Electric Car Batteries Shari Prange c. 1993 Shari Prange The battery pack is one of the electric car's biggest components, in many senses of the word "big". It is the bulkiest and heaviest component. It is one of the most expensive. And it is one of the most important to performance. Which kind of battery? Designing the battery pack involves a series decisions. The first decision is which kind of battery to use. If you study the literature about EV batteries, you may feel overwhelmed by myriad possibilities. In reality, though the realistic choices are not that many. Miracle batteries? The "miracle" batteries that receive so much attention Ä sodium sulphur, lithium polymer, nickel iron Ä are simply not commercially available to individuals. They may be planned for production for manufacturers only, or available only as laboratory test prototypes, or they may be nothing more than vaporware. The same is true of fuel cells, such as hydrogen or zinc air. Therefore, the first question to ask about any type of battery is, "Where can I buy one?" The answer for most of the exotics will be, "Nowhere--they aren't available yet." How much will it cost? The second question to ask is, "How much will it cost?" Some batteries are available--like nickel cadmium, or silver zinc Ä but only at costs equaling or exceeding the entire rest of the car's conversion to electric power. Unless you have special contracts, or a lot of money, the answers to these first two questions will probably narrow the field to conventional lead acid batteries. Battery performance characteristics The third, and most complicated, question to ask is, "Are the performance characteristics of the battery well matched to the needs of an electric vehicle (EV)?" This question has several different aspects. Lead acid batteries a constructed for a variety of uses. The shape, number, and spacing of cell's plates, composition and thickness of lead paste used to make the plates, and the ratio of electrolyte are all critical items that vary depending on the intended use of the battery. For example, the normal battery used in a gas car is a starting battery. It is intended to supply high amperage for a very short time Ä just long enough to start the engine. It is discharged by a small percentage (about 1%), then immediately recharged by the alternator. Starting batteries are not intended to be deeply discharged, ever, and especially not repeatedly. As many of us have found out, a starting battery that is run "dead" too often, perhaps by leaving the lights on accidentally, will soon refuse to store power. An EV needs a deep cycle battery. This means it can be discharged to 80% of its capacity and recharged repeatedly. Conventional starting batteries and other non-deep cycle batteries such as gel cells can provide high short-term performance for a race car, but not continuous daily use. Traction Batteries Not all deep cycle batteries are appropriate. Marine batteries or standby power batteries are not intended to handle the occasional brief high amperage draws a car requires. They will not provide as much range or cycle life as a true "traction" battery. A "traction" battery is designed for both high current draws and repeated deep discharges needed to move an electric vehicle down the road. The type of battery most commonly used in EVs is a golf cart battery. This is a 6 Volt deep cycle battery, typically rated at 220 to 240 Ampere-hours. This type of battery uses three series connected cells assembled into a single battery case. This is an excellent choice because it is well developed, easily available, and affordable. There is also now a true "traction" 12 Volt battery (six series cells) available from U.S. Battery. This is the model 1450, and can be ordered through U.S. Battery or Interstate dealers. While the battery is too new to have full life-cycle testimonials from EV owners, it was developed by a company with an excellent record of producing quality batteries suited to EVs. This battery would be highly recommended for very small cars where space is limited, and may well eventually take over the market for larger conversions as well. Ni-Cads Nickel cadmium batteries are probably the second most common in EVs, far behind lead acid. There are several drawbacks to ni- cads. One is high cost, even for reconditioned batteries. Another is low power density. It is necessary to have several strings of ni-cads in parallel to have sufficient amperage capacity for acceleration. This means a lot of space filled by batteries. A third problem is that ni-cads come in 1.2 Volt cells, which requires many more cell interconnects Ä potential failure points Ä as golf car batteries. For most people, the conventional 6 Volt lead acid deep cycle golf car battery is the optimum choice today. If a better battery becomes available next year, you can always upgrade. How many? Once you know what kind of battery to use, the next decision is how many batteries does the EV need. In very simple terms, Amps equal torque and voltage equal speed. In the electric motors used in EVs, torque is governed by the battery ability to deliver high current and speed is limited by the battery's voltage. The higher the battery's to deliver current, the more torque power it will be able to supply to the EV's electric motor. More torque results in faster acceleration and hill climbing. The battery must be able to deliver the sustained high currents (over 200 Amperes) required by a hard-working EV motor. Higher battery voltage gives the resulting EV a higher top-end speed. However, you can't simply add as many batteries as you want. Each one is the size of a toaster and weighs close to 70 pounds (32 kilograms). Both space and weight are limited and carefully planned for in a successful electric vehicle. A question of voltage Early EVs often ran 48 Volt systems. The EV industry is still trying to live down their poor performance. A 72 Volt system is the bare minimum for a road-going passenger car. This will give performance comparable to the original gasoline-powered 1200 cc VW Bugs. It will only be adequate for a very lightweight car that is never intended for sustained highway speeds. Most electric cars now have a 96 Volt system, which means sixteen 6 Volt batteries. This seems to be an optimum weight/power balance. A typical steel-bodied conversion will have a range of 60 to 80 miles (96 to 129 kilometers) in average commute conditions: mostly flat roads, some freeway time, some stop-and-go in-town traffic. It will have a top speed of about 60 mph (95 kph). Some of the more aerodynamic cars will do better. A lighter weight fiberglass car will have a range of 80 to 100 miles (129 to 161 kilometers), and a top speed of about 85 mph (137 kph). If there is room for the batteries, the system can go as high as 120 Volts. In fact, this is recommended for pickup trucks, where some payload capacity is desirable. Beyond 120 Volts, there is a lack of controllers and chargers. It is debatable whether the extra voltage offsets the handicap of the extra weight of the additional series cells necessary to raise the battery's voltage. Since most EVs use a single series string of batteries, the actual number of batteries making up the battery pack is determined by the voltage of the EV system. A 96 Volt EV will use sixteen 6 Volt golf car batteries weighing 1120 pounds (509 kilograms). A 120 Volt EV will employ twenty 6 Volt golf car batteries weighing 1400 pounds (636 kilograms). When To Buy Batteries Although you have chosen the batteries you will use, and decided how many you want, don't buy them until you are ready to install them. Get precise dimensions and use cardboard or foamcore mock-ups for designing your car. That way the batteries won't be getting stale sitting on your garage floor for weeks or months Ä and you won't be tripping over them. Next time we'll talk about battery placement, boxes, racks, and hold-downs. Access Author: Shari Prange, Electro Automotive, POB 1113, Felton, CA 95018 ù 408-429-1989