I scuff sand the top of the cell at the connection spot. Heat the solder on the iron just until it flows onto the cell then pull away. I then tin the connection wire and hold it on the cell. Apply heat to the wire and touch a little solder to the iron. When the solder flows on top of the cell, I pull away heat keeping the connection still while blowing on it. This works well for me. The solder conducts and transfers the heat well by closing the gap between the iron and connection and I don't have to hold the iron on the cell for any length of time.