Electric Vehicle Frequently Asked Questions

BATTERY CHARGERS

What chargers are available?

Chargers range from homemade "bad-boy" chargers to computer controlled, high power onboard units.

The homemade kind usually consists of a rectifier connected either to the output of a variac or directly to the AC input line. This kind of charger requires constant monitoring and manual control. It is called a bad-boy (or a vari-bad-boy, to quote John Wayland) because it is completely unregulated. If you watch every battery and the AC line like a hawk, and tweak the output appropriately, this can be a fine charger. However, this isn't always possible.

The chargers discussed below are available from evparts.com and the other suppliers listed elsewhere in this FAQ.

Chargers range from small, simple onboard units to large, offboard high-power units. The top of the line chargers are both small and high powered. The most popular are discussed here in order of increasing utility. The prices tend to follow the same trend.

At the bottom of the line, there is the K & W BC-20. It is a small, lightweight, SCR based charger that runs off of 120VAC. It has a built in ammeter and has adjustable current to avoid tripping breakers. It applies maximum current until a preset voltage is reached, at which the current tapers off. The maximum set voltage is then held, with no automatic shutoff. It charges packs from 96 to 144 Volts, although to charge over a 108V pack, it requires an available booster transformer, which adds approximately 15 lb.

Next up is the Russco. It is also a 120VAC input transformerless unit, with a current display, but is available with a timed shutoff, and has a slightly better charging algorithm. It also has the ability to charge up to a 120V pack without a booster. Booster transformers are available to allow the Russco to charge a 132 or 144V pack. The Russco will charge packs from 84 to 144V.

Two types of large, transformer isolated chargers come next. These are less common, but are available. First is the Lester. It can be ordered as either a 120VAC or 240VAC input, and has high power handling capabilities.

The Bycan is similar, but has both 120VAC and 240VAC inputs. It automatically selects between them. It can charge pack voltages of 120, 132, or 144V. It has a switch to automatically run an equalization charge.

The K & W BC-250 is next, although its very high price tag (more than the PFC-20 discussed later), makes it much less common. It can charge 72 to 156 volt packs from 120 or 240 VAC. It will interface with the badicheq battery management system, and employs the same basic charging algorithm as most of the other chargers.

The most popular EV charger is the Zivan. It has computer control, and comes with the user's choice of 12 charging algorithms preset into its memory. Models are available with either 120VAC or 240Vac input (high power model only has 240VAC input), and it has an available temperature probe for temperature compensation. It also has several outputs for relay triggers. It is available in up to a 4.3kW power handling capability. The user can not adjust any of the settings, and no instrumentation is included on the unit.

Of all those listed above, the Lester is slightly more efficient, due to its slightly better power factor. None of the chargers available to hobbyists have ever employed active power factor correction, until now.

The PFC-20 (and its higher power cousin, the PFC-50) is considered the top of the line and state of the art. They are small and light enough to be carried onboard. It combines the best features of the units discussed above. They both have high power handling capability, can charge any pack voltage from 12 to 336 volts, adapt automatically between 120 and 240 VAC, and employ active power factor correction for high efficiency. They have a shutoff timer and the user can select how it is used. It comes preset with a good general purpose charging algorithm, but has a computer control option that lets the user alter the charging profile in any way they want. It also interfaces with the most popular battery regulators (made by the same company).
The PFC line of chargers is offered by evparts.com. Some special modifications are available on request, for a slight extra charge

What wiring is required to charge in my garage?

EVs can be recharged from any normal AC outlet; special outlets are not required. However, the higher power the outlet, the faster you can recharge. For example, recharging from a US standard 120v 15amp outlet can take 8-16 hours; recharging from a 240v 30amp outlet can recharge in 1/4 the time.

As with any outdoor outlets, the outlet used for recharging your EV should have a ground pin and GFCI (Ground Fault Circuit Interrupter) for safety.

