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EV Success Driven by Battery and Charging Solutions

Feb. 21, 2018
Sponsored by: Texas Instruments. New charging methods and intelligent battery management help make electric vehicles practical.

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All-electric vehicles (EVs) are available right now. So, what’s keeping you from buying one? Of course, the answer is travel range—the range of even the best of the EVs is around 200 miles on a full charge.  That’s roughly half of what the typical internal-combustion-engine (ICE) vehicle can do. That limitation is going to put off potential EV buyers until something better comes along. 

Better batteries are the ultimate answer to that problem, but it hasn’t happened, yet. For now, the real solution is more charging stations and faster and easier charging methods. You will be glad to know that progress is being made, and superior electronics is making it happen.

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Transportation of the Future Already in Sight

We’re now on the path where we’re evolving from ICE vehicles to full EVs. It will be decades before all ICE vehicles abandon the roads, but the world is moving more quickly to make EVs the primary transportation appliance.

For example, the UK has banned ICEs by 2040, and France will end sales of ICEs by 2040. Germany is more aggressive, saying only zero-emissions vehicles will be sold by 2030. China mandates at least 5 million zero-emission vehicles by 2020. India has stated that there will be only EVs for sale by 2030. No U.S. policy has been stated yet, but there will be one. Most expect the U.S. to adopt a more conservative and extended transition from ICE to full EV.

In the meantime, hybrid vehicles provide a middle ground for the development and testing of the electrical systems that will lead to full EV nirvana. A total EV state will certainly improve the environment. However, some projections indicate that the existing electrical grid may not be able to support the massive power requirements of recharging. It’s expected that an extensive electrical grid infrastructure update will be needed. The question is will the inevitable increase in power station emissions offset any reductions gained with the all EV condition?

To achieve the desired zero-emission state with optimized vehicle safety goals, the average vehicle is experiencing a massive increase in the number of electronic systems and components. This is happening now with the roll out of the popular advanced driver assistance systems (ADAS).

Further development of hybrid vehicles and full EVs has revealed the need for an upgrade of the vehicle electrical systems from their current 12-V-only configuration to hybrid 12- and 48-V systems. Next is a full EV system powered by a 200- to 800-V battery system. A major focus today is on four key goals: improving batteries, enhancing vehicle battery-management and charging systems, aggressively building out the charging station network, and making sure the electric power grid can handle the tidal wave of power needed to support an all-EV paradise.

Intelligent Battery Management and Charging for EVs

The battery is the only source of power for an EV. For that reason, it must be treated with extreme care.  The battery in a modern EV consists of many voltage cells connected in series and parallel to get the desired voltage to drive the motor.  Voltage levels are in the 200- to 800-V range.  To make sure the battery is working as intended, EVs incorporate a battery-management system (BMS). The BMS is made up of a battery-monitoring capability, multiple power-conversion stages, and one or more intelligent embedded microcontrollers.

The battery-monitoring function is performed by a dedicated IC (BMIC) that monitors the voltage of each cell in the battery. Yes, each cell. It only takes one bad cell to kill the whole battery. The BMIC looks at the voltage of each cell and checks for overvoltage or undervoltage conditions. Temperature is also monitored at several points to signal out-of-range temperature conditions. All of the collected data then goes to a controller, where the state of the battery is computed and reported. The controller then determines the battery’s charge condition so that it can calculate the range of the vehicle for the current charge level of the battery or respond to deficiencies in the battery.

EV batteries are made up of multiple battery modules where a module consists of multiple cells connected in series and/or parallel to get a desired higher voltage. These modules are then connected in series and/or parallel to develop the needed high voltage (HV) in the 200- to 800-V range. 

1. Here’s a simplified view of the TI bq76PL455A-Q1 battery-monitor IC for observing up to 16 cells in a typical EV battery module.

Figure 1 shows a simplified battery-monitoring system for one module. Cables connect the individual cells to the battery-monitor IC. Switching, filtering, and protection circuits are provided at the inputs.  These input voltages are sent to a multiplexer that passes them on to an analog-to-digital converter (ADC) for data conversion. These signals are then sent to the system embedded controller whose software does the evaluation of each cell.  That controller typically has two processors, a standard RISC plus one for DSP for higher math computations.

One example of a battery-monitor IC is Texas Instruments’ bq76PL455-Q1. It monitors up to 16 cell inputs, meaning that multiple chips are used to fully monitor the battery. Each system uses 8 to 12 modules. Overvoltage and undervoltage comparators provide critical warnings, and a 14-bit ADC provides the data conversion.  

The outputs of the battery-monitor IC are transmitted via a UART serial interface that may be daisy-chained to provide a single path to other modules. The BMIC is usually paired with a TI C2000 series MCU that features both ARM RISC and a 32-bit DSP. The software in the embedded controller tells the system what to do. Any low voltage cell will be detected, overtemperature conditions will be sensed, and then action will be taken to prevent a fire or some other catastrophic failure. An evaluation module using this IC is available from TI.

Charging Ahead Toward an EV Support Infrastructure

New battery technology and a full network of charging stations will eventually make the zero-emissions EV-only dream come true. There will be a variety of EV charging stations, including home systems, workplace facilities, shopping-area systems, and roadway systems. The Society of Automotive Engineers (SAE) currently defines three levels of charging. These systems are ac-to-dc power supplies, but vary in capacity and speed. Table 1 summarizes the features. Note that the charging circuitry for levels 1 and 2 resides inside the vehicle.  The charger electronics for level 3 is inside the station itself.

A key design objective is charging station efficiency. Remember, chargers convert ac power line input to a dc suitable for charging. An essential requirement is the type of rectifier used. This is especially important in the design of level 3 chargers. A simplified diagram of a level 3 charger is shown in Figure 2.  The three-phase AC input is filtered then applied to the rectifier. A special form called the Vienna rectifier is used. It’s a switching type of rectifier using pulse-width modulation (PWM). When paired with MOSFET, GaN, or SiC FET switches or IGBTs, the Vienna rectifier can achieve an efficiency of 98%.

2. This concept for a level 3 EV charger leverages a three-phase AC input, an efficient Vienna PWM rectifier, and a dc output in the 300- to 800-V range for recharging.

TI offers a reference design (TIDM-1000) for a three-phase level 3 charger using a Vienna rectifier with a PWM switching frequency of 50 kHz. Power factor correction (PFC) is incorporated. With a 208 V three-phase input, the output is 600 V dc at a power level up to 1.2 kW.  A C2000 series MCU provides the control.

The Wireless-Charging Option

As the electric grid infrastructure evolves to meet the needs of an EV world, a wireless-charging capability becomes desirable. Rather than connecting your vehicle to the charger via a massive cable, you can park near a wireless charging area and “refuel.” A charger coil usually buried in the ground aligns with a coil in your vehicle and through transformer action (near field), energy is efficiently transferred.

While inductive transformer action is common now, capacitive energy-transfer methods are being developed. These chargers operate at higher frequencies. One plate of the capacitor is in the vehicle and another is in the charger. Phased arrays of capacitor plates have been shown to greatly boost energy transfer.

As more EVs emerge, wireless charging will become a major alternative to physically plugging in.  Wireless chargers will be built into parking lots and garages as well as roadways. Wireless chargers at intersections can provide a partial recharge while waiting for a green light. This technique can be extended by installing a series of coils along a road. Such an arrangement would provide a continuous transfer of power for charging as an EV drives over the coils (or plates).

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