• Batteries and The Maze! Understanding Your Boats DC Electrical System.

    For many of us our first boat is not likely to be a brand new mega-yacht fresh off the showroom floor. More likely it will be a used boat that has experienced a few owners with each one adding their personal touch of electrical devices from trolling motors, depth sounders to extra batteries. One of the most intimidating aspects of boat repair for most people is tackling the marine electrical system. This is typically not a problem for most people with basic mechanical skills and a willingness to learn. But when you as the new owner uncover the batteries and the picture before you resembles the inner workings of the space shuttle this is where things begin to get complicated. The key to a trouble-free marine electrical system is an understanding of what, why and where. What the wire is connected to, why is it there and where does it lead (the 3 W?s). The wiring on some of the larger boats can be quite complex with both AC and DC circuits along with inverters and generators. There is enough information to fill several books should we try and cover all the relevant topics. However this article is only going to deal with getting power to the batteries then to the circuit breaker panel. To understand a basic system we have three things to consider, a power source, a way to store that power and a way to distribute that power. An alternator, a battery and a circuit breaker panel are the common components that accomplish these tasks.
    Attachment 65
    Alternators produce AC current which is then rectified into DC current. A voltage regulator controls the voltage output of the alternator. Most voltage regulators are internal while some are mounted on the outside of the alternator yet are still considered internal. Some of the external regulators are adjustable, this along with the fact that they can be wired to sense the voltage at the battery makes them preferable in many cases and lead to a more efficient charging setup. The more expensive higher output alternators regularly require an external regulator, and to get the full benefit of these alternator?s capabilities a multi-step voltage regulator may be used. A multi-step regulator allows your alternator to behave very much like a multi-stage battery charger, providing various charging stages as needed based upon the battery?s condition. To determine what type of setup you have consult the service manual and check the identification plates on the alternator.
    Attachment 944
    Figure 1 Multi-Step Regulator

    Attachment 67
    Figure 2 Voltage Regulator

    Caution: the alternator to battery connection must never be broken while the alternator is generating current (open circuit). If this happens the energy stored in the alternator?s inductive coils may cause a voltage spike which could cause damage to the alternator and/or other electronics. It is common on marine engines for the alternator output wire (usually red or orange) to be routed to the starter where it connects to the same post as the positive lead from the battery. During start current flows from the battery, through the battery cable to the starter. Once the engine is running, the starter disengages and the alternator begins producing current that returns down that path through the battery cable to charge the battery.

    Other Components

    Battery ON/OFF Switches

    These switches are relatively simple rotary on/off selector switches that can be used in a multitude of applications to switch the power supply on or off. If alternator charging output is routed through this switch, do not place the selector to OFF while the alternator is producing power.
    Attachment 68
    Figure 3 Battery On/Off Switch

    Battery Selector Switches

    These switches are normally used for dual battery installations. They allow you to choose either battery 1 or 2, or both in parallel for starting purposes. They also allow you to disconnect both batteries when placed in the OFF position. Some of these switches have a ?make before break? feature. If alternator charging output is routed through this switch, this feature allows the user to switch from battery 1 to the both position then to the battery 2 position or vice-versa without breaking contact. This prevents an open circuit if the user switches positions while the engine is running as long as the OFF position is not selected or passed through. Other versions of selector switches have a ?field disconnect? feature. Field disconnect switches have a few extra terminals that allow the field connection between the voltage regulator and alternator to be broken if the selector switch is placed to the OFF position thus saving the alternator.
    Attachment 70
    Figure 4 Battery Selector Switch

    Battery Isolator
    A battery Isolator is in essence a one way check valve for current flow. Isolators consist of diodes mounted on a heat sink with cooling fins. On boats with more than one battery, the isolator allows several batteries to be conveniently charged from a single alternator meanwhile preventing a discharged battery from draining the other battery. There are also versions of battery isolators designed to be used with boats having two alternators and a multiple battery setup. A common drawback to battery isolators is the voltage drop encountered as the current passes through the diode, this drop varies from 0.6 ? 1.0 volts. On boats with common alternators this voltage drop is robbing the batteries of valuable charging current. This leads to chronically undercharged batteries and shorter battery life. This can be corrected with alternators that have adjustable voltage regulators or external regulators that can be wired between the isolator and the battery. However using an external regulator cannot fully solve the problem since the regulator can only sense the voltage at one of the isolators output terminals. This makes it difficult to protect the second or third batteries from constant under/overcharging.
    Attachment 71
    Figure 5 Battery Isolator

    Automatic Charging Relays (ACR)
    Automatic charging relays may also be referred to as ?combiners? or ?series relays? however they may not have all of the features and functionality of an ACR. An ACR parallels (combines) batteries during charging and isolates them when charging has stopped. Similar to an isolator it allows one alternator to charge two batteries and disconnects when the charging has stopped. This isolates the batteries preventing a discharged one from draining the other battery. Some ACR?s are dual sensing, which means if either battery is receiving a charge it will parallel them so that both receive charging current, this is a useful feature on twin engine boats with larger battery banks. ACR?s are more versatile than isolators without the associated voltage drop.

