Thought I'd share my method of making Paralleling Adapters. These are made with 5.5mm contacts from, where else? OffshoreElectrics.com.

I use Copper Tubing to provide a solid joining of the three contacts.

I drill an appropriate sized hole all the way through the middle contact, and just through one side of each of the outside, then press them together.

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Then I solder them together, filling in the wire barrels as well, and cover with heat-shrink. Pretty basic. Seem to work very well.

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