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.

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.

The Batteries Arrive

After a little excitement with UPS losing my batteries for a day – they were loaded onto the wrong delivery truck – the batteries are here. I’m really glad they got here in one piece. With all of the cells laid out beside the car, I am wondering if they are really going to all fit.

The battery is considered fully charged at 3.6 volts, and empty at 2.8 volts. These cells shipped at 60% full, at 3.3 volts. There is a little variation in voltage from cell to cell.

Before the cells can be installed into the car, they need to be balanced. The process is to wire up all of the cells in parallel, and charge up the pack. The goal is to ensure that each cell has the same voltage when the pack is fully charged. When this occurs the pack is said to be in balance. Balancing is required because each cell has a slightly different capacity, and as the pack is discharged, cells with less capacity will have lower voltage than cells with higher capacity. If the cells are balanced when full, the charger will be able to more reliably stop the charge process when the pack is full. There is less risk of a single cell reaching the full state ahead of the other cells and being over charged. The problem with top balanced packs is that on discharge it is difficult to know when cells are empty. The voltages will vary a lot, and there is risk of discharging a cell too low. For long battery life it is important to be conservative with charging and discharging the pack.  I will also have a circuit on each cell (battery management system) that monitors for a low voltage condition and gives a warning to stop driving before damage occurrs.

Cooling Loop

The controller is very efficient - somewhere between 94 and 99%. But when a lot of power passes through a system, small percentages can become significant. The controller is rated for 1000 amps peak when air cooled or 1000 amps continuous when water cooled. 1000 amps at 150 volts produce 150,000 watts. At 97% efficiency, the controller generates 4,500 watts of waste heat. My motor and the police will not allow me to draw that kind of power for more than a few seconds. On average, I expect the car to use about 280 watt hours of energy per mile. At 50 miles per hour, we are talking about 14,000 watts of power through the controller. At 97% efficient, the controller needs to dissipate 420 watts. The controller components are mounted on a heat sink with water channels. Water cooling is an effective means of exporting heat from the controller. Cooler electronic components run longer and more reliably.

Clockwise from the top left:  Controller, braided hose, reservoir tank, circulation pump, radiator with fan.

The cooling loop consists of a water circulation pump, a radiator, reservoir tank, and plumbing. The pump is recommended in the owner’s manual of the controller – a Laing D5 hot water circulation pump. It is designed for solar hot water heating applications and runs on 12 VDC. The radiator is an aftermarket automotive part used to supplement the stock radiator capacity, typically for towing. The reservoir is an expansion tank. It holds 1.25 quarts (1.2 liters) of fluid. It will be located at the highest point in the cooling loop to purge air bubbles and for filling. The fill neck accepts standard radiator caps. I selected a cap rated for 7 psi. The plumbing is steel braid jacketed rubber hose with -6 AN type compression fittings (http://en.wikipedia.org/wiki/AN_thread).