Energy storage is playing an increasingly important role throughout the energy infrastructure, from powering hybrid and electric vehicles to offsetting the inherent intermittency of renewable energy generation. Unlike batteries for electronic devices, which can be charged using a pre-determined protocol simply by plugging them into the wall, many of these applications are characterized by highly variable charge and demand profiles. The Energy Storage Group led by Craig Arnold is working to characterize how such variability in charging power affects battery behavior in order to improve overall system efficiency and lifespan. This past year, work has been focused on understanding the effects of variable charging on different battery chemistries and methods of controlling/improving the capacity.


Lithium iron phosphate batteries under variable charging conditions

The optimization of energy storage in variable charging conditions requires a thorough understanding of how semi-unpredictable charging affects measures of battery behavior and performance, including lifetime, efficiency, charge acceptance, and voltage behavior. In this work, we have been studying the effects of variable operating conditions on different battery chemistries with an eye toward understanding their applicability for grid level and other applications.

We find that of the standard chemistries (lead-acid, lithium cobalt oxide, lithium nickel managenese cobalt oxide, and lithium iron phosphate), the lithium iron phosphate is the most reliable in response to variable charging. However, through this work we have also identified an important feature of these batteries in that, similar to lead-acid cells, these systems exhibit a ‘memory’ effect, where the available capacity depends on the charging-discharging history of the cell. Over time and cycle, the storage capacity of the battery decreases as expected.

However, we find that when the cell is charged completely through a constant voltage step (e.g. charge the battery to the cut-off voltage at constant current and then hold at that voltage), it is possible to ‘reset’ the capacity and provide a temporary increase in the storage capacity (Figure 3). These results suggest that constant-voltage charging of lithium iron phosphate batteries could help maintain grid-level storage capacity at optimal levels, as well as extend the lifetime and reduce lifetime costs of batteries in electric/hybrid electric vehicles.

Figure 3. Battery discharge capacity versus cycle number for cells undergoing constant current charging/ discharging. At cycles 40 and 80, the cell is held at constant voltage after charging and we see a corresponding increase, or ‘resetting’ of the cell capacity to its original level.