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  • The Official Dual Battery Thread

    I see this topic come up many times so I think it would be best to create a "Dual Battery Bible" for newcomers and others interested in adding a second(or more) auxillary battery to their boat. I will post my experiences and we can get other member's input as well. Hopefully we will all learn somthing.............
    05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

  • #2
    Since I have posted a few times on the topic I have had quite a few requests and PMs regarding dual battery setups and isolation. As a result I have composed the following guide to help you wire up a dual battery system using a heavy duty isolation relay. I have been using dual battery relays for over a decade in all kind of vehicles and I have found the relay to be the most effective and trouble free method yet (and yes I have tried just about every other method!). This is a very straight forward, easy installation that just about anybody should be able to accomplish in a few hours. Please use this at your own risk and please understand the following information has been obtained solely from my own personal experiences. If you have any questions or need help please ask. Also if I left anything out please let me know. With that said here we go…….

    The easiest way to add an additional battery to your boat is to simply hard wire it in parallel. To connect the batteries in parallel you simply wire the negative post of battery 1 to the negative post of battery 2 and the positive terminal of battery 1 to the positive terminal of battery 2. In this configuration the two batteries act as one large 12volt battery and you get double the capacity. There are a few problems with this method. First, when not in use the batteries will continually charge and discharge into each other eventually ruining both batteries over time (I am speaking from experience here!). Second, since the batteries are permanently connected you run the risk of running down both batteries while the engine is not running. Yes, you get double capacity but after a day of running the stereo at the beach you still risk having two dead batteries and not enough juice to get back to the dock. The solution: a dual battery relay.

    The two batteries are still wired in parallel but a relay is inserted in the positive cable to isolate the batteries when the ignition is OFF or in the ACC position. When wired properly the relay provides complete automatic control of the battery bank by simply using the ignition key. With the key in the OFF position or in the ACC position the relay is not energized and the positive circuit between the batteries is open. There is nothing to remember, switch, and no fear of accidentally disconnecting the main battery with the engine running potentially harming the charging system. In theory it works the same as an on/off Perko switch but instead of manually turning the switch the ignition key acts as the switch. Once installed you can forget about your batteries and enjoy the water! I like this because I do not have to remind or provide specific instruction to my wife, father, friend, etc….. when they drive my boat.

    Below is a schematic that details the connections required to install an isolation relay. I have used a Borg Warner R3098 for many years in both my boat and jeep. The BW R3098 is identical to Stinger’s SR200 and can be found at most local auto parts stores. It was used in late 80s early 90s GM C/K3500 trucks for battery isolation and was mfg specifically for this purpose. It has a continuous duty rating of 200 amps and a peak of 650 amps and usually costs around $30. In its natural state (not energized) the relay is open which provides isolation. If your alternator has the potential to output more than 200AMPS you will need to find a different solution. Currently the largest alternator MC has offered from the factory has been the 05 and later models at 90amps.

    Start by hard wiring the negative terminals between the two batteries using 2 Gauge cable (negative to negative). If possible try to keep the batteries as close together to save on battery cable expense. I have found most auto part stores have pre-made battery cables on the shelf in various lengths that work very well if the batteries are fairly close to each other. On marine Optimas (as well as any other dual post marine battery) I prefer to uset the 3/8 stud on top or on standard Optimas you can use the side posts. This will keep the aux negative connection out of the way of the MC factory top post connectors (engine) and make it easy to unhook the system during storage.

    The positive connections are also wired positive to positive but a Borg Warner R3098 is used to open the connection to isolate the batteries. The relay has for posts on the top, two 5/16 suds (large) and two #10 studs (small). The 5/16 studs are connected to the positive terminals of the two batteries. It does not matter which way they are connected. Most BW relays do not come with 5/16 nuts so you will have to find a couple. I prefer Nyloc stainless steel 5/16 – 18 NC nuts to prevent corrosion. My local auto parts store stocks these as well and are in blister packs out on the floor. Here is a pic of the relay:



    The small #10 studs are used to energize (close) the circuit. One small stud is wired to a ground and the other is wired to the ignition circuit of the boat. Again it does not matter which stud is used for the ground and which is used for the ignition circuit. For the ground I usually just use the closest battery and use a large ring terminal to hook it up. The other small terminal will be connected to the ignition source.

