Battery options for starting and accessories, outboard has an alternator - SailNet Community
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post #1 of 10 Old 05-15-2012 Thread Starter
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Battery options for starting and accessories, outboard has an alternator

I've got a Merc 9.9 prokicker with a 6amp alternator. and a single battery.

Does anyone know how long it takes for the alternator to recharge what it uses on a typical cold start?

Is it feasible for me to use a single battery for starting and up to say 10 hrs of powering the radio?

After running the engine for ~20 min I kill the engine and start sailing. I like running the radio and occasionally the bilge does some work. A couple hours of radio tends to drain the battery too low to start the engine. Luckily the prokicker has a pretty easy pull start option.

I just boat the boat a few weeks back and still haven't taken a detailed inventory to note the specs of the battery. So I don't know if the battery is bad or I'm overusing it, or if I really just need a starter and a deep cycle.
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post #2 of 10 Old 05-16-2012
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Re: Battery options for starting and accessories, outboard has an alternator

I have an 8 hp merc on my boat, looking for an alternator but for right now i made a home built solar panel to charge it.
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post #3 of 10 Old 05-16-2012 Thread Starter
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Re: Battery options for starting and accessories, outboard has an alternator

Can you tell me about your solar set up? materials, cost, does it work well? I'm very interested in solar charging: trickle charge for near term, and high power solar for long trips away from port. Kind of off topic from my original post.
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Re: Battery options for starting and accessories, outboard has an alternator

Mainsail did a nice write up on his sight to which i do not have the link right now





But the 40 watt panel and Genasun control are working well and even MY inboards 51 AMP alt really cant bring the batteries past about 85% charged and the panel tops them off very nicely as it puts out 2+ amps for at least 4 hours a day

It is one of the more usefull upgrades i have done

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post #5 of 10 Old 05-16-2012
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Re: Battery options for starting and accessories, outboard has an alternator

The alternator should replace what it takes to start an outboard pretty quickly, even with a 6 amp alternator. To start a small diesel it takes less than one AH, usually a lot less.

Here's the link to Maine Sail's solar install:Installing A Small Marine Solar System Photo Gallery by Compass Marine at pbase.com

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Re: Battery options for starting and accessories, outboard has an alternator

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Originally Posted by sccndc View Post
I've got a Merc 9.9 prokicker with a 6amp alternator. and a single battery.

Does anyone know how long it takes for the alternator to recharge what it uses on a typical cold start?

Is it feasible for me to use a single battery for starting and up to say 10 hrs of powering the radio?

After running the engine for ~20 min I kill the engine and start sailing. I like running the radio and occasionally the bilge does some work. A couple hours of radio tends to drain the battery too low to start the engine. Luckily the prokicker has a pretty easy pull start option.

I just boat the boat a few weeks back and still haven't taken a detailed inventory to note the specs of the battery. So I don't know if the battery is bad or I'm overusing it, or if I really just need a starter and a deep cycle.
I don't think a couple of hours of radio should drain a battery to that extent. How big is the battery (approx. dimensions would suffice)? It's hard to imagine a radio in receive mode using more than a couple of amps, so a 50 Ah battery should run it for 10 hours and still be 60% charged. Sounds like the battery is on it's last legs.
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Re: Battery options for starting and accessories, outboard has an alternator

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I don't think a couple of hours of radio should drain a battery to that extent. How big is the battery (approx. dimensions would suffice)? It's hard to imagine a radio in receive mode using more than a couple of amps, so a 50 Ah battery should run it for 10 hours and still be 60% charged. Sounds like the battery is on it's last legs.
I'm thinking the same thing. I wish I had a chance to inspect teh battery before posting. The seller didn't do much maintenance the past couple years. So teh water levels may be low, or I just need a new battery. Based on your post it seems reasonable to run the radio while day sailing and start the 9.9 using a single battery.

I plan to take the battery to advance auto and have it tested this weekend.

Thanks
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Re: Battery options for starting and accessories, outboard has an alternator

The best approach is to measure the current your radio etc. is using, and then size the battery so you don't go below say 50% after a long day sailing.

That's because going below 50% battery capacity shortens the life a lot.

Note that your alternator won't take the battery over 85%. So that actually gives you 35% (from 85% down to 50%) of the battery capacity to power the radio for a day.

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Re: Battery options for starting and accessories, outboard has an alternator

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I plan to take the battery to advance auto and have it tested this weekend.

Thanks
Steve
They are not able to test it for capacity. They will test to see its starting ability which doesn't mean much. Even a battery with little remaining capacity can start an engine. In your car as the battery gets older and loses capacity the engine can still be started until one day it won't even do that.

There are testers that can tell capacity but they are very expensive and I doubt you will find many. The way to test for capacity is to fully charge the battery and put a small load on it - the battery capacity divided by 20 in amps. In other words if the battery is 100 AH you put a 5 amp load on it. If at full capacity it should last 20 hours before it reaches 10.5 volts. If it lasts 10 hours it has half its capacity left.

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post #10 of 10 Old 05-17-2012
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Re: Battery options for starting and accessories, outboard has an alternator

I would guess your battery is low on charge, or due for replacement. A full charge on a typical Group 24 deep cycle battery would power a VHF radio for dozens of hours. But at only 6 amps from your alternator, it's going to take a long time to fully charge it back up. If you are at a slip you can keep the battery topped off. If not, consider hauling it ashore periodically and putting it on a charger to top it off. If you do this and it still doesn't hold a charge for a decent period, its time for replacement.
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