    Attachment 72
    [B]Figure 6 Automatic Charging Relay

    Wiring Examples
    Before beginning any wiring modifications or additions it is imperative to have a copy of the manufacturers service manual and associated wiring diagrams. Even armed with the manufacturer?s wiring schematic use caution since previous owners may have changed things considerably.

    It is worthwhile to invest in an electronic label maker that is available at most office supply chain stores, be sure and get waterproof labels with a white background to make them easily readable. Using the manufacturers wiring diagram, identify all the wires, switches and components and label them. If you notice any differences from the manufacturer?s wiring diagram, remember the 3 W?s and explore until you find all the answers. Make notations of the changes on the original wiring schematic or if the differences are extensive consider drawing your own schematic.

    This setup is commonly found on smaller boats. It usually suffices for boats with small electrical demands. It is designed to allow the battery to be disconnected from the electrical system to avoid discharging while the boat is not in use. Drawbacks: The ON/OFF switch should never be placed in the OFF position while the engine is running or damage to the alternator may occur.

    This example shows a dual battery setup with the selector switch used to control starting/charging. This setup allows the batteries to be isolated from each other. While starting or charging it allows either battery to be selected or both in parallel. This is typical of a boat with a dedicated starting battery and an additional battery to run house loads or trolling motors. If the c/b panel does not have an on/off switch then one should be installed to prevent house loads from draining the battery while not in use. Drawbacks: The selector switch should never be moved to the OFF position while the engine is running or damage to the alternator may occur. With the selector switch in the both position, a discharged battery will cause the other battery to discharge as well.

    Here is another example of a dual battery installation. This setup uses an isolator to charge both batteries without user input and alleviates the worries of open circuiting the alternator. The isolator also prevents one battery from discharging the other. Drawbacks: The house battery cannot be paralleled with the start battery to assist in starting the engines. This isolator causes a voltage drop so the batteries do not receive the full charging benefit from the alternator.

    This is the same setup as above but with a Battery Selector switch. The same advantages regarding the isolator are realized in this setup as well. The battery selector adds advantage of being able to use either battery or both batteries in parallel to start the engine. Drawbacks: Once again the isolators? voltage drop restricts the batteries from receiving the full benefit of the alternators output. With the selector switch in the both position, a discharged battery will cause the other battery to discharge as well.

    This is a 3 battery setup, 2 start batteries and 1 house battery. This setup also uses an isolator and battery selector with the same benefits listed above. This setup has the added benefit that house loads are isolated from the starting system and can never drain the start batteries. The ON/OFF switch on the house loads allows the user to completely depower the c/b panel, this is useful when maintenance is required on the panel itself. Drawbacks: The voltage drop on the isolator for charging the batteries. With the selector switch in the both position, a discharged battery will cause the other battery to discharge as well.

    As you can tell there are a number of ways to wire the starting and charging system. To determine the setup that works best for you will depend on the equipment already installed, the electrical demand of your boat, how easy you want the management of the system to be and finally how much money you want to invest. Regardless of the setup you use always ensure your bilge pumps will have an uninterrupted power supply, either by wiring them directly to the batteries or installing a bus bar that is always powered and wiring the bilge pumps to that.

    Caution, always consult your engine or boat manufacturer prior to changing your existing wiring layout.

    This is the wiring diagram of my personal boat. It is a twin engine boat with 3 batteries (2 deep cycle house batteries/ 1 start battery) and uses an Automatic Charging Relay. Some engine manufacturers don't recommend this wiring setup, so check with them before applying this layout.

    On my system my house batteries are wired in parallel with an ON/OFF switch that allows me to isolate them from each other when not in use. The battery selector controls which batteries are used to start the engines. The alternators are essentially wired directly to the batteries, port alternator charges the start battery while starboard charges the house batteries thus eliminating any chance of open circuiting the alternator. With the use of the ACR either engine can charge all batteries. I also added positive and negative busbars. The positive busbar is always hot and is the feed for my bilge pumps. I also have an ON/OFF switch in the feed line to my DC electrical panel.

    Normal operation of my electrical system is extremely easy. When I arrive at the boat I turn the battery selector to position 1, turn my battery parallel switch ON and turn my house power selector switch ON. When I leave the boat I turn all switches OFF which leaves my batteries isolated from each other.