If your charger is NOT in the vehicle, then NEC (National Electric Code) section 625 outlines the requirements for high-powered EV chargers. Since many local building codes reference the NEC, you may have to meet its rather conservative requirements for new construction.

Can I build my own charger?

This question has not yet been answered

What is power factor correction in a charger and how is it done?

Most low-end high frequency battery chargers today use a simple rectifier circuit consisting of a diode bridge followed by a capacitor bank. Rectified voltage is then brought to the high frequency DC/DC power stage where it is converted into regulated output current or voltage. The rectifier's capacitors significantly affect the current waveform. As the input voltage reaches the stored level in the capacitor, the rectifier diode conducts, allowing the current to flow as long as the line voltage is greater than capacitor's voltage. While the load current is continuously drawn from the capacitor by the DC/DC stage, the capacitor is recharged only during the interval when the input rectifiers conduct. No current flows into the capacitor from any point along the voltage waveform where it is below the capacitor's DC voltage.

High power factor results when the current and voltage have low distortion and are in perfect phase. Low power factor results when either the load current is drawn over only a part of each line cycle (current harmonic distortion) or when the line current is out of phase with the line voltage. The first problem is the result of off-line rectifiers where the input diode does not conduct until the peak of the rectified line voltage waveform exceeds the DC level across the input capacitors.

Power factor represents the ratio of real to apparent power. Low power factor is caused by the apparent power being higher than the real power. Apparent power is the current read on an ammeter times the line voltage. A low power factor is characterized by a higher current than the load actually needs to satisfy its real power requirement. The difference between the current that produces the real power consumed by the load and the current measured on an ammeter is known as the circulating current. It is so called because even though it does no real work, it continuously flows back and forth between the line and the load. This circulating current is at a different frequency and/or phase than the line voltage and does nothing to supply power to the load. The circulating current does heat up the transmission lines supplying the power and it will open fuses and breakers at less than the rated power because the current is real but delivers no real power to the load.

For example, a high frequency charger with 85 percent efficiency and a power factor of 0.60 can produce only 734 watts of real power to a load with 12 amperes from a 120V AC utility mains (12 amperes is the maximum continuous rating of a standard 15 ampere branch circuit). On the other hand, the maximum power that can be used by a load with unity (1.000) power factor is 1440W. Thus, only about half of the power in our example is being used by the load.

Resistive loads have a power factor of one, since current flows through the load proportional to the voltage across it.

Power Factor Correction (PFC) circuits are used in order to improve the poor power factor of standard rectifiers. Various PFC circuits are employed to actively force the input rectifier to conduct over the entire cycle of the input waveform. Most commonly used PFC circuits are in the form of a high frequency boost converter that precedes the input filter capacitor. With a slight reduction in efficiency and by almost doubling the complexity, the PFC boost converter increases the power factor to something between 0.95 and 0.9999. This circuit also operates at an efficiency of 92-95 percent. Equipping our previously discussed 85% efficient charger with a 92% efficient PFC circuit will yield a charger with an overall efficiency of 78% and a power factor of 0.97. At the example conditions of 120V and 12A current, this charger will be able to provide 1,089W to the battery, a 48% increase in the available power!

Another advantage of PFC is that at RMS current of 12A, peak line current will be only 17A. Without PFC, peak current can easily reach 35-40A, causing line voltage distortion and may negatively affecting other equipment connected to the same circuit.

How can I quick charge my batteries, will it harm my batteries?

This question has not yet been answered

What are battery regulators and balancers for?

Batteries are the "fuel tank" of an electric car. Most vehicles have one big fuel tank, so it is easy to remove and add fuel, and tell how much fuel is in the tank. But most EVs use many batteries. In theory, they are all identical, and always charged and discharged equally.

But in practice, there are always small variations even between new batteries, and these differences get larger as the batteries age. There can also be temperature variations, or differences in the load that each battery sees. This means that different batteries are at slightly different states of charge. It becomes difficult to know exactly how much "fuel" you have, or when they are full or empty.

