1983 Porsche 911 SC Targa

Monday, December 31, 2012

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.

Tuesday, December 4, 2012

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.

Sunday, December 2, 2012

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.