Background: In the joint project "Mobile Care Backup'' funded by the German Federal Ministry of Education and Research, the smartphone-based app "MoCaB'' was developed in close cooperation with informal caregivers. It provides individualized, algorithm-based information and can accompany and support caring relatives in everyday life. After a multi-step development, informal caregivers tested the MoCaB app in a home setting at the end of the research project. Objectives: The goal was to find out how the test persons evaluate MoCaB and in which form the app can provide support to informal caregivers. Methods: Eighteen test persons caring for relatives participated in a four-week test of MoCaB. Guideline-based qualitative interviews to record usage behavior and experiences with the app were conducted after two and four weeks of testing, transcribed and analyzed using qualitative content analysis. Results: The test persons described the care-related information as helpful. The individualized, algorithm-based mode of information delivery and the exercises provided for family caregivers were generally rated as helpful, but their use depends on the individual usage style. Three dimensions can describe the effects of MoCaB: 1) expansion of care-relevant knowledge, 2) stimulation of self-reflection, and 3) behavior towards the care recipients. Discussion: With few exceptions, the testing caregivers felt that the MoCaB app was enriching. The support dimensions have an effect at different points in everyday life and vary in intensity, depending on the duration of the existing care activity and the individual preferences of the users. Conclusion: The way in which caregivers used the app was not always consistent with the expected behaviors. This demonstrates the relevance of open-ended, qualitative research methods in the evaluation of health apps.