It sounds like your battery switch is only an "On/Off" type switch with one terminal (that's the correct name for the place where the electrical cables
attach. A "lug" is a type of terminal) for the battery and one terminal for the ship's loads (goes to circuit breakers, alternator, etc.). The terminal where the ship's loads are attached is the "common" terminal. Most larger boats have more than one battery and use a battery switch that is label with a version of "Off", "1", "Both", "2". Each battery is connected to either the "1" or the "2" terminal and the ship's loads (c/b panel, etc.) are attached to the "common" terminal. To be able to discuss marine
electricity you need to keep the names of things standard.
- - Any switch should not show a voltage on the "load" side when it is turned "off". If it does and there is nothing else in the boat that is generating/supplying DC power, then the switch is bad and should be replaced. The purpose of the battery switch is to first allow you to disconnect the battery from the ship's load in case of an electrical fire. It is also used to be able to disconnect the battery(s) from the ship's loads to keep the battery from being drained during boat storage
- - So you may have two separate but related problems. One the battery switch is bad - so replace it.
- - Second there may be a short or "leak/drain" on the battery that should not be there.
- - As I suggested in step 5 the next thing to do is to turn on the battery switch which will allow full voltage to be measured on both sides of the battery switch. Then turn off every circuit breaker and disconnect all "hot wired" loads (if any) to the battery by removing their fuse(s). Then test each circuit breaker for a bad circuit breaker.
- - Now you have eliminated all possible drains except two. One is a stray wire that is touching a positive terminal/lug/whatever somewhere and draining off power. The other possibility is that there is a leak in your alternator diodes.
- - To determine which is the problem, disconnect the alternator feed wire that runs from the alternator (+) terminal to the battery switch.
- - This is where you need an ammeter in the circuit between the "common" terminal of the battery switch and the cable/wire running to the ship's DC panel. With all the c/b's OFF there should be no current
shown. If there is there is a short somewhere - find it (see step 6).
- - Reverse the procedure now by removing the ship's loads(DC panel) wire/cable, from the ammeter and push it aside. Then hook the alternator feed cable to the ammeter. If there is a current
showing then the alternator is leaking. Get it tested and fixed.