Do I really need 2 isolated banks?

[hriehl1]

Hinterhoeller HR 28 with 15 HP 2-stroke outboard with a 6 or 10 amp alternator (not yet sure which).

Prior Owner had 2 separate battery banks with the typical 1-2-Both-Off switch. There is no Automatic Charging Relay, so I presume he ran the selector on Both when running the engine.

I am questioning the need for the two separate banks. With the outboard's pull-start as a backup, doesn't that mitigate much of the rationale for isolated house and starting systems?

I am planning for 2 6V deep-cycle batteries wired in series totalling around 200 +/- amp-hours. My daily consumption will be very modest... I estimate probably 25 to 40 Amp-Hours daily for lights, VHF, depth sounder, stereo and fresh water-macerator-bilge pumps. If I go with all LED lighting, consumption will be even less. There is no windlass, refrigeration, watermaker, chartplotter, radar or other amp-hog.

Does this single-source power strategy sound viable or would you recommend keeping the 2-bank setup? If so, why? If the systems are kept separate, is this the proper use case for an Automated Charging Relay?

Note: I am aware that the small alternator on the little-used engine will not keep up with consumption. I am planning to augment this with a 30 watt solar panel which I figure will replenish 10 to 15 amp-hours per day during the week when the boat is idle. Again, I'd appreciate your thoughts.

[ILikeRust]

Sounds to me like you don't need the two separate banks.

I've been reading Nigel Calder's "Boatowner's Mechanical and Electrical Manual," and he's got a whole chapter on batteries and charging systems. In that chapter, there is a whole section titled "Is it Better to Have One or Two Battery Banks for House Use?"

I am no expert, by my understanding is that the reason for separate banks is either to have multiple house banks or to have house batteries separated from a dedicated cranking battery for the engine. This is necessitated by the differences between deep-cycle marine batteries and cranking batteries, which require different charging regimens for maintaining battery life.

On smaller engines that don't require a lot of cranking power and that start right up without a lot of cranking (like your outboard, or my Atomic 4), you actually don't even need a cranking battery and can use a good deep-cycle marine battery. Although the deep-cycle battery is not designed for heavy cranking, we're not starting up diesel backhoes, here, so two seconds of cranking won't hurt them.

My boat came with three batteries - two set up as "house" batteries, and one set up as the engine battery - although all three were marine deep-cycle batteries. I actually have been considering changing it so that all three are just connected as one bank, since my engine starts right up and doesn't require a lot of cranking. Plus, as you say, my boat is very simple and I don't have a lot of electronics or appliances or anything, and I also am in the process of obtaining a solar panel for topping off.

[mitiempo]

I wouldn't bother with a start battery if your outboard is easy to pull-start.
The main reason for a separate start battery is that whatever happens to the house bank you will have a charged battery for engine starting.

In the case of an inboard engine that can't be started by hand I think it is a good idea to have a separate start battery.

Charging regimens for a start battery and a deep cycle house bank are the same as long as they are both of the same technology - flooded, gel, or agm.

A house bank of a reasonable size will have more cranking amps than a start battery - no problem at all starting a diesel up to 50 hp or even more. Engine starting will use about 1 AH or less.

[hriehl1]

Thanks... absent any additional replies to the contrary, I think I'll go with one 200 AH golf cart battery bank.

I too have read Calder's and Casey's books and the 12-Volt Bible (probably 10 times each), but I just get buried in all the advanced power management methods and devices they describe. I want a simple, safe but effective system for my modest needs. I have yet to find a book that does not go off into powering a 50 footer that has every electric gizmo and device imaginable.

[mitiempo]

Make sure you fuse the bank close to the positive post - ABYC calls for within 7".