Don't give up on your battery yet!
Tom; Don't give up on your battery just yet. Marine Electrical systems are our specialty at Spindrift Marine. It may suprise you how circuits get cross connected into your starting battery circuit that shouldn't be there, and that you don't realize are there. The first thing you should do is fully charge your battery before you leave your boat for a week. Then disconect the fully charged starting battery at the battery terminals, not the battery switch. This way you will be absoluely sure that there is no load connected to it. (VERY IMPORTANT: Connect your bilge pump circuit to your working house battery system until you get this problem solved so that your boat will not sink if a leak developes while you are away!) When you return, check the voltage on your still disconnected starting battery with your volt meter. If it is dead or low again, then there is a problem in the battery and it must be replaced. (There may still be other problems as well. The dead starting battery could be a secondary problem caused by this battery being permanently damaged by being left dead too long.) If you have to replace the battery, and the new battery no longer goes dead when you leave the boat, then I would guess the problem is solved. If your old battery is still fully charged after sitting disconnected for a week, or if you replace it with a new battery, and still have the same problem when you return to the boat after leaving it with the starting battery connected, then you must start checking your wires. Many boats have badly designed circuits, even new from the factory! First, make sure that your starting battery is, indeed, being charged while your motor is running or your onboard battery charger is on. If the charging is fine, and you are actually leaving the boat with the starting battery fully charged, then you will need to follow every wire connected to your starting battery, and every wire branching out from the wires connected directly to the starting battery, to see what they are connected too. If you bought this boat used, it will be most efficient to start from the battery and work outward. If it was brand new when you bought it, first check any 12v equipment you may have added to see if it accidently got connected into the starting circuit, then start checking from the battery outward. Don't assume that a circuit is properly designed just because the factory put it in your boat! The starting battery should be only for starting the motor and powering motor related equipment while the motor is running (gauges, electric fuel pump, etc). It should have no other equipment connected to it other then, perhaps, a battery combiner or battery isolator diode. I would recomend, assuming you have only one alternator on your engine, that the alternator be connected directly into the starting battery circuit to be sure that your starting battery is always properly charged. Then use a battery combiner (not a battery isolator) between the starting battery and the house battery. This way your starting battery gets a full charge as soon as the motor starts, and power is taken away to charge your house battery only after the starting battery is charged. Where the problems come in is when someone installs some piece of equipment and needs a convenient 12v source to hook it to. They check around and find a 12v connection on the back of an engine gauge, or where the battery cable connects to the starter motor, or some other similar place that is actually part of the starting battery circuit. They hook the new equipment to this. Now this equipment drains the starting battery. If it is a large enough drain, or if it is something that turns itself on automatically, then you run into problems. Sometimes, wires can also get cross connected between your starting circuit and your house circuits, so that even when the battery switch is turned to off, all your batteries can be drained. I would guess that you do not have this problem, as you stated that your house battery is fine when your starting battery is dead, but it is a situation that can happen very easily and should be watched for. After you have a fully charged and properly functioning battery, and have traced all of your circuits and switched any non engine related circuits to your house system, your problem should be solved. If the battery still goes dead, my only other sugggestion would be to call in a reputable professional that understands electrical systems.
