I have successfully made some spot-soldering attempts with the E-Caps.
2 x 2200uF / 35V in parallel charged with 30V. Thin-stranded wire to be welded to an old battery. I strongly recommend some tests to find the right balance: I needed several attempts varying capacity and voltage. The wire must be thin enough, otherwise, too much energy must be applied and a hole is burned in the battery case.
The devices shown above may be too powerful unless they are widely adjustable.
Important: the wires must not be connected to the HA. It would be destroyed by the high voltage applied when welding the 2nd connection.