...yeah with 25watts, you need some patience, not recommended to solder cells tho, you have to let both the lead and the battery terminal warm up so the solder can fuse to the terminal. It is advised to use flux, better stronger hold, and easier to solder on.
Sand the surface, rub a bit of flux on, hold the iron with some solder on to the terminal and wait until the solder starts to fuse with it. Do the same to the connector. Then hold the lead and terminal together and use the iron to heat both until the solder melts and fuses.
Also make sure the two batteries you add on are of the same size and capacity.
I usually use my 120/200watt gun takes about 3 seconds