Good article – myth busted!
The surface “grime” (acid, dirt, moisture, etc., on a battery case), being ever so slightly more conductive than a clean surface, could allow a tiny bit of current to flow between the + and the – terminals, thereby speeding up the self-discharge of the battery.
A storage battery on a trickle charger is a completed circuit – of course; the battery couldn’t be charged unless there is a compelted circuit. The charger takes energy from the ac wall outlet, converting ac to the suitable dc voltage and current, and “pushes” the charges into the battery. The dc side of the charger circuit (two leads, + and -) forms a complete loop with the battery.