On-Board Battery Charger with Integrated 12V DC-DC to Power Accessory Loads

Vehicle Requirements

Electric utility vehicles such as on and off-road electric vehicles, custom golf cars and scissor lifts will typically be fitted with 2 to 12 batteries that are deeply cycled on a regular basis. These batteries need to be charged at the end of each shift, or whenever possible so that the vehicle is ready to use the next time it is needed. This charging can be provided by off-board or onboard chargers, using either ferro-resonant, SCR (silicon controlled rectifier) or HF (high frequency) technologies. These vehicles may also have accessory loads such as lights, controllers, and other luxuries typically run at a different voltage from the battery pack. This creates a need for two separate voltage busses in the system.

There are important design considerations applied when designing an electric drive vehicle’s charging system. These considerations become much more complex when a 12 Volt bus is also required. Various charging and accessory load solutions will evaluated against these design considerations and an ideal solution presented.

Design Considerations

Today’s vehicle market is competitive and OEM customers are becoming more sophisticated. New technologies need to perform better, be easier to use, and be cheaper to own and operate. Engineers must address space, weight, parts count, cost, cabling, thermal management, maintenance and ease of use constraints. Simple solutions to providing 12V accessory power alongside the vehicle’s higher voltage electrical system may meet cost and wiring constraints at the expense of much higher maintenance costs. Other solutions may meet the ease of use and maintenance constraints with significantly increased size, weight, cabling complexity and cost. Truly efficient design lies in the balance of all the constraints.

Battery Charging Solutions

The challenge with electric vehicles has always been charge time and charge convenience. While charge time depends on electro-chemical limits such as the size of the battery pack and supply power available, charge convenience is greatly improved by simply placing the charger onboard the vehicle.

Off-board Charging

Having electric vehicles return to the same location for charging works well for most fleet and maintenance groups. Predictable shifts and easy access to storage areas means the vehicles always spend large amounts of time in the same location. It therefore makes sense to charge the vehicles there too. Rotating shifts can lead to a slight economic savings by having fewer chargers than the number of vehicles - some vehicles are in use while others charge. In these situations, off-board charging is efficient as well as convenient.

We are, however, dealing with vehicles – moving vehicles. They may find their way around a facility or wander far from the centralized charge location. These vehicles often are not in use the entire time, and may find themselves parked next to electrical outlets that could conveniently extend their runtime for that usage. Retrieving a charger for use becomes a hassle, and carrying one with the vehicle is an inconvenience. In many applications, it may also be advantageous to be able to leave a vehicle and return to it after some time, perhaps even overnight. In these instances, centralized charging is not so convenient.

Onboard Charging

With vehicles that may be in motion for hours, venturing far from their chargers, having the charger with the vehicle provides great convenience. This gives the user the flexibility to use a commonly available electrical cord to plug it in to a nearby electrical outlet and charge while the vehicle is not in use.

Large, ferro-resonant charger, however, make this a difficult engineering task. Their sheer bulk not only consumes valuable space on vehicles, but also adds to the mass that the electric motor must move around. This reduces the very runtime that the onboard mounting was designed to improve! Issues also arise when non-sealed chargers are used in harsh environments where dust and moisture can enter the charger. Performance can be quickly reduced by a build up of material inside ventilated chargers, increasing the amount of heat and reducing lifetime. Moisture may also enter the unit and begin to corrode parts, eventually causing a malfunction. All of these factors add to the cost of operating the electric vehicle, and may offset the convenience gained by having the charger onboard.

Recent technological advancements, however, have created a solution for both onboard and off-board charging. The advent of high-frequency technology has created compact, sealed, lightweight chargers that excel in onboard use, and operate equally well when used off-board. Despite their small size, they still compete for space onboard vehicles because of the increasing number of other components.

Already in use by a number of neighbourhood electric vehicle manufacturers, on-road electric vehicle converters, and electric utility vehicle OEMs, the QuiQ charger has well recognized benefits. Its rugged, sealed nature, along with universal input, provides reliable operation in any application. The QuiQ is also intelligent, with a number of selectable algorithms loaded in memory, and many more that can be field-programmed. Its high-frequency design makes it approximately 80% lighter than conventional ferro-resonant chargers. High efficiency and power factor correction means that the QuiQ is also more than 20% more energy efficient. As an onboard or off-board charging solution, the QuiQ adds value and performance to electric vehicles.

