There can often be a large difference between something working and something engineered to be optimal. The cigar lighter socket was designed for just that. It doesn't have any locked connection, relying on a sprung positive contact and little more than friction for the return connection, limiting the current it can handle. It's location under dash further adds to the likelihood of connection breaking. Clearly not best suited for alternate tasks.
The supplier of my highly recommended
Maypole 4 amp charger and some others, provide a small loom to allow permanent connection direct to the battery. This minimises volt drop by reducing the electrical path and many unseen connection points in between. It terminates in a polarity sensitive connector that matches that of charger and includes an insulating cover.
