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Author Topic:   Dual Battery Replacement for Montauk
RabAye posted 07-22-2011 07:48 PM ET (US)   Profile for RabAye   Send Email to RabAye  
I have two group 27 batteries in the center console with a Perko. Lately, I am not able to cut the engine while fishing without a quick voltage drop, with either of the batteries selected, or even with both. I realize it is likely time for battery replacements. My electrical use is GPS, fish finder, VHF, AM/FM radio hooked to iPod, bait tank pump. Should I be able at full charge on relatively new batteries to sit with the engine off for long periods of time, either with just one of the batteries selected or with both, or is this too much draw? Can I do just as well with group 24 batteries as I can with group 27?


'85 Montauk
'09 Suzuki 80

jimh posted 07-22-2011 10:20 PM ET (US)     Profile for jimh  Send Email to jimh     
The BCI Group number just tells the physical size of the battery, and it tells little about its capacity to store electricity. The capacity of a battery to store electricity is measured in Ampere-hours.

Except for the pump in the bait well, the loads you described are low current loads, and I estimate that your current load on the battery is probably no more than 5-Amperes. If you have an 60-Ampere-hour battery you ought to be able to run a 5-Ampere load for over ten hours.

Jefecinco posted 07-23-2011 10:43 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Two batteries with a selector switch provide redundancy which is mighty handy when a battery fails which we all know they will do sooner or later.


newt posted 07-24-2011 06:09 PM ET (US)     Profile for newt  Send Email to newt     
With two batteries, you have ample power to do what you describe, just make sure to use dual purpose batteries. Starting batteries don't like to be deeply discharged.
RabAye posted 07-24-2011 09:10 PM ET (US)     Profile for RabAye  Send Email to RabAye     
Thanks for the responses. It is obvious that both my batteries are at the end of their useful life. As it is now, my voltage is dropping with this relatively small load even when both batteries are selected on the switch. So, if the battery group number has little significance, what type of marine (dual purpose) batteries are people using? A group 24 size battery will be easier to maneuver into my console than the group 27, but what minimum amp hour rating should I be looking for? And if I have a 5 amp drain for five hours, how long will it take at cruising speed for the alternator in my four stroke to top the battery off for the next day? Is it common to alternated between the two batteries during daily runs? Thanks again.


jimh posted 07-24-2011 09:35 PM ET (US)     Profile for jimh  Send Email to jimh     
The primary purpose of a battery on a small boat is to crank over the outboard engine for starting. Consult the owner's manual of your outboard engine and find the recommended battery specifications. Usually the cranking battery will be specified by the minimum rating in Amperes for cranking the engine at normal marine temperatures. This rating is called the MCA or Marine Cranking Ampere rating. The engine starting battery must meet or exceed this minimum recommended rating.

Some outboard engine manufacturers, notably Mercury, are specifying specific battery types for engine starting, notably a recommendation for an AGM battery by Mercury for their VERADO engine, in order to have an engine cranking battery that can absorb very high charging currents. (This is due to a lack of charging output at low speeds; these engines can only charge the battery at higher engine speeds and must depend on the battery being able to absorb high charging current to restore electrical energy to the storage battery.) If your outboard engine owner's manual specifies a specific type of battery, it is wise to follow those recommendations.

If you cannot pull-start your outboard engine, you should have a second battery that also meets the outboard engine manufacturer's recommended minimum cranking Amperes and type.

jimh posted 07-24-2011 09:47 PM ET (US)     Profile for jimh  Send Email to jimh     
The time needed to restore a charge on a storage battery depends on many factors, including the type of battery, the amount of charging current available, and the condition of the battery. The best source of information about the characteristics of a battery is the manufacturer of the battery. Get a data sheet for your battery from the battery manufacturer. It should have information about the recommended charging procedures with regard to charging voltage, charging current, and the ability of the battery to accept a charge. To speculate about an unknown battery being charged by an unknown charger is pointless.

The charging voltage and current available from an outboard engine charging system typically vary with the engine speed. The greater the engine speed the more charging current will be available.

In a boat with more than one battery, distribution of charging current from the outboard engine can be handled either manually or automatically. If there is no automatic charging system for the second battery, the second battery must be periodically rotated into use as the primary battery so that it can be charged. This can be done with electrical switches. Automatic charging can be done with many techniques. Research the archives here for the terms "automatic charging relay," "voltage sensitive relay," and "automatic combiner relay." Also search on the acronyms ACR and VSR. You will find many prior discussions.

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