    The easiest way to find the ignition circuit is to gain access to the back of the key. In all the MC boats I have done the ignition circuit has been a purple colored wire. Keep in mind there are many other purple wires in the harness so it is best to use some type of circuit tester or Fluke. If your batteries are in the back of the boat and you are confident in finding the ignition circuit you can open the harness near the engine to find the ignition circuit to save the aggravation of running a wire all the way from the dash. By far the easiest way is to locate the ignition terminal on the back of the key and run a wire from the dash to the remaining small terminal on the relay. Once wired up you can test the relay with a Fluke or you can just listen for an audible “click” from the relay when the key is turned to the ON position. If using a Fluke to test it should only show continuity (closed circuit) when in the ON position. In ACC and OFF the circuit should show no continuity and should be open. If using the your ear to test, the relay should click once when the ignition is turned to the ON position and click again when brought back to the off position. It should not click when the key is turned to the ACC position.

    The only thing left to do is to relocate all the accessory components to the auxiliary battery. This usually can be accomplished by moving only the amplifier battery cables to the aux battery. In most systems I have not had to rewire the head units to the aux battery. If you are using external amplifiers the amp draw from the head unit is so small that it would take a long time to draw down the main battery. If you do run into a situation where your head unit is draining the main battery I would start by relocating the constant lead of the head unit to the aux battery and leave the ignition wire and ground hooked up to the factory wiring. The ignition circuit on the head unit should only pull a few mA which should not drain the main battery.

    Diagram 1:

    Last edited by Diesel; 06-24-2005, 05:09 PM.
    05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

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    • #3
      The above method is how I have my boat wired and has served me trouble free for many years. It works well and is completely automatic but if you require more control over your batteries you can add a DPDT switch in your dash to manually control the relay. I used a DPDT switch in conjunction with the relay in my Jeep so I could manually override the ignition circuit in certain circumstances. I have never had the need for more control in my boat but I outlined the details below for those who are interested.

      All the wiring is the exact same as described above except at a DPDT switch is added to the ignition circuit. A schematic has been drawn below detailing the circuit. The relay is connected to terminal A of the switch and a jumper wire is then connected from terminal A to terminal F. The ignition circuit from the key is then attached to terminal B. A new wire is sourced from a constant positive source (back of the key also contains a constant positive) and connected to terminal E on the switch.

      Now you have three options depending upon where the DPDT switch is located. With the switch in the middle position all circuits are open the batteries are always isolated regardless of key position. With the switch down the system is identical to the automatic mode in the original installation. The key automatically controls the isolation. With the switch in the up position the batteries are connected 100% of the time regardless of key position. The addition of the switch does provide an extra level of control since you can select how you want the batteries to be isolated manually. Plus it still gives you the option to put the system in an automatic mode. I have found when installing the switch in boats they stay in the automatic position 99% of the time anyway so I usually do not bother with one.

      Last edited by Diesel; 06-24-2005, 05:29 PM.
      05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

      Comment


      • #4
        Great read, Diesel!

        I've thought about adding a second battery to my boat, but the solutions out there in the world always seemed excessively expensive (certain high-end "battery isolators"), excessively complex (various home-brew cross-wiring systems, with/without Perko switches), or unreliable.

        This one wins the cheap, effective, robust, and elegant awards.

        Thanks,

        /frank
        1998 Maristar 200VRS

        Comment


        • #5
          I also put in a high amp relay for my two battery system upon advice from Diesel. Working great so far, and the peace of mind of being able to start the engine after hours of just listening to the stereo is priceless. Thanks again Diesel, good info.
          "Filling my potential would really cut into my sitting around time!" - Bamford

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          • #6
            Nice thread Diesel. Very detailed. My only concern is what effect will this cause on your alternator. I'm not an EE, but what do you think is the minimum amperage alternator you could safely use with this? Obviously, by putting the batteries in parallel you are charging both simultaneously and you have doubled the demand on the alternator.
            Brian

            Current: 2000 ProStar 205

            Former: 1987 ProStar 190

            sigpic

            Comment


            • #7
              Originally posted by BriEOD
              Nice thread Diesel. Very detailed. My only concern is what effect will this cause on your alternator. I'm not an EE, but what do you think is the minimum amperage alternator you could safely use with this? Obviously, by putting the batteries in parallel you are charging both simultaneously and you have doubled the demand on the alternator.
              Theoretically it should not matter nor harm the alternator. The alternator only sees one large battery when the relay is energized. As a result it will have to run longer to bring the system up to a full charge. Any amp alternator will work but a higher amp alternator will charge the system faster, thus work less.
              05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

              Comment


              • #8
                Originally posted by BriEOD
                Nice thread Diesel. Very detailed. My only concern is what effect will this cause on your alternator. I'm not an EE, but what do you think is the minimum amperage alternator you could safely use with this? Obviously, by putting the batteries in parallel you are charging both simultaneously and you have doubled the demand on the alternator.
                If I am correct, the relay solves this problem.. as the relay charges both the batteries evenly.. and in theory its just one large battery.

