If using one battery you can use a "jumper" which will make a connection between the two wires of the open second battery connector by plugging the jumper into the unused battery connector. Basically shorting the + & - of the other battery connector, the two connectors are run in series allowing you to connect say two 2s for a final of 4s collectively, or two 3s for a final of 6s collectively. This works well with RCz that use two batteries giving the RC better balance weight wise, and also better "C" discharge capability, it wont increase or double your "mAh" that will remain the same but your voltage and "C" rating will double. You need to use two batteries that are equal and the same... preferably two LiPos that are the same mAh, same "C" , same model, same brand, and same Cell Count, and bought at the same time and used the same amount over time (i number lik/same batteries and keep track of their use rotating them each time to keep them relatively the same, although I try to keep dual packs used with ESCs with dual connectors as dual packs and not ever use them individually, this will keep them from deviating in spec as much as possible).