Two Batteries, but which loads for each?

[JDavis]

I’ve been reading about installing a second battery in various boats, but there appears to be a difference of opinion on what loads should be connected to each battery. Obviously this depends on the type of vessel in question.

In a motorboat with an inboard engine for example, many advocate only the starter motor be connected to the starter battery, and all other loads be connected to the 'house' battery. Others stipulate that loads such as engine instrumentation, fuel pump, ECM, bilge blower, windscreen wipers, nav lights as examples be connected to the starter battery as these are loads mainly in use when the engine is running.

Any thoughts?

[john61ct]

The design does depend on many different variables, and you won't find a consensus.

More detail as to **your** setup is required, including usage patterns, how long away from shore power etc.

Personally I think Essential loads, including cranking, should be capable of being switched to either bank.

Even if one is just held as a Reserve bank, should be cranking off that regularly to confirm all is well.

Non-essential Auxiliary loads should be on separate circuit(s), ideally with LVC cutting out when SoC drops too low.

House is usually quite a bit bigger than Reserve (starter), but IMO both should be robust quality and designed for deep cycling, if not actual composed of identical model units.

Finally, a portable jumpstarter LI pack can act as belt and suspenders, but needs charge maintenance, regular testing and if used for gadgets, the get two and rotate them against Murphy's Law.

[ranger58sb]

Originally Posted by JDavis

I’ve been reading about installing a second battery in various boats, but there appears to be a difference of opinion on what loads should be connected to each battery. Obviously this depends on the type of vessel in question.

In a motorboat with an inboard engine for example, many advocate only the starter motor be connected to the starter battery, and all other loads be connected to the 'house' battery. Others stipulate that loads such as engine instrumentation, fuel pump, ECM, bilge blower, windscreen wipers, nav lights as examples be connected to the starter battery as these are loads mainly in use when the engine is running.

I suspect your second scenario is -- or has become -- quite common, especially given that sometimes gauges actually come bundled with the engines. Those aren't usually consider house (hotel, service, whatever) loads.

Electronics, on the other hand and often installed without regard to (old, mechanical) engines, are often considered house loads, along with DC fridges, interior lighting, etc.... and would usually be installed on separate "house" batteries.

There's a common hybrid in twin-screw powerboats: two dual-purpose banks, where each bank starts an engine and runs about half the house, engine gauges on one bank, electronics on the other. Works about as well as any, near as I can tell... especially when augmented by parallel switches on those two banks, and a separate genset with its own battery.

-Chris

[GordMay]

Greetings and welcome aboard the CF, JDavis.

[Stu Jackson]

Originally Posted by JDavis

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In a motorboat with an inboard engine for example, many advocate only the starter motor be connected to the starter battery, and all other loads be connected to the 'house' battery. Others stipulate that loads such as engine instrumentation, fuel pump, ECM, bilge blower, windscreen wipers, nav lights as examples be connected to the starter battery as these are loads mainly in use when the engine is running.

Any thoughts?

Without understanding how the banks get charged, isn't what the loads are connected to immaterial?