                I did not know the Borg piece was available.. and ended up buying a 60 amp (rated) Isolator from Napa.. it works great. If I was to do it all over again, I would probably just use the Borg piece.. Tho finding the Acc wire on older boats that aren't MC's might be a pain..

                Comment


                • #9
                  Originally posted by Diesel
                  Theoretically it should not matter nor harm the alternator. The alternator only sees one large battery when the relay is energized. As a result it will have to run longer to bring the system up to a full charge. Any amp alternator will work but a higher amp alternator will charge the system faster, thus work less.
                  Is there any difference between a marine rated alt.. (or does such a thing exist..) and your basic high amp Ohio Generator or.. stinger alt? Since these are standard GM blocks you should be able to bolt up just about any alternator your heart desires, assuming you can use a car one. But I'd imagine there would need to be some sort of spark arrestor or SOMETHING..

                  Comment


                  • #10
                    Originally posted by zberger
                    Is there any difference between a marine rated alt.. (or does such a thing exist..) and your basic high amp Ohio Generator or.. stinger alt? Since these are standard GM blocks you should be able to bolt up just about any alternator your heart desires, assuming you can use a car one. But I'd imagine there would need to be some sort of spark arrestor or SOMETHING..
                    Yes, a marine rated unit is mandatory. The automotive units will bolt right up and function the same but they do not have the proper spark arresting screens and or brush less armature. A non-marine alternator will pose an explosion hazard.
                    05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

                    Comment


                    • #11
                      Dual battery systems do not have to isolated to be effective. If the system you use works, then stick with it. I, too, debated about the best way, and with 30 years experiance in the automotive repair field I know that, for example diesel trucks with a dual battery set up use no relays to seperate the batteries. What they do, that is different, and the way my batteries are connected is very simple. The positive cables to the boat are connected to the positive post of the main battery. The negative cables to the boat are connected to the negative post of the second battery. Then, the batteries are connected together in parallel. This does one very important thing. The batteries are discharged and charged evenly. The biggest enemy in a Marine battery's life, are number of cycles (discharging and recharging) and heat. When you isolate one battery and discharge it completely, it has not only been thru one deep cycle, but also creates more internal heat in the recharge process.
                      Diesel, I am in no way saying your system is wrong. I decided after my own experiance and research, and a pow wow with my Interstate Battery rep.(who knows batteries inside and out), that this system works the best for me. Even after a day at the beach running 2 amps, I have never had an issue. You are right about one thing, if you poop both batteries, you are screwed, so I just keep a booster pack charged and on board just in case. However, in 3 seasons with my 2002 maristar 210 have never had to use it. The other important thing for a marine battery is to keep it fully charged in the off season. When a battery becomes discharged, the acid absorbs into the plates causing sulfation, and leaves the water in a state that will freeze.

                      Comment


                      • #12
                        Originally posted by gregg
                        Dual battery systems do not have to isolated to be effective. If the system you use works, then stick with it. I, too, debated about the best way, and with 30 years experiance in the automotive repair field I know that, for example diesel trucks with a dual battery set up use no relays to seperate the batteries. What they do, that is different, and the way my batteries are connected is very simple. The positive cables to the boat are connected to the positive post of the main battery. The negative cables to the boat are connected to the negative post of the second battery. Then, the batteries are connected together in parallel. This does one very important thing. The batteries are discharged and charged evenly. The biggest enemy in a Marine battery's life, are number of cycles (discharging and recharging) and heat. When you isolate one battery and discharge it completely, it has not only been thru one deep cycle, but also creates more internal heat in the recharge process.
                        Diesel, I am in no way saying your system is wrong. I decided after my own experiance and research, and a pow wow with my Interstate Battery rep.(who knows batteries inside and out), that this system works the best for me. Even after a day at the beach running 2 amps, I have never had an issue. You are right about one thing, if you poop both batteries, you are screwed, so I just keep a booster pack charged and on board just in case. However, in 3 seasons with my 2002 maristar 210 have never had to use it. The other important thing for a marine battery is to keep it fully charged in the off season. When a battery becomes discharged, the acid absorbs into the plates causing sulfation, and leaves the water in a state that will freeze.
                        I've seen this used this way in car audio applications but never in a boat app..