Accessory Power Solutions

Different approaches to the provision of the lower voltage for electric vehicle accessories vary in their complexity and effectiveness.

Battery Pack Tap

The battery pack tap (shown in Figure 1) is the simplest method of extracting accessory power from a large battery pack. Older heavy-duty trucks are a good example of a 24V system, containing two 12V batteries, one of which is wired to the 12V system. The same can be done with any battery pack to “tap” it for accessory power. Because no special wiring is required, the battery pack tap is common when loads are minimal.

The consequence of this simplicity is that even a minimal current draw on one portion of the battery pack will cause the entire pack to fall out of balance. This means one or more batteries are not only more discharged than the rest when it comes time to recharge them, but those batteries will be cycled more deeply over their lifetime than the rest of the batteries in the pack. This can lead to battery damage as a charger attempts to charge the entire pack and either undercharges or overcharges some of the batteries. Over a vehicle’s lifetime, this may result in the cost of several extra battery packs.

Separate 12V Battery and System

Another uncomplicated solution sometimes employed is the use of a completely separate 12V system. This will include a separate 12V battery with a dedicated 12V battery charger, or a 12V battery and a larger DC-DC converter or battery equalizer to charge the battery. Though containing more components than the battery pack tap, with higher cost and cabling complexity, this method is attractive due to its conceptual simplicity.

Such an arrangement, shown in Figure 2, is necessary if there is a large 12V load on the system, such as hydraulics, inverters, or other high-power electronics. However, the battery is underutilized if there is no load this large (>20A draw). It is also likely that the charging system for this 12V battery is not used efficiently as the battery may not see large discharges. The additional battery also means unwanted weight and presents another consumable maintenance item. All this adds additional cost over the life of the vehicle.

Separate DC-DC Converter

The installation of a separate DC-DC Converter unit in the vehicle is a common practice today. These are usually small, efficient pieces of hardware that take the battery pack voltage as input (typically 24V to 72V) and convert it to a clean 13V to 14V for the accessory loads. The converter acts as a bridge between the two voltage busses, supplying the 12V system with all the current it needs from the larger battery pack. The configuration is simple, and components are kept to a minimum.

Many converter choices are available, ranging in output from 5A up to 120A. Efficiency and output performance also vary widely with the choices on the market. A limiting factor for DC-DC converters is their ability to dissipate heat from the conversion process yet remain sealed for on-vehicle use. Larger units may resort to liquid or forced air cooling, while smaller ones rely on convection and radiated cooling

This thermal management is critical to a DC-DC converter’s ability to continue operating under heavy loads. A converter which does this poorly will run too warm, consistently under-perform and ultimately have a shorter lifespan resulting in frequent replacement.

If an electric vehicle has a 12V accessory system, it will typically have a moderate load (15-30A) to make best use of the wiring that exists. This is an ideal range for a compact DC-DC converter to be used, resulting in savings from better battery health, and reduced maintenance from an efficient use of components.

Utilizing a DC-DC Converter to handle the 12V accessory loads easily, coupled with an onboard charger to simply and conveniently charge the higher voltage battery pack, offers tremendous benefits over the other options presented so far. However, some problems present themselves in this configuration. When the DC-DC converter remains active while the battery pack is charging, it creates a parasitic load on the battery pack that the charger is unaware of. The charger may misinterpret this load as a battery problem, undercharge the batteries, or even stop charging altogether.

The Integrated Solution

Though a great improvement over existing methods, onboard charging with a separate DC-DC converter has even more potential for improvement. While charging, the DC-DC converter is typically under-utilized or even inactive. While in operation, the vehicle may make full use of its DC-DC converter, but the battery charger remains dormant. Because these units are ideally sealed for environmental reasons, they rely on large metal fins to provide convection cooling. The potential then lies in the sharing of this cooling between the units.

