it works out that the long cable runs to the cells in the back of the car can connect to adjacent cells. The dotted lines are only there to show the electrical current flow.

Battery Boxes, part 3

The first 12 cells are mounted. It was a bigger chore than I expected avoiding all of the potential interference issues. The rear crossbar (the one with the clamp) will be welded in place once the second box is completed and mounted.

Here is a close up of the cross bar mount.

There is only 3/4 inches (19 mm) of clearance between the motor and the battery box.  If there is any rubbing I will switch to a stiffer racing style motor mount.

The cells are mounted below the rear motor mount bracket.

The box sticks out beneath the bottom bumper and valance. This is mostly a cosmetic issue, but if mounted too low, dragging could be an issue driving up the drive way. From most angles, they box is not visible, but if you are low enough, or far enough away, it will be visible under the rubber bumper. Painting the rack black should help it blend in.

A clearance issue with a curved part of the frame forces me to offset one of the mounting points.

One corner of the box is close to a tie down plate on the frame of the car.

The boxes were skinned with sheet metal.

Electrical Odds and Ends

My electrician brother was visiting from out of town. I took advantage of his skills and we wired up a dedicated charging outlet in the garage. Sharing a plug with the clothes dryer would get old fast. We wired up a 240 volt, 30 amp circuit using a NEMA L6-30 twist lock outlet.

I ordered bunch of odd parts that I will need to complete the project. 

 The proximity sensor will be mounted near the motor tail shaft and will sense the motor speed. The signal is sent to the motor controller to limit the motor RPM and drive the tachometer in the instrument cluster.

The electrical terminals will be used to connect up the 12 volt systems. The translucent insulated ring terminals are marine grade.  A professional crimper can run $300, but for occasional use, a compound ratcheting crimper can be had for $35. The dies in the jaws are color coded to match the terminal insulation, and cover a range of wire sizes. The crimp quality is much better than you get with a combination stripper/crimper.

The rubber mounts will help isolate the controller from the jarring automotive environment.

The accelerator pedal position sensor is a potentiometer – the resistance of the device changes as the input shaft rotates. The throttle linkage connects to the potentiometer through a ball joint and a control arm. 

Adjacent batteries will be connected with braided straps. The braid is flexible to minimize any stain on the terminals of the battery. M8 bolts, and Nordlock washers fasten the braided straps to the battery terminal. Nordlock washers are a marvelous invention. They are a pair of washers with sloped steps that must slide past each other, expanding the thickness of the washer pair, in order to loosen the bolt. This mechanism prevents vibration and thermal cycling from backing out the bolt over time.

Longer connections are made with 0/2 AWG welding cable. The cable is 0.63 inches (16 mm) in diameter. 1,235 strands of 30 AWG copper makes for a flexible cable. The cable is sized based on average current draw and is rated for 190 amps at 90 degrees C. The terminals must be crimped with a hydraulic crimper.

Battery Boxes, part 2

The first battery box is taking form. The iron has been cut, sanded, and tack welded. I like sanding the parts before welding. After the structure is assembled, it is difficult to get access to clean inside corners. A clean surface is import for a strong welded joint.

There are a couple of interference issues that need some attention. I need to maintain at least half an inch clearance between the battery box and transmission adaptor plate. The transmission will flex and rotate some under load.

The batteries must stay below the rear motor mount bracket to prevent the battery terminal from being shorted out across the car frame.

Battery Boxes, part 1

The batteries will be housed in four battery boxes. The boxes are constructed of 1.25” x 1.25” x 0.125” (31.75 mm x 31.75 mm x 3.175 mm) angle iron for the frame, and sheet metal encloses the frame. The box top will be acrylic (Plexiglas). The exterior of the boxes will be finished with a high build under carriage coating. The interior will be primed, painted, and lined with a thin layer (3 mm) of foam. The first two boxes will be located on each side of the motor, and hold 12 batteries each.

The bottom frame of the battery box is cut and clamped to a piece of plywood, ready to be welded together. My grandfather, a retired carpenter, gave me that framing square for Christmas when I was five years old. I still use it on a lot of projects around the house. I like to think that I got my mechanical inclination from him. Someday, I hope to pass it on to my young son.

Cross bars of 1.25 inch (31.75 mm) square tubing are mounted in the engine compartment. Metal tabs will be welded to the car’s frame, and the cross bar bolts to the tabs. The boxes must be removable to preserve the ability to maintain the car – like replacing the shocks. Vertical lengths of angle iron at the corners of the box will connect the bottom of the frame to the cross bars. A cardboard box mock-up was constructed as a light weight stand in for 12 batteries. There are lots of curves in the car’s structure that I still have to negotiate to square up the battery box frame.  The motor is wrapped in black plastic to protect if from debris during fabrication.

The upper right corner of the picture is a detail of the tab that will be welded in, and the bolt securing the rear cross bar.