    Download a printable copy of this article Here!
    JAT, The Skipper and Ana like this.


    Comments 20 Comments
    1. nnyerges's Avatar
      nnyerges -
      I have some questions regarding your last chart of the article. See: http://www.boatinghowto.com/showthre...&p=968#post968

    1. The Skipper's Avatar
      The Skipper -
      First off, the diagrams are great and very helpful, but can anyone explain, preferably diagram how the bonding system should be tied in to the boats wiring system?

      thanks all,
    1. Roger's Avatar
      Roger -
      Your bonding system should all tie into a common bus. This bus should have all wires from through hulls, struts, shafts, etc., and the wire to the zincs. It should also have a wire going to the ?Common Ground?. This is a pic of my bonding system bus bar taken while I was wiring mine up last year(not complete at the time of the pic).

      Attachment 2273

      Ideally your boat should have a ?Common Ground? point. This is where all of the ground wires tie in together including the wire to your bonding system.
    1. arthor's Avatar
      arthor -
      Greetings, I'm new here. Just been speaking to someone at Bluesea about my add a battery kit. Apparently, the alternator output lead goes straight to the positive side of the crank battery. However, I have also read that it can go to the post on the starter that has the positive lead from master switch (and ultimately the battery, I guess). Both make sense but which is best? Any help would be much appreciated.

    1. Cap'n Ray's Avatar
      Cap'n Ray -
      All else being equal, the shortest, cleanest run is best. That being said, if you have anything other than a battery switch (ACR etc) then it's best to follow manufacturers instructions... Most are available online.
    1. Ana's Avatar
      Ana -
      Thanks for the article, Tim. When it comes to electricity, I need all the help I can get and more. Question for you;
      At the very end, you say: "When I leave the boat I turn all switches OFF, which leaves my batteries isolated from each other."
      With your switches OFF, do you still have power to run things (e.g. lights, fresh water pump, electronics, etc.)? If you wanted to, would you be able to run these things with all your battery switches in the OFF position?
    1. TimG's Avatar
      TimG -
      Ana the answer to your question is no. This is just a personal preference but the only DC item i used to leave powered when I left the boat was my bilge pumps. I have since managed to go through all of the wiring and install a galvanic isolator so I am more confident that the system is wired correctly and does not have any stray current etc. So now I leave the DC side of the boat powered as well.
    1. Ana's Avatar
      Ana -
      I see. Well, do you think there's anything wrong with my system? I have two multiple battery banks; starter and house. When, for instance, I am anchored out there for the night, I turn both my battery switches to OFF as soon as I stop the engine. But then, even with the battery switches to OFF, I can still run lights, electronics and whatever else I need. Shouldn't this be the case?
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Ana, I'd say no- that is not correct- batteries off should mean just that. Like Tim said, the bilge pump is typically powered before the switch, CO detectors, smoke detectors and auto fire extinguishers and radio memory sometimes are also wired before the switch but everything else should be off when the battery switches indicate 'off'. The main reason for this is to prevent discharged batteries, and reduce risk of stray current damage, it's also a good troubleshooting tool. Hopefully this is helpful!!
    1. TimG's Avatar
      TimG -
      Ana he is correct, with both battery switches off you should not have power to those items. If I were you I would begin with a pencil and paper and begin drawing out a basic electrical diagram of your boat. It would probably be easier to start at the batteries and do the starting/charging circuit first (batteries to engine, alternators to batteries, charger to batteries etc). Then draw from the batteries to the battery shutoffs. The only things that should be wired in between the batteries and the battery shutoffs would be the safety critical items he mentions.
    1. Ana's Avatar
      Ana -
      Thanks so much, both of you; you have confirmed what my gut feeling has been telling me for quite some time now. Everything works well, but when I look at the cables behind my switch panels, what I see resembles a spider web more than anything else, and I just have to do exactly what you just said. It will be a big project, but a necessary and also a rewarding one, as I have no doubts I'll find a lot of enjoyment in the midst of all the frustration along the way
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      I would add to 'rewarding' that it will give you peace of mind knowing your electrical system is safe and reduce the likelihood of surprise issues. The process will also help you get to know your WHOLE boat as you trace wires, remove splices and learn how to get into some of the little hidden spaces to find that one last little connection.
    1. RickD's Avatar
      RickD -
      Quote Originally Posted by Cap'n Ray View Post
      I would add to 'rewarding' that it will give you peace of mind knowing your electrical system is safe and reduce the likelihood of surprise issues. The process will also help you get to know your WHOLE boat as you trace wires, remove splices and learn how to get into some of the little hidden spaces to find that one last little connection.
      Splices are the ban of electrical systems.