The clever addition of a DC-DC Converter to the QuiQ Charger, shown in Figure 4, means Delta-Q can offer even more benefits to vehicle manufacturers. A combined DC-DC converter and sealed, high-frequency, onboard battery charger is a revolution in electric vehicle system components, and meets many common design constraints on electric vehicles. It is an efficient design that adds value to the vehicle while increasing its performance.

With a 60A peak and 30A continuous output, the QuiQ’s 400 Watt DC-DC Converter provides ample power for almost all 12V applications. It outperforms many existing stand-alone DC-DC Converters in its ability to operate under high loads, thereby keeping the vehicle operating.

Reduced Space

Designing a vehicle’s infrastructure to be as small as possible allows more space to be used for the practical purposes it was designed for. This design philosophy should trickle down to suppliers, so no part of the vehicle design is compromised.

High-frequency battery chargers already represent a significant space savings when compared to traditional ferro-resonant chargers. Combining a DC-DC converter in the same package as the charger means the entire DC-DC converter, whose volume is mainly made up of cooling fins for thermal management, can be eliminated.

What is truly astonishing is that the QuiQ-dci incorporates an isolated DC-DC converter into the existing housing of the Charger, making full use of the cooling available while the charger is inactive. This space savings results in more design flexibility for the rest of the vehicle.

Reduced Weight

Weight is always a critical factor in vehicles. A vehicle’s weight directly relates to the amount of energy required to move it. Though some ballast is sometimes necessary in lift applications, weight reduction of any component creates more flexibility in design. A weight reduction typically leads to increased energy efficiency for the vehicle, and increased flexibility in placing ballast in optimal locations.

High Frequency chargers typically offer significant weight reduction compared to conventional ferro-resonant chargers. Further weight reduction is achieved by combining the converter and the charger, since much of the weight of a stand-alone DC-DC converter is in the heat sinks. As an integrated unit, this means more vehicle performance from the same amount of energy cost.

In a 48V configuration, the QuiQ-dci saves about 30 pounds compared to a traditional SCR Charger and DC-DC combination. The incremental weight of the DC-DC converter is less than 3 pounds, substantially less than a typical DC-DC converter. This improves the energy efficiency of the entire vehicle.

Reduced Parts Count

Reducing parts count means less inventory management, less warehouse space, and less vehicle complexity. It also means more efficient production and simpler maintenance, improving the quality and overall success of the vehicle.

A combined DC-DC converter and onboard charger reduce two previously separate components to one. It is also possible that this will eliminate a vendor from the vehicle OEMs list, adding to the already improved efficiency of the system.

Reduced Wiring

With each electrical connection on a vehicle comes a risk of corrosion, poor contact, and manufacturing defects. Poor connections can cause hours of troubleshooting time and reduced reliability of the vehicle.

Wiring reductions, resulting from more compact harnesses with fewer connections, are not only less costly to produce, but have fewer points of potential failure. With the DC-DC converter and onboard charger integrated, connections are saved and the vehicle’s harness can become simpler. Using one single node for two functions means a simpler, more reliable machine.

Reduced Cost

Combining an onboard charger with a DC-DC converter results in design and manufacturing efficiencies that can reduce the overall cost compared to separate converters and chargers. Vehicle OEMs may see reductions in raw vehicle parts costs, but will certainly see reduced costs in cabling, manufacturing labour, inventory management, battery maintenance and service.

Improved Ease of Use

The goal of any product is to help the end user achieve their productive goals. Any complexity that may hinder this effort becomes a liability, and increased ease of use is always a valuable asset to a product.

An electric vehicle with an onboard charger increases the vehicle’s ease of use as the end user simply plugs in a standard AC cord to charge it. Charging can also happen anywhere, at any time, at the user’s convenience. It provides the flexibility to continue focussing on their goals.

An integrated DC-DC converter and battery charger also eliminates the problem of parasitic loads. A well designed unit will be able to detect DC loads while charging and compensate for this, avoiding undercharge of batteries and further improving health and lifetime of the battery pack.

Conclusion

The QuiQ-dci has been designed to meet all of the electric vehicle engineer’s constraints. It offers savings in size, weight, system complexity and wiring; improvements in vehicle performance; and savings in component cost, cabling, manufacturing, inventory management and battery maintenance. The result is a more efficient, more economical electric vehicle.

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