Advancing battery technology and performance has become imperative for automotive applications. The function of automotive batteries has transformed from just an auxiliary power source to a system providing considerable contributions to the performance of the vehicle, especially for fully electric vehicles where it is the sole power source. This has been triggered by the need to reduce carbon emissions and the high cost of fuel. Therefore, the behavior of automotive batteries needs to be understood well.
Hybrid electric vehicles have also imposed a more significant change to battery operation. In this type of vehicle, batteries have been used in conjunction with internal combustion engines such that the two provide traction power. There are a number of possible configurations but the principle of operation remains the same. These hybrid electric vehicles can be driven by either the batteries or the internal combustion engines, or both combined.
In order to maximize the effectiveness of the hybrid electric vehicle drive train, maximum energy should be recaptured and stored in the course of all regenerative braking periods. However, charge acceptance of the batteries at high-rate partial state-of-charge is the principle limiting factor for energy capture. The batteries are needed to provide more of electrical power to the vehicle, therefore they are required to recharge rapidly and it is important that their performance under these conditions is known.
A number of methodologies, such as Dynamic Charge Acceptance, have been adopted to characterize the performance of these batteries. Therefore, understanding the dynamic charge acceptance performance of automotive batteries is a critical requirement for developing electric vehicles. Researchers at The University of Sheffield’s Centre for Research into Electrical Energy Storage & Appications, Mr Matthew Smith, Dr Dan Gladwin, and Professor David Stone, investigated how varying the parameters and conditions of the standard dynamic charge acceptance test arrangement would provide an excellent analysis of the dynamic charge acceptance performance. The purpose of their studies is better understanding of the behavior of the cell under real world conditions. Their research work is published in Journal of Energy Storage.
The research team tested both standard and carbon-enhanced lead-acid cells, together with lithium iron phosphate cells over a range of state-of-charge, temperatures, and rest periods. They observed a clear correlation between dynamic charge acceptance and both temperature and state-of-charge. Dynamic Charge Acceptance was observed to improve at high temperatures and lower state-of-charge. The cells could have exhibited a memory effect resulting in improved Dynamic Charge Acceptance after a discharging period. For the case of rest period, reducing the rest period improved charge acceptance.
According to the results obtained in their study, when selecting a battery based on Dynamic Charge Acceptance, it is necessary to take into account the range of state-of-charge over which the battery is operated. Going for a narrow state-of-charge window would lead to a suboptimal performance under particular conditions.
The magnitude of the recuperation current is another parameter to consider. Under these operational conditions, carbon-enhanced lead outperformed lithium cells. The findings of the study indicates that dynamic charge acceptance is a non-static parameter and is dependent on history of operation, environmental conditions, and the electrochemical balance within the cell at a particular time.
“This work has provided deeper insights into the fundamental factors influencing DCA performance, and forms the basis for ongoing research into methods by which it may be improved”. Said Matthew Smith, first author of the paper.
M J Smith, D T Gladwin, and D A Stone. Experimental analysis of Dynamic Charge Acceptance test conditions for lead-acid and lithium iron phosphate cells. Journal of Energy Storage, volume 12 (2017), pages 55–65.
Go To Journal of Energy Storage