Consumer batteries, when bundled by an aggregator, can provide flexibility by being used for consumers self-consumption and by offering Frequency Containment Reserve (FCR). Combining these two can also generate additional revenues for consumers since the aggregator pays an allowance to the consumers for FCR, called FCR price. The aim of this paper is determine the optimal share of consumer batteries that should be reserved for self-consumption and FCR to minimize the consumer's cost, while considering various FCR prices and investment cost of batteries which is paid by either consumers or the aggregator. For this purpose, an optimization model is presented, and applied to a case study in the Netherlands. The results show the consumers annual cost is minimized by finding the optimal FCR share of batteries. Despite that, investing in the battery is still not profitable for the consumers, unless the aggregator invests in the batteries.