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  1. #1
    Petty Officer 2nd Class
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    Nov 2003
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    Default Battery Isolator, Charging, 2-Battery System

    Hello all,I have read the numerous threads about 2-battery systems and isolators, but have not found anyone addressing my specific questions.My boat is a 15' tri-hull with a 70hp 1978 Mercury O/B. I use the boat to run around local lakes and bays, and I would like to be able to go just offshore (1-2 miles out). It is likely that the motor will never be at WOT for more than an hour at a time.I currently have a single "Cranking" battery. All other electricals (lights, VHF, sonar, etc.) are attached to this battery.I want to add a second, "Deep Cycle" battery to operate a 12v trolling motor.The guys at Mercury tell me that the O/B charging system puts out "40 amps no load, 100 amps cranking."Question 1: is this enough juice to charge both batteries through an isolator, especially considering my usage pattern?Question 2: what capacity isolator would I need? Is 42 amp enough, or will this burn out during cranking?I know I should put down the bucks for a 2-bank onboard charger; but I already have a portable 1-bank 2 amp/10 amp charger I ought to be able to use.Question 3: can I charge the batteries through the isolator using the single-bank portable charger? If so, do I have to disconnect anything else? Can I connect the negative posts of the two batteries together to ease hookup of the charger?Question 4: should I connect the electricals (lights, VHF, etc.) to the cranking battery or the deep cycle?Thanks.

  2. #2
    Admiral
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    Apr 2002
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    Miami Fl
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    7,499

    Default Re: Battery Isolator, Charging, 2-Battery System

    In my opinion the alternator on any outboard is no substitute for a regular charger when batteries are low. The best thing is to keep the batteries fully charged, use distilled water and check the charge with a hydrometer. The alternator on you motor will keep them that way.You are on the right track by planning on a deep cycle & charging battery.I would not connect batteries together when charging. You will always wind up with the charge being equal to the poorest of both. I run two starting batteries and have twin engines. I charge one on the way out and the other on the way back.

  3. #3
    Lieutenant
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    Oct 2003
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    Savannah, GA
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    Default Re: Battery Isolator, Charging, 2-Battery System

    Solittle,I have a two battery set-up on my boat with a battery selection/shut-off switch. I usually use both batteries to crank up because the more juice, the more flywheel spin and the quicker it starts. Sometimes I forget to select one battery or the other, and end up running around with both batteries turned on. My question is this: Will the charging system on the outboard keeo both batteries at a good charge (if they started out that way)?Thanks Knightgang

  4. #4
    Petty Officer 2nd Class SingleShot's Avatar
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    Default Re: Battery Isolator, Charging, 2-Battery System

    Most alternators unless there is multiple outputs don't due to well on charging multiple batteries through Isolators. The reason is in the voltage regulator. Most that I have seen anyway have a sense input that measures the battery level. As the battery is charging it alows the regulator to adjust the charging current. If you have the ability to move this sense lead between the batteries as you select one or the other then you would not have a problem. Where you may have problems is when charging both together if you left the switch on both. If one battery is weak or going bad and the other is good, you would may damage the good battery and/or your voltage regulator.Singleshot

  5. #5
    Petty Officer 2nd Class
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    Default Re: Battery Isolator, Charging, 2-Battery System

    So the answer appears to be...Forget the isolator, and forget charging both batteries simultaneously with a single-bank charger; either lay down the cash for an onboard two-bank charger, OR trickle charge each battery separately with my existing single-bank portable charger.Is this the consensus?Any recommendations for a good brand of 2-bank onboard charger?I lean toward Xantrex, as I have heard good things about this brand. I lean away from Guest/Marinco, as all Marinco products I have ever bought have been complete, utter, total, outright, undivided, thorough, and unqualified CRAPOLA. I see ******** has a sale on "Charles" brand chargers...anyone with experience there?My boat usually sits in the driveway 1-2 weeks between outings...would a solar charger (or two) be applicable?Finally, should I put all the "other" electricals on the deep cycle, and keep the cranking battery ONLY for starting?

  6. #6
    Petty Officer 2nd Class SingleShot's Avatar
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    Default Re: Battery Isolator, Charging, 2-Battery System

    Here's my setup,I have the cranking battery alone by itself being charged by the outboard. Completely separate. I've never had to mess with it 'that is charge it'I have 2 deep cycle batteries for all my electronics with a battery switch to select either one. Both batteries are charged with an on board dual battery charger. I'm at work but I believe it's a Marine Pro or somthing Pro. I've had absolutly no problems with it and it charges each battery independent of one another. I just plug my boat in and it charges then maintains once charged until it's unpluged.This setup works for me and I'm sure some will comment on theirs.SingleShot

  7. #7
    Captain
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    Default Re: Battery Isolator, Charging, 2-Battery System

