Both Batteries Drain Even When Only Bank Is Selected

[mitiempo]

Some of the problems can be caused by grounds being attached at many different points. What I do is attach the common battery ground to a negative bus (Blue Seas maxibus) and from there to the engine block. That should be the only ground wire attached to the engine block. Everything else that is grounded is attached to the negative bus. That way there can be no electrical difference as they are effectively at two ends of the same heavy wire. My guess is that there are wires attached to various places for ground as so often happens after many years and owners additions.
As far as "death by a thousand cuts", even an older boat that has been maintained well has seldom been upgraded continually. You will when finished have a boat that is well outfitted and up to date as electrical scenarios have changed somewhat over the years. In the case of my current boat, a CS27 I purchased last October, the AC was new but the DC was original (32 yrs old) so is being replaced totally - it's not as bad as it sounds as there were only 6 interior lights. But the engine was in serious need of an upgrade electrically. When purchased the Yanmar started ok, but the cables were maybe 6 or 8 gauge, very stiff and original for starting. These were the largest cables on the boat! After building a new engine panel with gauges and moving it to the forward wall of the cockpit well, rather than shortening the harness I tore every wire off and replaced all with tinned marine wire of the proper gauge. Where the engine used to start in 10 or 15 seconds most times, after hooking everything up I turned the key to check my wiring and it started immediately. This was after not running at all over the winter as the fuel tank was replaced as well! It's not too tough a job and there is a huge difference in starting performance and charging. I find that instead of trying to sort out a mess left by somebody else and then having another problem crop up in the future you're often better off to replace all and there aren't that many wires on a diesel anyway.
Good luck.
Brian

[roline]

If an isolator is not used, the alternator could be connected to the common on the 1 2 battery switch. Power usage and potential alternator charge would go to the chosen battery, or both. A zap stop diode should be added to the alternator to prevent swirching to an open circuit condition that can blow the alternators diodes. Some older 1 2 battery switches may not be make before break and go open for a brief moment...