Impact of Current Ripple on Li-ion Battery Ageing

The aim of this paper is to investigate the impact of the current ripple, originating from the dc-dc converter of e.g. a PHEV powertrain, on the ageing of Li-ion batteries. Most research concerning batteries focuses on very low (μHz) to low (Hz) frequencies and low current ripples to create very accurate battery models which can determine e.g. the State of Charge of the battery. On the other hand the design of dc-dc converters tries to reduce the current ripple by using multiple phases with interleaving technique and capacitors in parallel with the battery. The interaction between the current ripple of the dc-dc converter and the battery has received little attention so far. A test set-up is build with two identical 304 V , 12 kWh Li-ion batteries and two 100 A dc-dc converters. The dc-dc converter can be connected to an LCL-filter or solely to the primary inductor of this filter, such that the battery current contains a small or large current ripple respectively. The batteries are discharged and charged to simulate the circumstances in which a plug-in hybrid electric vehicle is used. After each month, during which the battery either experiences a small or large current ripple, characterization tests are performed to establisch the ageing of the batteries. Based on the test results, the current ripple does not appear to have a measurable impact on the battery resistance and the Discharge and Regen Power. There is an increase of the resistance and a decrease of the Discharge and Regen Power, but this is to be expected as the battery packs are submitted to 3 months of Combined Cycle Life Testing. The temperature of the battery turns out to be far more important for the resistance and attained power levels of the batteries. The absent effect of the current ripple on the ageing of the batteries may be due to the intrinsic double-layer capacitor. This capacitor at the surface of the electrodes carries part of the current ripple and reduces the current ripple as experienced by the actual charge transfer reaction which carries the dc-part of the current.