We have investigated preservice science teachers' qualitative understanding of circuits consisting of multiple batteries in single and multiple loops using a pretest and post-test method and classroom observations. We found that most students were unable to explain the effects of adding batteries in single and multiple loops, as they tended to use reasoning based on current and resistance where reasoning based on voltage is a necessity. We also found that problems such as thinking of the battery as a source of constant current resurfaced in this new context, and that answers given were inconsistent with current conservation. We describe the curriculum we developed that enables students to model circuits with multiple batteries qualitatively. Post-test results show that the majority of students were able to apply their newly developed model to make accurate predictions for complex circuits.