1983 Porsche 911 SC Targa

Sunday, September 30, 2012

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.

Thursday, September 27, 2012

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).

Friday, September 14, 2012

Counting the Costs

All of the major components for the project, except the charger, have now been purchased. I’ve kept records of my expenses. Not surprisingly, the cost of the batteries and the car itself top the list. All supplies, materials, and services I’ve incurred are included. Not included are the tools I’ve purchased along the way.  I also have not sold the gas engine, yet.  I still need to trace out some of the electrical connections on the wiring harness.  Selling the old gas related parts could recoup quite a few dollars. 

Some have asked when the project will pay back. Not for a very long time, at least in financial terms. I once estimated about 250,000 miles, making some wild assumptions about future energy prices. But that is not the point of a project like this. If cost and operating expense was truly the parameter to be optimized, the project would look quite a bit different. Just looking at the cars I see around town, price isn’t the only priority of most gasoline car owners, either. It will be nice to drive by the gas station and not require their expensive product. It will be even nicer to take a ride on some winding country roads, on a nice spring day, with the top down and enjoy the silent performance of an electric car that I converted in my garage at home.

Tuesday, September 11, 2012

Ordering the Batteries

It is time to order my batteries. I’ve selected the CALB CA 180. This is a new product that recently became available. It will give the same range as the previous battery (100 miles per charge), but it can deliver more current (1800 amps in a 10 second burst and 540 amps continuously), and it lasts 3000 charge cycles. 
Batteries store and deliver energy through a reversible chemical reaction. The chemistry of choice for my electric car is lithium iron phosphate. This chemistry provides a good balance between mass density, capacity, lifetime, and safety. This chart shows the progress of battery technology.
Chart is modified based on this Wikipedia source.
Within the Lithium category, lithium iron phosphate is at the lower end of the range for current rating and energy density, but the trade-off is worthwhile – better thermal stability under high load reduces the risk of a fire, and longer lifetime. The only real downside is the price.