Empty batteries are easy to recognize However, it is much more complicated to know the charge status between full and empty. A completely new approach with ultrasound pulses offers a precise and simple method.
Batteries are the snag in many “mobile” technologies. Smartphones, drones, or electric cars – in many cases the time between battery charges is much too short for many people. This is why it is important to determine the exact state of charge. But this is more complicated than you would imagine.
Currently, battery management systems (BMS) carry out the necessary measurements. They calculate the state of charge for each cell based on the parameters current and voltage. But since the calculations are partly based on standard values, they reflect the current state only approximately. In particular, this is very inaccurate in case of frequent partial charges. The battery management systems also consume some of the energy that was actually to be used for the next song or mile.
Battery management with ultrasound
In the future this will be more reliable, more energy saving and cheaper with sensor systems that are being developed in the SoCUS project at the Fraunhofer ISC. They measure the density of the negative anode with the help of ultrasound pulses. This changes as the state of charge of the cell changes.
The method has several advantages:
- there is a direct linear connection between the state of charge and the measurement signal. This makes the evaluation simpler and more precise than with the technologies currently in use.
- The new battery sensors can be easily integrated into existing systems.
- One evaluation unit can monitor several battery cells simultaneously and measures the state of charge only during charging and discharging. The fact that this system does not check the charge continuously saves energy and, consequently, costs.
- Since the ultrasound signal correlates directly with the mechanical properties of the cell, all aging processes are taken into account better. This allows more accurate statements to be made about the current remaining capacity and, hence, the performance.
Battery management for all types
The new measuring method is suitable for almost all types of battery. However, to date only lithium ion batteries have been tested. In particular, electric vehicles should benefit from reliable recording of the battery charge status. After all, the distance covered between charges is the key factor for further development. But reliable monitoring of the state of charge is also important for drones that monitor industrial plants and wind parks or that manage agricultural land.
The ultrasound method could be especially profitable for stationary storage systems with a large number of connected battery cells. A sensor that works only when required and records the state of charge of several cells simultaneously can save energy and also costs. In this application, flame retardant battery types are often used where the state of charge cannot be determined accurately with current methods.
In the future, the new method could extend existing measurement methods of battery management systems, especially also in electric mobility with a reliable, energy-saving, inexpensive variant.
Scientific publication in the Journal of Power Sources, Volume 343, March 1, 2017: Probing lithium-ion batteries’ state-of-charge using ultrasonic Transmission