Package of 4 D-Cell Battery Holders for series connections. Series connections allow you to increase voltage by adding additional cells. If you use a single battery holder, you will get roughly 1.5 volts. By interlocking multiple battery holders top to bottom, you will increase your voltage by roughly 1.5 volts for every battery you add to the configuration. For example, if you use all 4 battery holders, you will have roughly 6 volts. These battery holders are wonderful for physical science experimentation in the classroom such as simple circuits demonstrations. These battery holders have color coded terminals (black and red) for simple wiring connections. Sci-Supply is a USA (Ohio) based seller. Please support small American businesses.