I bought a 2007 246. Trying to figure a couple of things out.
There is a thick, red wire in the engine compartment that is not attached to anything. It passes through the bulkhead, on the port side, heading forward. The end of the wire can reach the batteries, but the eye on the crimped fitting is too small. It does not appear to attach to the onboard charger; there are two smaller red and a yellow attached to the charger on the charger's bottom left side and no open studs.
However, the onboard charger doesn't seem to work. Is there a specific setting the batteries or panel need to be on for it to work?
Also, the panel will not energize unless the onshore power is plugged in. Again, is there a specific setting I might be missing?
I have 2005 242 the predecessor of 246. I too have a 0 gauge red wire cable that runs pass the batteries but it is attached to a terminal block on the transom, that wire originates from under the captains chair where my battery selector switch is located, that's your 12VDC bus. In your case it may be a cable that wired a windlass, perhaps it was installed there by previews owner and subsequently removed/replaced. It's impossible to tell without inspecting your setup.
The on-board charger only works when you have shore power connected and on the AC panel and both the "AC Main" and the "Battery charger" breakers are on. The battery charger converts 110VAC in to 12VDC and will charge and maintain both batteries regardless of the battery selector switch position when the AC panel is energized and the battery charger breaker is on. Hope this helps.