The simple solution is to pretend that all batteries are identical. The "fuel" gauge simply displays the average state of charge of the whole pack. When driving, the vehicle loses power when the weakest or least-charged battery goes dead. Thus, your range is limited by the "weakest-link" battery in the chain. And when charging, the charger deliberately overcharges the entire pack, so even the battery at the lowest state of charge gets fully recharged. This is fine for lower-tech deep-cycle flooded lead-acid and nicad batteries; they tolerate deep discharges and modest overcharging with relatively little loss of life.

But higher-tech batteries are not so tolerant. They are damaged by excessively deep dishcarges, and excessive charging. Battery regulators and balancers are devices to monitor batteries individually, and add or remove charge from them to keep all batteries "filled up" to the same level.

The simplest type is a Regulator (example: BatPro or Rudman Regulator). One goes across each battery. If the voltage during charging indicates that the battery is full, the regulator bypasses any further charging current through a resistor, to prevent that battery from overcharging. The excess charging power is burned up as heat.

A Balancer is a bit more sophisticated (example: Powercheq, Badicheq, Zizan Smoother). These systems also monitor individual battery voltages, but use a small DC/DC converter and switching network to transfer charge from one battery to another. Balancers can thus work to balance batteries even while parked or driving; not just while charging.

Regulators and Balancers thus extend battery life. They are helpful for flooded lead-acid and nicad batteries, highly desirable for sealed batteries, and mandatory for high-tech batteries like nimh and lithium batteries.

What do battery regulators do, and how do they do it?

As described in question 13.6, there are different types of regulators. They will be described separately because they do different things.

First and most common is the dissapative regulator. These are inexpensive devices that wire across the terminals of each battery. They protect the battery from overcharge during charging in order to maximize battery life. This ensures that each battery is fully charged, because you can overcharge a bit to fill up the batteries that need it more without damaging the rest.

When a battery is fully charged, any more energy put into it goes into heating it up and electrolyzing the water in the electrolyte. This is fine with flooded batteries. They heat up slow, and have plenty of water to keep the plates immersed. But more expensive sealed batteries are typically "starved electrolyte," which means they do not have nearly as much water to give up. What's more, once it's gone, there is no getting it back without disassembling the battery.

Dissipative regulators typically have an adjustable voltage setpoint, such that above that point, they attach a load across the battery until the voltage drops below the setpoint, which it does in a second or two. This way, the battery is held at the setpoint, which is chosen based on the battery type and service (usually 14.7 to 14.9 volts per 12V battery), for the rest of the charge. They are still being charged, just not to the point where they exhibit extreme voltages and vent their precious water vapor. Any gassing that occurs is kept at such a low rate that it is within the battery's capacity to recombine within the case. The most common dissipative regs are available with a number of options, such as communication with the charger, low battery detection, and the capability to use external loads to handle higher currents.

The other type of regulator is called an additive regulator. As of this writing, these types of regulators do not protect the batteries from overcharging. Instead, they measure the voltage level of each battery, and give it an individual charge either from neighboring batteries or from the pack as a whole. This way, the lower capacity batteries are kept closer to the same level of charge as the higher capacity batteries. Thus, these types of regulators protect the batteries from excessive discharge. Since they usually involve a DC/DC converter and some sort of electronics for control, these types of regulators are usually more expensive than dissipative regulators.

Some brands of additive regulators consume power all the time, and can drain a pack if it is left unused for an extended time. Most kinds only use voltage as a measurement of state of charge, when in reality, batteries at equivalent states of charge may be at slightly different voltages due to the same miniscule differences between the batteries that makes them go out of balance in the first place. So the regulator tries and tries to get the voltage up, all the while it is draining the rest of the pack or the neighboring battery.