      My background is communications work. We once had our security systems (the stuff I maintain) that was all copper (before fibers and such) with modems. Each wire had something like 42 connection points between one end and the other.

      Troubleshooting a communication system running digital data at 4800 baud, down copper wires, with multiple SPLICED connections was the biggest pain in the butt I've ever had with electrical and electronics. ACK.
    1. TimG's Avatar
      TimG -
      Ana don't give up we will get your uploading pictures error fixed it may take some time, p.s. I forgot to mention try another browser.
    1. rloo's Avatar
      rloo -
      TimG, nice article! I have been slowly leading to more or less the same setup as you have except here's what I'm trying to confirm so hopefully you can help. I have a main inboard (Volvo Penta 5.0gi) and a Yamaha T8 kicker, 1 Starter battery and 2 House batteries in a parallel bank. My second house battery (H2) is about 13 ft from the first (H1). I would like to avoid the expense of the larger (2/0+? AWG) cables connecting the 2 so wondering if I can add a switch to disconnect H2 from H1 in the event I need to use H1 as a backup starter? My electronics would be connected to H1 through the Blue sea switch/ACR. H2 would be used mainly to run pot puller and other accessories as needed. If this makes sense, do you think 6 or 4 AWG would be sufficient to connect the 2?

      Thanks in advance,

    1. TimG's Avatar
      TimG -
      okay you have me a bit confused, you say your house batteries are in a parallel band, but then say they are 14 ft apart and ask about cables, are they already parallel connected OR is that what you are looking at doing? If in case you are planning on installing cable to parallel them (house) and want to use House 1 as backup for starting but not house 2, then you can look at this chart Allowable Amperage in Conductors - Wire Sizing Chart - Blue Sea to get an idea of what amperage the cables can handle vs length, download the chart so you can see the notes and select the engine room column.

      What you are trying to do sounds feasible but without a diagram it is hard to say.

      Warning though, if you ever forget to un-parallel them and try to use H1 to start while in parallel with H2 you stand a good chance of burning up the cable and possibly fire.
    1. rloo's Avatar
      rloo -
      Tim, had to take a "sabbatical" from playing with the boat...back on trying to get this project complete. As far as the 2 house batteries, I'm looking to connect them in parallel. I'm now looking at probably having them no more than about 10-12 ft apart. I looked over the chart a few times and haven't quite figured out what "Engrm" is referring to so if you can point me a little more in the right direction, that would be helpful! As far as a diagram goes, it's close to what you have as your setup so edited a copy of yours (hope you don't mind). I currently have my kicker connected directly to the starter battery so not sure if I should route that through the switch like you have for your starboard engine? I'm sure I have mistakes so please let me know what should be corrected or if parts of it even makes sense.

      Appreciate the help!

    1. TimG's Avatar
      TimG -
      Where are you seeing Engrm? Looks like abbreviation for engine room, but I couldn't find it. Just looking over it everything looks fine, start batt selector can parallel "start" with 1 or 2 "house" batts. The cutoff switch between the batts i'm assuming is a 4 way (1,2, both, off). I see you are pulling some loads off of H2 only. Keep in mind that when paralleling if one battery has more volts than the other, the higher will charge the other, they want to equalize, so to keep them at same health, i'd parallel them when pulling that load, they will act as one. Otherwise I don't see anything wrong.
    1. rloo's Avatar
      rloo -
      Tim, yes, guessed that it was abbrv for engine room but not sure how to interpret the different temps and numbers in the table. With H2 being about 12 ft from H1, do you think 6 AWG would be enough to parallel them? I figured the cutoff switch between H1 and H2 would simply be a on/off, on all the time except for when I need to switch the Start batt selector to H1 (backup start), this is when I would cutoff H2. The idea would be to keep H1 and H2 paralleled all the time when pulling the loads off either one. I read somewhere that ideally the loads should be pulled off the negative off one battery and the positive off the other? Is there a "general" battery terminal fuse size that you would be able to "recommend" for each battery? Thanks!
    1. TimG's Avatar
      TimG -
      That 6 awg should be okay for normal use depending on the temp rating of the insulation looks like appx 80 amps, but if you use them to start your engine you may burn up a wire. As far as the loads pulling off I can't answer that, it will work either way and that may be better but I don't know. The terminal fuse size would depend on the load, if you ever plan on starting the engines off of them then you are looking at a larger fuse size sometimes up to 300amps. However if you go with that 6 awg, then plan on an 80 amp fuse or less to protect the wiring. One thing limits another!
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