I think there are a lot of variables that need to be established before your question can be answered properly. My '97 had 2 4D batteries from the factory, the selector switch selected either battery 1, battery 2 or both. No separate starting battery. I've since installed a custom system from Ample Power with a higher capacity alternator. The 2 4D batteries are paralleled, on the #2 switch. Separate starting battery with it's own, separate keyed switch. A solenoid connects the starting battery to the charge system only while the engine is running, otherwise it is completely isolated from the house bank.

Your system is likely different. It sounds surprising to me that the start battery would be on the same switch position as the house bank. Maybe something is amiss???

I have two AGM batteries connected in parallel as house supply and one engine starter battery. I set the selector switch to 1 for starter battery for engine, or 2 for house batteries to engine, or both for both to engine.

The battery combiner ensures that once one battery bank is full, the charge is diverted to the other bank.

Hope this helps.

The Mk II has two factory-installed 4D batteries located beneath the stbd table's aft seat. As the factory wired them, each battery goes to the selector switch, one as #1 and the other as #2. If your system matches this installation, running on BOTH is an almost sure way to eventually have two flat batteries after some amount of time on anchor.

The proper procedure is to select EITHER #1 or #2, always keeping the other battery in reserve.

Using this factory-standard installation means that your house bank (battery #1 or #2, you decide which one is which, since they are identical) is small for your anchoring needs, and your standby battery is very large for back-up engine starting needs.

Most Mk II owners have converted their electrical systems as follows: Connect both 4Ds together in parallel as one large house bank, typically called #2 (although the name you give it isn't really important). Install a separate small battery [I don't remember what size it is, but the size of a car battery] under the forward seat of the stbd table. Call this one #1, if you wish, and it will be your back-up engine start battery.

On our boat, we ALWAYS use Battery #2 (the very large "House Bank", the two paralleled 4Ds), and NEVER use Battery #1. Simple as can be. No switching to worry about, ever, unless you run the House Bank flat and then have to switch to the #1 to get the engine started.

The conversion to this layout isn't too difficult, although it does demand being done correctly to ensure the safety of the vessel. I believe we have an article in the Upgrades library on this conversion.

Which battery layout do you have: (a) the factory-installed two 4D batteries, one going to #1 and the other to #2, or (b) the two 4Ds paralleled as Battery # something, and a separate engine-start stand-by battery installed as Battery # otherthing???