    Absolutely split the batteries. When you add the deep cycle, ALL the electronics goes on it and only it. Think of each battery as an independent system. Ground (common) is common (ground) everywhere. BTW, add a true deep cycle, not a dual purposed battery, which is what you should have been using in your single battery config.A battery isolator is a diode...a one-way only path for electrons on their way out of the battery, never to return again. It’s commonly used for applications involving solar or other onboard charging systems, including generator/alternator charging, where battery bank amp monitoring is available. In your application you would only ‘hope’ everything is charging the way you want it to.Xantrex products are common and these folks know a lot about battery charging. There are always a lot of them on Ebay. You can check out the marine product line on their web site www.xantrex.com. Recently, their PCB manufacturing has shifted from NAFTA (not Canada) to Asia. Local Xantrex repair folk say there is a noticeable difference in quality. While the product quality is still exceptionally good, it used to be indestructible. I have a Xantrex inverter/charger. It is a tad beyond the scope of your application. I have never heard anything negative (ptp) about the Charles chargers or inverters.Someday when you have a little extra time, go back and add an extra On/Off battery switch to combine the + terminals of the two systems. Not that you want to or should ever have to but your deep cycle can then be used as a starting battery should it ever become necessary.Alternators don’t want to, but are made to...charge that is. You should presume they will never get your starting (or any other) battery fully charged...unless you are running your motor non-stop for 30+ hrs. I recommend any name brand of 2 or 3-stage/2-bank charger. I have a Guest 2-stage/2-bank charger on a little 18-ft’r in the drive. It keeps a couple group 27’s alive. BTW: Perko does not compete with Guest’s line of HD battery switches. (You won’t need the HD’s )

  8. #8
    Admiral
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    Default Re: Battery Isolator, Charging, 2-Battery System

    Great thread with lots of info - I bought one of these and it does what it is supposed to so far: http://www.batterymart.com/battery.mv?p=VDC-12112

  9. #9
    Cadet
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    Default Re: Battery Isolator, Charging, 2-Battery System

    I've been running outboards since I was child and have followed the advances in charging systems and isolators just about as long. My current inland fishing boat is a 17 foot Tracker Targa with 75hp 4 stroke merc. I often spend two to four days camping and fishing away from a power source to charge my trolling motor batteries. I was carrying three deep cells to be sure I had enough battery for the entire time. I finally got around to installing a battery isolator and found as usual that with an outboard it can be very difficult because often there is no external alternator wires. So I did what I do best found the answer to quick easy isolation. It's called a voltage sensitive relay and all you do is connect your starting battery to your other batteries through this device and a common ground. No need to locate or reroute the alternator or charging wire. It works like magic when your motor is running and the alternator is producing power (usually over 13.5 volts) the unit senses the voltage of your starter battery and once it exceeds 13.5v from the alternator the relay closes and connects your extra battery and charges it also. When you turn of the motor the voltage will drop below 13 volts and the relay opens disconnecting the starter battery from you extra battery so that the starter battery stays full to start the engine. Basically it does the same as a battery switch but you don't have to remember to turn it. It's all automatic and you do not need typical battery cable wire to connect it instead you only use wire substantial enough to handle the charging amps usually a 10 or 12 gauge wire is enough. Just check the amps of your alternator and get wire to match. These units also will allow you to shore charge all your batteries simply by connecting the charger to the starter battery. It took me less than 10 minutes to install this and I now keep my trolling motor or accessory batter charged simply by when I run the main motor. Of course I still trickle charge the batteries when I am near a power source but this will get me through several days without shore charging. Total cost about $80. I have recently installed two of these in my 28' seafox to keep the house batteries charged too. I have a link to a site that will set you up with a unit and instructions if you want to go this route. They are small and compact less than 3 inches square. Email me and I'll get you the information. REMOVED EMAIL ADDRESS
    Last edited by Don S; April 29th, 2010 at 07:50 PM. Reason: Removed email address, not allowed on forums.

  10. #10
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: Battery Isolator, Charging, 2-Battery System

    Automatic Charge Relay, Voltage sensitive relays, battery combiners, all the same thing, just a different name, and already out and have been used for many years now, and sold by several manufactureres.
    Don S.


    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.

  11. #11
    Supreme Mariner bruceb58's Avatar
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    Default Re: Battery Isolator, Charging, 2-Battery System

    OLD OLD THREAD

    Can't believe Don didn't catch it either!
    1998 Wellcraft Eclipse 24 Cuddy
    Volvo Penta Duo-Prop 7.4L "LK"

    2006 Sun Tracker Party Barge 21
    Mercury 90 4-Stroke FI
    "Common sense is not very common"
    "Failing to prepare is preparing to fail." -- John Wooden

  12. #12
    Petty Officer 1st Class
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    Default Re: Battery Isolator, Charging, 2-Battery System

    The guys at Mercury tell me that the O/B charging system puts out "40 amps no load, 100 amps cranking."Q

    This makes no sense at all.

  13. #13
    Supreme Mariner Silvertip's Avatar
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    Default Re: Battery Isolator, Charging, 2-Battery System

    The alternator does not put out 100 amps cranking. In fact it puts out very little while cranking. Depending on what engine you have, it may put out 40 amps but only at wide open throttle. And all the BS in the above thread about charging systems is simply wrong. You can have 15 outputs off the alternator and you will not get 15 outputs with 40 amps each. It outputs 40 amps period via one wire that feeds the start battery via the positive battery cable. Simply connect the batteries in parallel via switch, ACR, VSR, combiner or isolator and go boating. There is one voltage regulator and it doesn't know or care if you have one battery, two batteries, or 10 batteries. The only issue with charging multiple batteries is the amount of time it will take. The voltage regulator monitors "system" voltage so if you have one weak battery, it will drag down the the other one until the "system" is equal. At that point the voltage regulator charges both accordingly. The only time you have an issue charging multiple batteries from a single charger or alternator is if they have different charge profiles such as a flooded battery on the same system as one of the high-tech batteries. Diesel powered vehicles have used multiple batteries in parallel since Hector was a pup. Those systems have one generator (abeit a big one) and one regulator. The last semi I looked at had six batteries in parallel. My diesel cars had two. The fact is, battery manufactures gang-charge huge racks of batteries all in parallel so lets forget this nonsense about parallel charging being a bad idea.

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