Battery Rating while You Recharge

Lithium-ion batteries are currently used in most consumer electronics such as cell phones and laptops because of their high energy per unit mass relative to other electrical energy storage systems. They also have a high power-to-weight ratio, high energy efficiency, good high-temperature performance, and low self-discharge.

Most of today’s Electric Vehicles use lithium-ion batteries, though the chemistry often varies from that of consumer electronics batteries. Research and development are ongoing to reduce their relatively high cost, extend their useful life, and address safety concerns in regard to overheating.

Batteries have various issues, such as:

• Thermal runaway

• Phantom drain

Thermal runaway is a chain reaction within a battery cell that can be very difficult to stop once it has started. It occurs when the temperature inside a battery reaches the point that causes a chemical reaction to occur inside the battery.

Battery drain is the energy lost from your car’s battery when it is idle. Among electric vehicle owners, this is popularly known as the phantom or vampire drain.

How batteries perform over time, and their residual life, depends on numerous factors:

• Cyclic life (frequently, infrequently, city, highway, etc.)

• Depth of discharge

• Climate/Temperature effect

• Recharge voltage and rate

The performance and residual life of a battery is a complex issue and cannot be determined easily. The scatter in the remaining range of an EV based on mileage is immense (image from teslarati.com).

What we propose is to use QCM technology – and complexity in particular – in order to institute a battery rating. The idea is to measure the battery’s state of health.

This is how it works. Batteries are made of cells. A healthy battery is one in which the various cells are used in a “balanced” fashion, so that each cell contributes in a way similar to the other cells. A complexity map of an unbalanced battery composed of 30 cells is shown below. It is clear which cells are almost “dead” and are not contributing to the power output of the entire battery.

A more balanced situation is shown below. In this case the cells “collaborate” more than in the previous case.

This means that battery 2 gets a significantly higher rating as shown below:

The respective ratings are, 73% and 85%, which is equivalent to a three and four-star rating (on a scale from one to five).

In order to obtain a battery rating one may use a variety of measurements, such as cell temperature, voltage, or state of charge, and this may be done onboard a vehicle or during recharging. A few minutes of data are sufficient. With this approach, one can get a battery rating each time one visits a charging station. Evidently, the QCM-based battery rating system will need to be integrated with the software that manages the charging station itself, as illustrated below (see blue QCM module).

Clearly, this approach can be adopted to any kind of battery not just EV batteries. Large batteries in power stations can be rated accoding to this technique which can help program maintenance cycles as well as indicated which cells need more attention (see the small squares in the above complexity maps).