                        The first boat stereo I did was for a very good friend of mine's grandfathers boat (his gramps has become older and isn't using it much but has a lake house) anyways, we added an isolator and a 2nd marine battery (biggest one walmart had..) I would have like to use all optima's but they just weren't in the budget.. someday maybe! Since this boat was going to be used by others and not just me and my buddy or his dad, this option was just not a viable one.. we wanted the extra off key (motor off) playing time.. and doing just 1 battery, or 2 in parallel was just not something we could do. Even in my own boat I think I would isolate them still.

                        I need to bring a volt meter out with me this weekend to see what kind of voltage we are getting when the boat is at full throttle.. but it works great, stereo is loud! I am hoping for around 12 volts in each battery before we start the boat.. some somewhere in the low 13's at throttle. My buddies dad did comment the boat has been running considerably better now that we have the dual batteries in.. could have been a grounding issue before, not sure.

                        We are adding a 12" Image Dynamics IDQ subwoofer undernear the rear seat in an enclosure we made out of 1/2 inch MDF and I am getting it line-x'd this weekend.

                        Comment


                        • #13
                          Originally posted by gregg
                          Dual battery systems do not have to isolated to be effective. If the system you use works, then stick with it. I, too, debated about the best way, and with 30 years experiance in the automotive repair field I know that, for example diesel trucks with a dual battery set up use no relays to seperate the batteries. What they do, that is different, and the way my batteries are connected is very simple. The positive cables to the boat are connected to the positive post of the main battery. The negative cables to the boat are connected to the negative post of the second battery. Then, the batteries are connected together in parallel. This does one very important thing. The batteries are discharged and charged evenly. The biggest enemy in a Marine battery's life, are number of cycles (discharging and recharging) and heat. When you isolate one battery and discharge it completely, it has not only been thru one deep cycle, but also creates more internal heat in the recharge process.
                          Diesel, I am in no way saying your system is wrong. I decided after my own experiance and research, and a pow wow with my Interstate Battery rep.(who knows batteries inside and out), that this system works the best for me. Even after a day at the beach running 2 amps, I have never had an issue. You are right about one thing, if you poop both batteries, you are screwed, so I just keep a booster pack charged and on board just in case. However, in 3 seasons with my 2002 maristar 210 have never had to use it. The other important thing for a marine battery is to keep it fully charged in the off season. When a battery becomes discharged, the acid absorbs into the plates causing sulfation, and leaves the water in a state that will freeze.
                          Good comments.............

                          The only issue I have with not isolating a parallel set of batteries is the continous voltage flow between the batteries. This is a non-issue in most vehicles (including diesels) because they are driven frequently. As a result the alternator has an opportunity to keep both batteries at full voltage to minimize discharge between the two batteries. Since a boat is not driven every day two batteries stored in parallel will eventually destroy each other.

                          In addition it is easy to see why you have never drained the two batteries since your stereo is only pulling two amps . At 90% my current system will pull 165 amps . I can drain two brand new optima blue tops in 3 hours very easily.

                          As a result, isolation not only give me the security of knowing I can make it back to the dock but it also gives me the security of knowing the two batteries are not going to kill themselves.
                          05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

                          Comment


                          • #14
                            Originally posted by Diesel
                            Good comments.............

                            The only issue I have with not isolating a parallel set of batteries is the continous voltage flow between the batteries. This is a non-issue in most vehicles (including diesels) because they are driven frequently. As a result the alternator has an opportunity to keep both batteries at full voltage to minimize discharge between the two batteries. Since a boat is not driven every day two batteries stored in parallel will eventually destroy each other.

                            In addition it is easy to see why you have never drained the two batteries since your stereo is only pulling two amps . At 90% my current system will pull 165 amps . I can drain two brand new optima blue tops in 3 hours very easily.

                            As a result, isolation not only give me the security of knowing I can make it back to the dock but it also gives me the security of knowing the two batteries are not going to kill themselves.

                            I think he may have meant 2 amplifiers. Maybe not, just my thinking. At any rate, thanks for the write-up Diesel! I am wanting to add a battery, this will come in very handy.
                            If my words don't make sense, try reading them backwards.

                            Comment


                            • #15
                              Originally posted by bcampbe7
                              I think he may have meant 2 amplifiers. Maybe not, just my thinking. At any rate, thanks for the write-up Diesel! I am wanting to add a battery, this will come in very handy.
                              I'm laughing at myself right now I knew there was no way he had a stereo that only pulled 2 amps . Your explanation makes so much more sense.........thanks
                              05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

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