Decide on the configuration first—series, parallel, or a mix. Always double-check polarity before connecting. For series, connect positive to negative in sequence. . Connecting lithium-ion batteries in parallel or in series is not as straightforward as a simple series-parallel connection of circuits. To ensure the safety of both the batteries and the individual handling them, several important factors should be taken into consideration. First, let's see why safety matters. There are two primary connection configurations: Series Connection: In a series setup, cells are linked end-to-end, with the positive terminal of one. . A series connection involves connecting the positive terminal of one battery to the negative terminal of another battery.