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Author Topic:   Boat Electrical System
hullinthewater posted 07-30-2013 09:46 PM ET (US)   Profile for hullinthewater   Send Email to hullinthewater  
I'm starting from scratch; boat is empty and new console is on the way. I'm contemplating a two-Group-24-battery system. I will likely keep [the batteries] charged by either a small removable solar panel or a permanent install multi-stage AC charger. The boat will be slipped, so I can either do bulk charge from the [shore power operated charger] or passive from a portable solar panel. The Group-24 batteries I am considering will be absorbed glass mat type. In my last boat (a 27-footer) I had two Group 24 [gelled electrolyte batteries from] which I got 10-years service, and they were still holding a charge when I sold the boat. For my planned use, I think AGM batteries will be fine, and will cost less in the short term.

My electrical system loads will be:

--navigation lamps (which may be converted to LED bulbs)

--compass lamp

--VHF Marine Band radio,

--two bilge pumps,

--depth sounder,

--outboard engine with power tilt,

--AM-FM radio,

--two LED lamps on deck

--power outlet for recharging handheld radio and telephone

I think there is a weak link in all of this: my outboard is a 40hp two-stroke-cycle engine. Comments and suggestions appreciated. Thanks.

jimh posted 07-30-2013 11:05 PM ET (US)     Profile for jimh  Send Email to jimh     
[Moved to SMALL BOAT ELECTRICAL. Please use SMALL BOAT ELECTRICAL to discuss this topic.]
jimh posted 07-30-2013 11:13 PM ET (US)     Profile for jimh  Send Email to jimh     
Does the outboard engine provide battery charging current?
jimh posted 07-31-2013 08:18 AM ET (US)     Profile for jimh  Send Email to jimh     
When you describe a battery as being of a certain BCI group, for example, a Group 24 battery, you do not describe any electrical characteristics of the battery. The group designators describe the dimensions of the case and the arrangement of the terminals, but they do not describe the electrical characteristics of the battery.

The electrical characteristics of a battery that are interesting are typically:

--for a battery used for engine starting, the maximum current available, usually specified in marine cranking amperes (MCA),

--the capacity of the battery to store electrical energy, usually specified in Ampere-hours at a particular load current, or

--reserve capacity, a measure of time for a fully charged battery to reach a particular terminal voltage at a particular current flow.

When you describe the electrical loads in your small boat electrical system, you need to provide a measure of the current each load will consume. The total load is then the sum of all the individual load currents. With the total current load determined, that figure is compared to the battery Ampere-hour rating. This gives an estimate of time for which the battery can operate the load current before becoming discharged.

The time to recharge a battery is proportional to the charging current and the storage capacity, but it is not as simple as the Ampere-hour rating divided by the charging current. Typically the charging current will be tapering during the charging period, so the time to recharge to full charge will often be much greater than the the initial charging current rate might suggest.

The current available from an outboard engine to charge a battery is typically quite variable and depends on the engine speed, as well as the design of the charging system. Some outboard engines with modern permanent magnet alternators can provide very impressive charging current even when operating at engine speeds near idle, while other outboard engines must be running at engines speeds of several thousand RPM before they can provide significant charging current.

Photo-voltaic (PV) charging sources that are practical for installation on small boat will likely have very limited current output, which, of course, will only be available for limited time, and will be highly dependent on the weather and orientation of the PV source.

As it stands now, we do not know any of the electrical characteristics of the batteries, details of the load, or the charging output of the engine. Without more information it will be hard to assess the suitability of the proposed system.

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