Regulators can be an important part of a battery management system. However, a true management system employs more accurate means of measuring SOC than simple voltage measurements, and helps each battery both during charge and discharge.

How do I charge my batteries using a variac?

This information was derived from the EV Discussion List. Special thanks to Lee Hart and Chuck Hurst.

Safety Precautions:

� This is a manual procedure. You've got to pay attention, or you'll wind up ruining your batteries.

� Only use a variac charger with flooded lead-acid batteries! SLA and AGM batteries require a different charging protocol, and are much more susceptible to damage. Charging AGM batteries with a variac charger will most likely destroy them.

� Use fuses all around the charger. Connecting the charger backwards (or otherwise incorrectly) is like short-circuiting the battery pack, and can lead to a lot of overheated, damaged components.

� Turn the variac down to 0 (fully counter-clockwise) before beginning. If something is incorrectly connected, you don't want to hit it with full voltage right off the bat.

� Build a timer for your charger. Overcharging the batteries can permanently damage them.

� Some variacs have been observed to destroy sine-wive inverters. If you have an inverter, you are advised to remove it from the circuit before charging with your variac. Required Instrumentation

� An ammeter and voltmeter are required to charge with a variac charger. An ?E-meter combines these functions, and can also measure amp-hours.

Process:

This description attempts to be generic. When numbers are used, they are based on a 144 volt, 100 amp-hour pack.

1. Dead batteries require a starting charge of just a few amps. Turn up the variac until your ammeter shows 2% of C. In one hour, the pack should show its nominal voltage (such as 144v). If not, some battery is damaged; use your voltmeter to check each battery individually; one is probably shorted or reversed, and it will need to be replaced.

2. Turn up the variac. You want to provide as much current as possible for a fast, thorough charge, but you don't want to burn anything up. Check the extension cords, plugs, and charger. If anything is too hot to touch comfortably, the variac is too high. Turn it down immediately. Use your ammeter to determine the maximum current you can safely provide during charging. Remember that using a variac from AC power produces transient DC "ripple" currents; at 12A, the ripple current can be as high as 20A. In fact, 12A has been cited as a reasonable charging current.

3. Watch the ammeter. As your batteries charge, their voltage rises. As the voltage rises, the current falls. To provide constant current, you'll have to turn up the variac. I recommend you check every commercial (15 minutes or so). Failing to turn up the variac will not damage the batteries; it will just take longer to charge.

4. Look for full batteries. When any battery reaches 2.5v per cell (15v on a 12v battery), it's full. You don't want to overcharge it, since that will shorten its life and release hydrogen into your battery compartment. You'll have to start turning down the variac to ensure that this battery doesn't get higher than 15v. When your pack isn't balanced, it's almost always the same battery that reaches the limit first; when your pack is balanced, they all reach the limit at the same time. (In that case, you can just use the whole pack's voltage instead of checking each individual battery; for 12 batteries at 15v limits, that's 180v.)

5. Fill the whole pack. You'll have to keep turning down the variac as more batteries fill up. As the batteries reach their full charge, less current will be required. When you reach 2% of C, they're full. You're done; turn the charger off.

6. Equalize the batteries. Let the batteries sit overnight. In the morning, measure each battery's voltage. They should all match (to within 0.05v for 12v batteries, 0.03v for 6v batteries). To charge the weak ones, you can either try charging them separately or running a 2% C charge to the whole pack for a few extra hours. Eventually they'll equalize.

A properly connected E-meter can tell when the batteries are full. This requires setting the minimum charging voltage, the maximum charging current, and the measurement time. When the charging voltage is above the minimum charging voltage at the same time the current is below the maximum charging current, and this condition persists for the measurement time (usually 1 or 5 minutes), the charge indicator on the E-meter will flash green. If the E-meter is equipped with a low-voltage alarm, it will go off at this point; you could use that signal to turn off the charger. Unfortunately, these conditions are difficult to meet with a variac, and almost impossible to meet with even a single damaged battery.