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Electric Vehicle Chargers

Shari Prange

©1994 Shari Prange

One of the most critical components for good electric vehicle performance doesn't even function while the car is being driven — the charger. It is also the component most likely to stimulate debate about appropriate specifications. While there is no consensus on the right answers, I can at least help you to ask the right questions, and evaluate the choices yourself.

Volts & Amps

The charger's job is to take alternating current (ac) from the power source and convert it to direct current (DC), as well as to alter the voltage to suit that of the battery pack.

Electricity flows "downhill", from a higher voltage to a lower voltage. For this reason, the voltage coming out of the charger must be higher than the nominal voltage of the battery pack.

Amperage (current) is also critical. Too little current will not achieve a full charge. Too much current will damage the batteries. The amount of current the batteries can accept is inversely proportional to the state of charge. Very low batteries can accept more current than nearly full batteries.

If a charger does not have enough output to bring the batteries all the way up, the car will not achieve its full potential for range. If the charger sends too much current to the batteries, it will cause them to gas, and will shorten their lifespan.

Home-Built Chargers

Early EV chargers were crude home-built affairs. They slammed juice into the batteries indiscriminately, and resulted in batteries that gassed a lot and required frequent waterings. This kind of abuse shortened the batteries' lives. If the charger was a little more

Above: A transformerless 110 volt ac input charger (bottom left); a booster transformer for 120 volt output (top left); and a transformer-type 220 volt input charger

(right). Photo by Shari Prange sophisticated, it might include a timer-controlled automatic shut-off, or a dial which the owner could adjust every hour or so to taper the amperage. Neither method provided very accurate control, and the second required frequent attention.

Today, there is no reason to use a home-built charger, since there are good EV chargers commercially available. There are also several reasons not to use a home-built unit. Commercial units include numerous safety features in the form of circuit breakers, fuses, and ground fault interrupters to prevent shocks or hazardous electrical faults.

They also provide internal shielding to prevent electrical noise and spikes from feeding back into the ac lines. This is especially important when so many of us have sensitive electronic equipment on those same ac lines.

110 or 220 Volt Input

For home use, chargers come in two sizes — 110 volt input, and 220 volt input. In general, a 110 volt charger will bring a 96 volt pack from completely discharged to completely charged in about 12 hours, and a 220 volt charger will do it about half that time.

At first glance, this would make the 220 volt charger seem to be the easy winner. However, there are other considerations. These chargers have traditionally been the size of a bread box and weighed 70 to 150 pounds, so they haven't been practical to carry onboard. They cost more too. Also, 220 volt outlets are not as readily available as 110 volt outlets, and there are several different plug styles as well. This makes it difficult to charge on 220 volts anywhere except at home.

At least the first drawback will soon be eliminated since K & W will be offering a compact and lightweight 220 volt input charger.

In recent years, the 110 volt charger has been the most popular, because it has been available in a package smaller than a toaster and weighing only 10 pounds. It is very convenient to carry onboard, and allows charging anywhere with a grounded 110 volt outlet.

The difference between the lightweight and heavyweight chargers is the result of the different technologies used. The heavy chargers use heavy transformers to change the voltage. The lighter units "chop" or "switch" the input current and use only a portion of it. They are similar in principle to chopper controllers.

There are also some chargers that will accept either 110 volt or 220 volt input. New charging technology is being developed that will decrease the charging time for 110 volt chargers through various techniques, such as more precise monitoring of the batteries' state of charge and rapid pulsing of the charging current.

Output Voltage

Output voltage varies from charger to charger. In some brands, it is necessary to specify the battery pack voltage, and that is the only voltage for which that charger will work. Other chargers offer the option of slight internal modifications to change from one size battery pack to another. The alteration is not as simple as flipping a switch, but certainly much less hassle than buying a whole new charger if you change your battery pack size.

Be aware that charging a 120 volt car from a 110 volt source may require a booster transformer in addition to the charger.

Set It & Forget It

Modern chargers require very little monitoring. They have the ability to sense the level of charge in the battery pack, and automatically adjust their output current to an appropriate level. They will start with an output of 20 to 30 amps for a completely drained pack, and gradually taper to a low amperage "finish charge" that equalizes the batteries. Some chargers will automatically turn off; others hold at the finish charge level.

Input Requirements

It's best to have a dedicated circuit for the charger, with no other load on it. This is generally a 20 or 30 amp service, whether it is 110 volts or 220 volts input.

Of course, when charging away from home, such a circuit may not be available. Many chargers have adjustable output current. The charger current can be adjusted to suit the circuit available, and the car will charge at a lower (and slower) rate.

Safety Features

Modern chargers also incorporate various safety features, such as fuses and circuit breakers. The heavier chargers use transformers, which provide internal isolation of the input and output current. This eliminates the possibility of high voltage current flowing into the chassis of the car.

The smaller chargers do not use transformers. To compensate for this, they include a ground fault interrupter (GFI). This system requires the battery pack to be isolated from the chassis, and not use the chassis as a ground. If there is any current path between the battery pack and the chassis, the GFI will shut off the charger. This is similar to the GFI required in many areas in new construction of bathrooms, spas, and other areas where electricity, water, and humans may meet unhappily.

Shopping for Chargers

When you are shopping for a charger, you will need to evaluate all of these factors, and decide on the unit that best meets your needs overall.

Some features, particularly those relating to safety, are not negotiable. As a minimum, a charger should have fuses or a circuit breaker, and a GFI if it is transformerless; it should have an automatic tapering feature on the output current; and it should come from a reputable manufacturer and be enclosed in a sturdy protective case.

Other features will need to be weighed to suit your needs. The questions to ask are:

1. How big is it and how much does it weigh?

2. What input voltage(s) does it accept?

3. What output voltage(s) does it provide?

4. How energy efficient is it?

5. How fast will it charge my car?

Charging the 12 Volt System

The final aspect of the charging system is the car's 12 Volt accessory battery. Some chargers have a separate circuit for charging this battery at the same time the main pack is being charged, but this feature is not common, especially on the smaller 110 volt units.

Of course, it's possible to use an ordinary automotive 12 Volt charger separately on this battery. However, there is a better way: the DC/DC converter.

The car no longer has an alternator to keep the 12 Volt battery charged. As you drive, this battery will deplete itself. A "12 Volt" system is really designed to operate at about 13.5 Volts. This is what a typical alternator — or a fully charged 12 Volt battery — puts out. Accessories that are designed to operate on this 13.5

Above: A DC/DC converter installed in a VW Rabbit conversion to keep the 12 volt Accessory battery charged. Photo by Shari Prange

Volts will not work as well when the battery falls to 12.5 Volts or lower. Headlights become dim, and turn signals and wipers become lethargic.

You could install an alternator to run off the motor, but a DC/DC converter is more efficient. A DC/DC converter taps off the main battery pack, drops the voltage down, and keeps the accessory battery continually charged, just like an alternator. The car's 12 Volt system stays strong all the time, and there is no need for another charger for the 12 Volt battery.

Be sure you are getting a DC/DC converter that is designed to be used in a vehicle. There are many converters commonly available for use with computer hardware, but they will not stand up to the temperatures, dust, moisture and vibrations of a normal automobile in motion.

No single charger combines the best of all features. Which combination you consider the best will depend on how important each feature is to you. The important points can be boiled down to the few basic qualities: effectiveness, safety, reliability, and convenience. The charge you get out of your electric car will only be as good as the charge you put into it.


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

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