Owners of electric cars and electric bicycles have experienced how the battery seemingly can go on and on and then suddenly go from half full of energy to totally discharged. The Danish led EU- project Hi-C has developed a new way of using ultrasonic soundwaves to measure the energy level of rechargeable batteries.
Americans are embracing the electric cars and the Europeans are catching up. According to Bloomberg New Energy Finance, the sales of battery-powered cars in USA and EU rose 49 and 38 percent respectively to a gross sale of 73.327 cars sold in total in USA, EU plus Norway in the first quarter of 2017.
Sales of electrical bicycles are also doing quite well, as one in seven bicycles sold is battery-assisted.
Electrical cars and bicycles have gained popularity due to being both fast and environmentally friendly, but usability of the electrical two- and four-wheeled vehicles depends on the battery, and a major drawback of the rechargeable batteries used in electrical cars and bicycles is the difficulty to measure the power level in the battery. LiFePO4-batteries, also called LFP batteries, have a stable discharge and retain the power for a long time, but the problem with this and other batteries of the same type is that they at first discharge very smoothly but then the energy level decreases steeply and all the sudden the battery is discharged.
Researchers from the EU-funded and Danish led research-project Hi-C have recently developed a method to measure the level of energy left in a battery by means of ultrasonic soundwaves. It will help preventing sudden power loss in electrical cars and bicycles, or more dramatic episodes like SAMSUNG 7, Boing 787 and hover boards, where batteries self-ignited due to latent defects that went undetected before emerging during normal use.
Listening to the battery
“Researchers from CEA developed an acoustic method of looking at batteries so they can extract a lot of analysis information from the battery by looking at the sound spectrum. The same method is used to look at wings for windmills”, says Professor Poul Norby from DTU, leader of the now finished Hi-C project, that had focus on energy storage, where the stability, storage and transport of ions and electrons is important.
One of the results of the project was Commissariat à l’énergie atomique et aux énergie alternatives (CEA)’s step forward on the acoustic measurement methods.
“CEA worked on the acoustic emissions for a while without any progress and we were all ready to move on, but the CEA researchers asked for a few months more. And then they got a breakthrough, enabling them to test and conduct non-electrochemical analyzes of electrochemical systems using sound waves”, says Norby.
The acoustic test methods do not affect nor interfere with processes inside a functioning battery.
“The acoustic method can in principle also be integrated as part of the battery to measure state of health, i.e. measure how much energy is left in the battery”, explains Nicolas Guillet, research engineer at CEA.
When a battery charges and discharges, lithium ions travel between the negative and the positive electrodes: one electrode swells whereas the other deflates. This produces mechanical stress that emits acoustic sounds. Materials changes and internal temperatures variations due to the electric current flowing through the battery lead to changes in the properties of the sound transportation through the battery. The Hi-C researchers developed both active and passive measurement methods, sending ultrasonic sound waves through the battery, while listening to wavelengths and background noise from the reactions inside the battery at the same time.
The passive acoustic method can be used to detect early signs of reversible or irreversible degradation mechanisms that may lead to the battery failure or safety issues: Materials cracks due to over-charge, over-discharge, or over-heating; Gas evolution due to electrolyte degradation.
Detect anomalies before it’s too late
The method also gives the possibility to detect anomalous working and emit warnings, in order to go back in safety conditions of operation before adverse consequences. (Increases SAFETY)The active acoustic method can be used to detect materials and interfaces changes inside the battery during real operation: Change of the density of the materials during lithiation and delithiation or temperature change; Delamination of the materials or swelling of the battery during ageing.
These battery monitoring techniques allows estimating more accurately the amount of energy stored in the battery at any time during operation. It can also improve the management of the battery in order to optimize its use according to the needs and the conditions of operation. Consequences could be an increase of its cycle life, reduction of the charging time and an increase of the safety.
A detailed picture of the battery’s electrochemical parameters can be created by combining the results from the two methods, making it possible to read exactly how much energy is left in the battery. And to detect any defect at early stage, before serious faults occur. CEA now has two patent applications pending on methods to read state of health on batteries by means of acoustics.