Optimizing Your EV Battery Storage System Design

What is the force that drives people to want to climb mountains? Or hike unfamiliar trails? Or even travel to new places? Some say there is a part of the human spirit that is inherently exploratory. Perhaps there is some truth to this, or perhaps there is a more basic and broader answer. Maybe humans simply have an urge to periodically exhibit their freedom, and it takes more to do so for some of us. However, for many of us, driving fulfills this need. This is perhaps why appreciable declines in individual vehicle sales have not occurred in spite of the ecological effects and the availability of less impactful public transportation options. Instead, an effective and more amenable response has emerged over the past few decades: the increasing use of electric vehicles, or EVs.

EVs can be broken down into essentially two systems: 1) the drive system that includes steering; and 2) the EV battery storage system. Although there have been significant improvements in the design of automotive electronics for EVs, the greatest factor in gaining near parity with combustion vehicles has been the extension of range autonomy to approximately 400 miles. Consistently supplying the power to drive the vehicle is an additional requirement for EVs in addition to supplying energy for starting, lighting, and other electronic systems, which is the limit of necessity for fossil fuel vehicle batteries. Let’s take a look at how the EV battery storage system is designed and how it can be optimized.

EV Battery Storage System Design

EV subsystems (source)

In the figure above, a block diagram that illustrates the major functions of an EV or hybrid electric vehicle (HEV). As shown, all of the vehicle’s operations; including the motor, transmission, steering, braking, and climate control units (air conditioner and heater) rely on power supplied by the EV battery storage system. The storage system itself is composed of the following primary elements:

EV Battery Storage System Elements

• Battery packs or modules

Most EV battery modules are either Sodium Nickel-Chloride (Na-NiCl2), Nickel Metal Hydride (Ni-MH), Lithium Sulphur (Li-S) or Lithium-Ion (Li-Ion) with Li-Ion being the most implemented to date.

• Cell monitor units

These devices are tasked with monitoring the voltage, current, and temperature properties of the individual cells. Cell leveling (the evening of the charge across adjacent cells) is an important function that these devices perform, as it prevents inaccurate readings of charge level. For example, for uneven charge levels across cells, the highest cell charge may be read as representative of all cell charges.

• Battery management unit

The battery management unit performs all of the data monitoring, comparisons, calculations, and control generation for the storage system. This includes regulating charging input, output distribution, and battery pack protection against overvoltages and state of charge (SoC) violations.

• CAN bus communication network

Communication throughout an EV is primarily over the CAN bus. The advantages of accuracy in a vibratory environment, high data rates, and protocol simplicity makes the method of communication ideal for all types of automotive vehicles.

EVs currently do not quite match the range autonomy of combustion vehicles, especially the larger SUVs with fuel capacities of 30 gallons or more. Therefore, maximizing the driving range is a primary objective (followed closely by charging speed) for EV battery storage system design regardless of capacity. By targeting each of the elements listed above, an optimal design for storage can be achieved as discussed in the next section.

How to Optimize the Design of Your EV Battery Storage System

EV battery storage system design is at least as unique as the EV type design. This is due to the fact that EVs (and HEVs) are no longer novelties, but are developed to meet a certain class of vehicle performance and capability. Some vehicles are intended for low occupancy and minimal trip distance, while others are meant for long-range and comfort. Nevertheless, there are some guidelines that if applied can help you achieve the best EV storage system that aligns with your design criterion.

EV Battery Storage System Design Guidelines

Step #1:    Design for the desired capacity

Capacity or the amount of energy that can be stored is determined by a number of factors, including available space for the storage module(s) and the type of driving (e.g. commuting, traveling, etc.) that targeted clients are predicted to base their buying decision on. Therefore, capacity should be an early determination and a compass throughout the design process.

Step #2:    Maximize range autonomy

Regardless of the EV’s storage capacity, the design should seek to provide the driver with the most distance between charges possible.

Step #3    Minimize energy consumption rate

The energy consumption rate, typically denoted in kWh/km (or kWh/mi), is a cost factor, and it is desirable for it to be as low as possible. Parameters such as vehicle weight, operating temperature battery architecture, voltage level (i.e. 400V, 800V, etc.), current level (the higher the current the more energy used per cycle), and others can have a significant effect on its value.

Step #4:    Minimize operating temperature

Higher operating temperature means more cooling is required and that translates into less energy available for other functions; therefore, the lower the better in terms of efficiency.

Step #5:    Optimize CAN bus routing

Effective CAN bus routing is another factor that can aid your storage system design. This affects signal latency and security.

Step #6:    Minimize charging time

One of the biggest knocks against EVs is charging time. Therefore, your design process should seek to minimize this parameter as much as possible.

Step #7:    Charging type(s)

The type of charging that the EV uses is an important consideration as charging station infrastructure and access to the different types of charging is varied and significantly limited in some areas.

Step #8:    Perform battery management testing

Finally, it is a good idea to test your battery management during development. For the best results, Hardware-In-Loop (HIL) testing should be done.