I used an 8 gauge copper fine stranded wire, crimmped the lugs and soldered them also
As a general rule, don't solder wires which are subject to vibration. A well done mechanical crimp is perfect, the barrel of the lug is slightly flared to avoid chafing the strands. The lug is held tight, but the strands can flex a bit to avoid mechanical strain. Smear a bit of silicon on, then add shrink tubing and you're done.

When you solder the lug the solder creates a hard edge for the strands. The individual strands will begin to fatigue and fail at this point.

This has nothing to do with your problem, just a general tip.

Regards, Mark