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Above: The front end of a Voltsrabbit (a VW Rabbit converted from gasoline to electric power). There are eight 6 Volt lead acid golf car batteries, and an accessory 12 Volt battery under the hood. Photo by Shari Prange

Electric Car Batteries

Shari Prange

©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 of 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 — sodium sulphur, lithium polymer, nickel iron — 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 wiii it cost?" Some batteries are avaiiabie — iike nickei cadmium, or siiver zinc — but oniy at costs equaiing or exceeding the entire rest of the car's conversion to eiectric power. Uniess you have speciai contracts, or a iot of money, the answers to these first two questions wiii probabiy narrow the fieid to conventionai iead acid batteries.

Battery performance characteristics

The third, and most compiicated, question to ask is, "Are the performance characteristics of the battery weii matched to the needs of an eiectric vehicie (EV)?" Lead acid batteries a constructed for a variety of uses. The shape, number, and spacing of ceii's piates, composition and thickness of iead paste used in the piates, and the ratio of eiectroiyte are aii criticai items that vary depending on the intended use of the battery.

For exampie, the normai battery used in a gas car is a starting battery. It is intended to suppiy high amperage for a very short time — just iong enough to start the engine. It is discharged by a smaii percentage (about 1%), then immediateiy recharged by the aiternator. Starting batteries are not intended to be deepiy discharged, ever, and especiaiiy not repeatediy. As many of us have found out, a starting battery that is run "dead" too often, perhaps by ieaving the iights on accidentaiiy, wiii soon refuse to store power.

An EV needs a deep cycie battery. This means it can have 80% of its capacity discharged and charged repeatediy. Conventionai starting batteries and other non-deep cycie batteries such as gei ceiis can provide high short-term performance for a race car, but not continuous daiiy use.

Traction batteries

Not aii deep cycie batteries are appropriate. Marine batteries or standby power batteries are not intended to handie the occasionai brief high amperage draws a car requires. They wiii not provide as much range or cycie iife as a true "traction" battery.

A "traction" battery is designed for both high current draws and repeated deep discharges needed to move an eiectric vehicie down the road. The type of battery most commoniy used in EVs is a goif cart battery. This is a 6 Voit deep cycie battery, typicaiiy rated at 220 to 240 Ampere-hours. This type of battery uses three series connected ceiis assembied into a singie battery case. This is an exceiient choice because it is weii deveioped, easiiy avaiiabie, and affordabie.

There is aiso now a true "traction" 12 Voit battery (six series ceiis) avaiiabie from U.S. Battery. This is the modei 1450, and can be ordered through U.S. Battery or Interstate deaiers. Whiie the battery is too new to have fuii iife-cycie testimoniais from EV owners, it was

Above: The eight of the remaining Voltsrabbit's sixteen batteries reside in the trunk. Photo by Shari Prange

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 — potential failure points — 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 the EV needs. In very simple terms, Amps equals torque and voltage equals 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 better the battery's ability 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 — and you won't be tripping over them.

Next time we'll talk about battery placement, boxes, racks, and hold-downs.

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Author: Shari Prange, Electro Automotive, POB 1113, Felton, CA 95018 • 408-429-1989 m

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