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Old 08-19-2007
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Charging mixed batteries with an outboard

I just recently purchased a 27 foot sailboat and have a question about the best way to set up my batteries. The boat has an 8hp outboard motor with a battery connected to it. Inside the boat are two deep cycle batteries that have no way of being charged. I eventually plan on getting a solar panel, but, ideally, I'd like to somehow use the outboard to both keep its starting battery charged and charge the two deep cycle batteries. I'm guessing that I'm going to need some kind of charge controller and would like to purchase one that will do this and allow for adding a solar panel into the system. I'd really like this setup to not involve disconnecting or reconnecting anything once it's in place. Any advice on this matter would be greatly appreciated. Thanks.
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Old 08-19-2007
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Presumably your outboard can be manually started?

If so then there doesn't seem much point in maintaining a separate battery bank for engine starting (you don't need the redundancy there) - you could simply lose the outboard battery and connect your outboard up to the main house bank and charge them without a charge controller.

Even though your house batteries are deep cycle, they should have plenty of cranking amps available for starting a small outboard.

I also have a 27ft boat with a 9.8hp outboard. I have a single 75Ah deep cycle battery that serves the house and can start the engine (though normally I hand start it). Like you my intent was to charge it from the outboard but I've discovered that the stator coil has burnt out and I haven't been brave enough to change it yet. Instead I went for a semi flexible 20W solar panel with a small charge controller and this keeps the battery nicely full. (Power needs are minimal - no fridge - just instruments, vhf, navigation lights, cabin lights and water pump.)

Last edited by BritAbroad; 08-19-2007 at 07:08 PM.
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Old 08-19-2007
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Actually, the outboard is only electric start. That would be great if I could just use my deep cycle batteries as the starting battery, but I've heard that's not a good idea. Another concern is running the deep cycle batteries dead and not being able to start the outboard when I really need to. That's why I was thining of maintaining a separate starting battery. Also, if I do use just the deep cycle batteries, can I directly charge them from the outboard, meaning I don't need a charge controller between them and the outboard?
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electric start ...

Is it really true that your motor soes not have a rope start? It is hard to believe.

But you shouldn't take seriously the idea that a deep cycle battery should not be used for starting. This advice is meant for starting diesel or other inboard engines where hundreds of cranking amps are required. Deep cycle batteries have a hard time supplying these currents. But your 8HP OB doesn't require anything like these currents to turn it over. In any case you won't harm the battery; it just isn't designed to supply large instantaneous currents.
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What make/model of OB do you have?

In terms of charging deep cycle batteries from the OB - their chemistry is no different from that of starting batteries - they just have larger, denser plates meaning that they can resist being deeply discharged in multiple cycles without dying. (I'm assuming you've got standard wet deep cycle batteries rather than AGM or Gel batteries).

So given that your OB already has a rectifier and regulator suitable for charging a starting battery, that same system will also work on a deep cycle battery.

If your OB really doesn't have a rope start (some have the rope hidden under the lid) then you are right to worry about keeping a redundant electric source for starting the engine if the house batteries are drained. In this instance, yes you will be able to use the OB to charge both banks, but you'd need some sort of isolating diode to allow charging of the banks, but not allow draining of the starter battery when running the house circuits.

Basically you'd be looking at a charging system every bit as complex as that on a large boat with an inboard - there are plenty of threads on that line....

p.s. I'd be really surprised if there was no way to start your OB manually.

Last edited by BritAbroad; 08-19-2007 at 11:31 PM.
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Old 08-19-2007
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You can go ahead and use your deep cycle batteries to start that motor, that size motor has a starter that only will draw between 30 and 50 amps. As an example, electric trolling motors designed for 12 volt deep cycle batteries draw between 30 and 55 amps (they draw about 1 amp per lb of thrust).

I'm new to sailing, but I know a lot about engines and batteries
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Old 08-20-2007
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This is my first outboard motor and now that you mention it it could very well have a rope pull starter under the lid. All this advice has been very helpful. Sounds like i can basically just charge my deep cycle batteries right off the outboard and start the outboard with the manual start that is probably hidden under the lid. Thanks for all the quick replies to my question.
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Old 08-20-2007
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I had a Yamaha T8 on a previous boat with electric start. There was an emergency start kit that came with the motor, with a plastic handle and rope like old time lawnmower.
I had it hooked up to a motorcycle battery which was entirely adequate for starting.
I believe that disconnecting the battery while running the motor might blow the alternator, so I would not switch batteries while running. But you could run it off either battery or bank
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dohenyboy has it right. If you disconnect the batteries while the outboard is running, you will probably fry the alternator....leaving you without a charging source and soon a motor....since a gasoline outboard requires electricity to operate. This is probably more the case with an outboard that has an electric start...which indicates it is supposed to be connnected to a battery.
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Quote:
Originally Posted by sailingdog View Post
....leaving you without a charging source and soon a motor....since a gasoline outboard requires electricity to operate.
I have to disagree - if you lose your alternator on an outboard of this size and your battery goes dead you will not lose the motor - it runs off a magneto. Larger motors, yes I'd agree, but not something less than 15 h.p.

I've been wrong before...well, actually I thought I was wrong but it turned out I was mistaken!
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