Thank uDepends on, a lot of solders have a flux core, meaning the flux is already within the solder tin..you want to check if that's the case.
Completely without flux, no.
Depending on the thickness of the cables, 30W might be a bit underpowered, too ... it's doable but not comfortably so.
Use flux. Flux and tin your wires, then flux and tin your bullets. Then insert your wires and apply heat and solder till all is melted together then remove your heat. Do not keep heat applied so long that it overheats your battery or electronics and possibly causes damage. If your wires are too hot to touch at the root, remove heat. Also, a hotter torch makes the job go faster and easier. Great investment here, been using one of these for years. You'll never touch one of those pencil irons again: http://www.ebay.com/itm/KKmoon-936A...189094?hash=item25cb0986a6:g:Z6wAAOSw44BYULyj