(737c) Optimal Charge Rates for a Lithium Ion Cell

Optimal Charge Rates for a Lithium
Ion Cell

Saeed Khaleghi Rahimian,Sean Rayman, Ralph E. White

Department of
Chemical Engineering,
University of
South Carolina,
Columbia, SC
29208, USA

Abstract

The optimum charge rate for a lithium ion cell at each cycle
is determined to maximize the useful life of the cell that is defined as the
number of cycles before the end of discharge voltage (EODV) drops below 3.0 V
or the cell discharge capacity becomes less than 20% of the original discharge
capacity. First, we showed that by applying a dynamic optimization routine the
number of cycles can be increased by approximately 29.4 % with respect to the
case with one optimal charge current. The dynamic optimization results
indicated that the optimum charge rates are the minimum currents at which the
constraints for the useful life are satisfied. This is due to the minimum
charge rate resulting in minimum side reaction rate and capacity fade. Then, a
new method has been developed to find the optimal charge without using
optimization algorithms. The cell capacity and the EODV are both a monotonic
ascending function of the charge current; as a result the optimal charge
current can be obtained by some iterations of the charge ?discharge processes
for each cycle. The new approach improves the useful cell life by approximately
41.6 % with respect to using only one optimal charge current in a few minutes
while the previous optimization algorithm takes at least one day. Thus, the new
technique enables us to find the best charge profile in online application in
reasonable computation time by using any rigorous model for the cell
simulation.