Introduction Li-ion batteries, after dominating the sector of portable electronics, are currently entering into a new market of the short and medium time stationary energy storage applications. Energy storage systems (ESSs) are considered to play a key role in the integration of the renewables in the grids of the high wind energy penetration and to participate in the future grids ancillary services. Nevertheless, they are currently missing market mechanisms and incentives for ESSs, which could attract potential investors and be economic viable. This paper will present results and one year field experience gained from operation of the 1.6MW /0.4MWh Li-ion battery energy storage (BESS) demonstrator used for providing primary frequency regulation on the Danish market. Moreover, the results from the periodic check-ups of the Li-ion BESS performance degradation and ageing will be presented. System description The ESS, located in Western Denmark, is composed of two Li-ion battery containers connected in parallel, through the power converter system, to the MV bus of the transformer, as illustrated in Fig. 1. The first ESS (ESS1) is based on lithium iron phosphate (LiFePO4/C) battery cells (3.3V/2.3Ah) and it can deliver 0.4 MW/ 0.1MWh. The second ESS (ESS2) is based on lithium titanate oxide (LMO2/Li4Ti5O12) battery cells (2.3V/60Ah) and it can deliver 1.2 MW/ 0.3MWh. Lifetime Periodical reference performance tests of the both Li-ion BESSs were performed for a period of one year in order to determine and analyze the performance degradation, which is caused by delivering the PFR service. The reference performance tests consists of capacity test, hybrid pulse power characterization (HPPC test) and electrochemical impedance spectroscopy measurements performed on some battery modules removed from the BESS for a duration of the reference performance tests. Economic analyses High prices of the primary frequency regulation service, low BESS utilization factor and low depth of discharge cycles (Fig. 2) are making this service potentially economic viable on the Danish market. Thus, economic calculations will be provided including revenues, predicted lifetime of the BESSs and auxiliary power consumption (cooling systems of the BESS, cooling of the converter, etc).