I sure hope you didn't heat the bus bar while it was on the packs.
Soldering to copper like that you need a much higher wattage iron, and/or you need a massive heatsink on the iron
Something like these:
You can kind of do this yourself if you take some bare wire and wrap it around the iron to build up the heatsink. This will just take longer to build up heat, obviously. The other is to use a fat tip, like 3158 and higher shown above as well. You might be able to make one yourself, or if your tip is long enough, wrap some bare wire around it as well.
The trick is to provide plenty of heat to transfer. A standard iron just can't do it as it can't keep up.
The other is to go with a higher wattage, like 250w iron or better.
And ultimately, the *best* option, is to spot weld the wires to the busbar. Takes a little getting use to, but that's the best option. You won't be wasting a lot of time and [literal] energy trying to solder the wires.
