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Author Topic:   Adding A Second Battery To A 2002 Dauntless 180
K Albus posted 12-06-2008 04:46 PM ET (US)   Profile for K Albus   Send Email to K Albus  
I have a 2002 Dauntless 180 with a 2002 Mercury Optimax 135. My boat currently has one battery which is used for starting and all other electrical circuits on the boat. The items on my boat other than the motor which consume electrical power include the following: combination GPS/fishfinder; VHF radio; AM/FM/cassette stereo system (cassette player rarely used); navigation lights; two electric downriggers; one livewell pump; one livewell/washdown pump; and one bilge pump. Since I bought the boat new in 2002, the electrical system has essentially been maintenance free. When the boat goes into winter storage, I take the battery out, store it in my basement, and charge it once a month. The battery has never needed a charge during the boating season.

I would like to add a second battery as a backup for starting purposes. However, I would also like to avoid having to switch between two batteries at various times, i.e., using one battery for starting, and the other battery for the house circuits. In other words, I would like for my electrical system to remain essentially maintenance free during the boating season. But I would like to have a second battery available for emergency starting, and I would like for the present electrical system to keep the second battery charged.

I have very limited knowledge and experience with regard to electrical systems. Before posting this, I reviewed several of the articles here in the Small Boat Electrical forum regarding ACRs and VSRs, and frankly, I remain a bit confused on the subject. I've also reviewed some of the information available at the West Marine and Blue Sea Systems websites, and they weren't much help to me either. It seems to me the Blue Sea Systems Add-A-Battery kit would do what I need. See However, the Blue Sea Systems website speaks of isolating the starting circuits from the house circuits. Is separating the two systems absolutely necessary? If not, is it desirable?

I ask for your advice here as to the best way to add a second battery to my system. Please keep in mind that my Optimax motor requires a 1,000 Marine Cranking Amp (750 Cold Cranking Amps) battery for starting. Although I cannot find anything in writing to confirm it, it is my understanding that the battery used for starting the motor should be a starting battery and not a deep cycle battery. I'm not certain whether the second battery must be the same as the first battery, so I'd like your advice in that regard as well.

glen e posted 12-06-2008 05:53 PM ET (US)     Profile for glen e  Send Email to glen e     
The engine [battery] must be a 1000 MCA "starting" [battery]. Everything that has to do with the engine goes to it. Everything else goes to the second [battery] which can be a deep-cycle type. Tie the two togeteher via a Blue Seas 7600 ACR. Wire with a OFF-1-BOTH-2 Perko-type switch. Keep the switch at OFF or !. Both [batteries] will get charged whenever the engine or charger is running. Call Blue Seas for specific information, they are more than willing to help.

That being said, if all you want is a battery to back up your starting [battery] so you never get stuck with a no-start sitatuion, the FAR easier way is to buy a battery jump pack from a marine store and just carry it with you.

jimh posted 12-06-2008 11:18 PM ET (US)     Profile for jimh  Send Email to jimh     
I have given some thought to this, but I have not settled on the optimum configuration. Glen's suggested configuration is one option.

In the Blue Seas line I tend to favor their

SI-Series Automatic Charging Relay

because you can configure it to always drop out the parallel battery connection during engine cranking. This keeps your electronics isolated from any transients during cranking.

If you don't already have a battery switch of the usual OFF-1-BOTH-2 configuration, Blues Seas also has a nice one.

Then there are the BEP Marine products. They have a some very nice battery management products. They're sold via a retailer that is local to you: Boater's World at SUMMIT PLACE MALL. htm?bct=t13046503%3Bcielectrical-boat%3Bcibattery-systems

I was really keen on BEP Switches, until I saw them in person. I was surprised by their size--they're smaller than I thought. But their three switch configuration gives you the most options in battery management and connection. It is a bit expensive.

We've had some long and interesting prior discussions about automatic combiner relays. See

which has a lot of details about the SI-Series ACR.

As the electronics on our boats become more sophisticated (and more crucial to the fun of operating the boat), I think it is a good idea to move to a configuration where a HOUSE battery is used to power them, and the HOUSE battery should be completely isolated from the engine cranking battery. This creates the problem of how to manage the charging of the house battery.

In terms of having two batteries of different types, this is not a problem as long as they have similar charging voltage tolerances. If one of them is a gelled electrolyte (or "gell cell") battery you can have problem. The gelled electrolyte batteries have different charging voltage requirements than a standard lead-acid flooded-cell battery.

Valve-regulated sealed lead-acid (V-R SLA) batteries are more sensitive to charging voltage than old-fashioned vented flooded-cell lead-acid batteries, but most modern motors have good voltage regulation and will charge them.

You can charge a V-R SLA absorbent glass mat (a V-R SLA AGM but often just called an AGM) battery in parallel with a flooded-cell lead-acid battery without much worry about any voltage problems, as long as the charging voltage is regulated to about 14.4-volts.

While having done a lot of research into this topic, I still haven't gotten around to changing my boat from its traditional set up with two identical flooded-cell lead-acid cranking batteries wired to a conventional OFF-1-BOTH-2 switch and managing the charging of them manually. It is the simplest and has worked well for me.

One thing I have done, and I recommend this highly, is to install a permanent on-board 120-VAC battery charger. This allows you to top off the battery charge before going to the launch ramp. I often pick up my boat from storage the night before I am going to use it, and I leave the batteries on charge for several hours. That way when I get to the launch ramp I know they're going to have plenty of cranking power.

K Albus posted 12-07-2008 11:35 AM ET (US)     Profile for K Albus  Send Email to K Albus     
If I add a deep cycle battery as the second battery and dedicate it to the house circuits, will the deep cycle battery be damaged if I need to use it for starting purposes on occasion?
K Albus posted 12-07-2008 11:41 AM ET (US)     Profile for K Albus  Send Email to K Albus     
Also, would the deep cycle battery need to be a 1000 MCA battery if it is only going to be used occasionally to assist with starting?
glen e posted 12-07-2008 01:45 PM ET (US)     Profile for glen e  Send Email to glen e     
[O]n occaision, no, but why skimp when on the water? Get a good strong deep cycle [battery]. Don't pinch pennies when it comes to the foundation of your electrical system. Delco Voyager is a good deep cycle battery.

Personally , I would buy two Sears AGM marine [batteries] and never worry about this again. When the boat is not being used, keep them charging 24/7 with a Promariner or three-stage equivalent charger.

jimh posted 12-07-2008 05:04 PM ET (US)     Profile for jimh  Send Email to jimh     
A battery is generally not damaged when connected to a load, unless the load causes an extremely deep discharge of the battery. Most batteries will last longer if their state of charge is maintained near full-charge. A deep-cycle battery is designed to be more tolerant of discharging to a low state of charge than the usual engine starting battery.

Starting the engine from a house battery which is a deep-cycle type battery should probably not be done too often, more in regard to the engine than the battery, since the deep-cycle may not be able to supply enough power for cranking.

The reason engine cranking batteries are used to crank engines is because they can produce high currents for short periods of time in a compact form. If you want to crank an engine with a deep-cycle battery, you will likely have to get a battery that is physically larger (than the cranking battery) in order to have the same current delivery capacity (as the cranking battery).

The concern in engine cranking with these modern engines is that the battery voltage be maintained at a certain minimum voltage in order to permit the electronics in the motor to operate properly. A motor that specifies it needs a cranking battery with 1,000-amperes capacity does not really need 1,000-amperes to be cranked over. Instead, the manufacturer has probably tested and found that a battery with less than 1,000-amperes of cranking capacity will not be able to maintain enough voltage to insure reliable operation of the electronics in the motor.

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