Thanks for all the postings. A couple of comments. First, it's consdered desirable in the sailing world to be able to use either battery to start the engine. Normally, one battery is designated as the starting battery, and is a thin plate type, as used in cars and trucks. The other battery is a deep cycle battery, used to power lights and instruments when the engine is off and you are sailing. However, just in case the starting battery doesn't work, it's desirable to be able to use the house battery to start the engine. I know from experience this IS useful ! Secondly, the cables running from the batteries to the earth (the engine casing), the starter, and the alternator, are about as thick as a man's finger. Moving them from their current connections would be very awkward. Thirdly, insurance firms insist that you have a battery switch. A sailing boat in water and using electricity is considered a dangerous thing ! (If you have ever seen a boat on fire, as I have, due to electrical fault, you'd probably agree !). Unfortunately, this morning, I don't have time to look closely at Brownout's latest solution, except to remark that the current draw for the starting battery must be quite high. I'm starting a 25HP diesel motor, and the recommended battery size is at least 600MCA. The highest battery draw from the house battery is probably 12amps at 12 volts, when a couple of lights are on and the frig compressor starts up. I'll be back later tonight, when I'll carefully review Brownout's latest solution. Thanks very much for the postings !