1983 Porsche 911 SC Targa

Sunday, October 21, 2012

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.

Monday, October 15, 2012

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.

Tuesday, October 9, 2012

Balancing the Cells

The first charge of the new batteries is the most important. Getting all of the batteries to the same voltage is the goal, because once they are connected in series, out of balance cells end up overcharged or undercharged. The cells are connected together in parallel – all of the positive terminals are connected together on one conductive path, and all of the negative terminals are connected together on another. Current will flow from batteries that are higher in voltage and into cells that are lower in voltage, arriving at a uniform average voltage on each cell. Then the pack is connected to a power supply that is used to charge up the pack. The voltage of the pack is monitored with a programmable volt meter. When the voltage reaches a set point of 3.42 volts, the voltmeter opens a relay that interrupts the charger. If the voltage drops below 3.4 volts, the volt meter closes the relay and the charger will resume. Eventually the pack will settle at 3.4 volts on each cell. I estimate this process will take 15-20 days. The charger I purchase for regular use will be more powerful and will only take 8 hours to charge.

In an ideal world, it would be just that simple. However, the 12 AWG wire I’m using and the 60 crimped ring terminals have small incremental resistance along the pack, and each cell will not rise in voltage uniformly. The cells that are wired closer to the power supply will rise in voltage faster. I have made several extra connections, evenly spaced along the pack, back to the power supply to try and minimize the voltage variation in the pack. Once the pack is nearly fully charged, I will need to allow the cells to stabilize with the power supply off, and over the course of several days the voltage of each cell should balance. 

As the cells are charged, the voltage of each cell increases, but resistance in the wire connecting each cell is causing the voltage to rise at different rates for each cell.  I added more wires in the middle of the pack to reduce the variation, but cells that are closer to the charger are rising faster.  The pack will need to stablize after charging, and the voltages will equalize.