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Old 11-03-2014, 06:47   #1
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Battery charger

How big a battery charger do we need for our 40' liveaboard trawler. Most lights are LED. Old charger was 50 amps and I want to replace it so that it doesn't destroy the two new 8d batteries. Thanks for any and all input.
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Old 11-03-2014, 07:36   #2
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Re: Battery charger

Doesn't matter all that much what the size the charger is, what matters is how long you are going to have the charger on. How long/short do you want it to take to charge the batteries. what construction are they, how are you charging them (shorepower or generator)?
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Old 11-03-2014, 07:46   #3
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Re: Battery charger

We spend time at anchor, and also at the dock, so charging both generator and shore power. we are old sailors, so are pretty miserly with power, but I guess the refer uses quite a bit, so if I don't want to be running the gen all day at anchor, better bigger than smaller.

My old charger keeps the batts at 13.6, is that too much given that the boat will be on the hard for 8 months for the summer?
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Old 11-03-2014, 07:49   #4
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Re: Battery charger

Agree with Sailorboy1. Very much depends on your boating style.

If you keep the boat at a marina and connected to shore power with an occasional day or weekend away then you can get buy with a much smaller charger.

If you have a large battery bank and stay at anchor so run a generator to charge batteries then you would need a much larger charger.

To get the best answer can you give us a little more information like:

- size of your battery bank
- boating style
- generator on board
- solar or other alternative charging sources
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Old 11-03-2014, 08:09   #5
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Re: Battery charger

If you rely on your generator to charge your batteries you want the largest charger your batteries can handle. Genset runtime is very expensive per amp hour generated; the larger the charger the less per amp/hour since fuel and hours are the same... For a 700a/h bank I'd shop a 200amp charger; not cheap but neither is running up genset hours.
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Old 11-03-2014, 08:13   #6
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Re: Battery charger

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Originally Posted by leadingedge View Post
My old charger keeps the batts at 13.6, is that too much given that the boat will be on the hard for 8 months for the summer?
13.6 volts is its float/maintenance charge (which is good). Your charger will go through a bulk charge at roughly 14.1-14.5 volts (depending on what type of batteries) to get it to 80% "full" the next 20% takes a lot longer to finish. And when it hits 100% charge it will drop to 13.5volts to maintain the batteries.

This is all a "general" idea of what happens battery composition and age can change this
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Old 11-03-2014, 08:35   #7
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Re: Battery charger

I'll just give answer. Assuming your 8Ds are flooded acid they have around 850 amp-hours of capacity. Flooded acid batteries normally will not accept more that 25% of their capacity as a charge current, and only will even accept that high a current when they are pretty low. So the largest charger of use would be a little more than 200-amps. If the batteries are AGM you could go larger.

But as a practical matter you could probably easily get by with a 100-amp charger and if your batteries are down real low it would just take longer. The better answer would be to know what your daily amp-hour loads are when at anchor and then decide how long you are willing to run the generator to get them charged. But for the time at anchor you also could just accept that you only recharge to 80-90% (at bulk charge that is fastest) and then plan to fully recharge to 100% at the dock.

If it were my boat I would probably get a 100-amp charger (I have 100-amp on my boat that has 1/2 your battery capacity, but it mainly is sized for the inverter).
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Old 11-03-2014, 10:15   #8
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Re: Battery charger

The new batteries may have sat on the shelf for a while and self discharged. If a hydrometer or resting voltage shows greater than half discharge, you can bring them to 100% charge as the sulfation will not be too great. To do this you will need to force charge the batteries using a charger that has a manual setting and run it at less than 8 amps for two size 8D batteries. Half that would be better as force charging "boils" the electrolyte. What is happening is the water is being broken down into hydrogen and oxygen. If the charger is not wired directly into the boat, always disconnect the battery charger from the 115 volt power source before disconnecting from the battery as a spark can cause an explosion spraying you with 32% sulfuric acid. If that happens the best thing is to dive overboard. As a rule of thumb, charge batteries at no more than ten percent of the amp-hour rating. For the 8D that would be about 200 amp-hours per battery so the 50 amp charger is sized about right. Make sure that the battery charger is a marine charger. These will usually have two charging circuits, one for the engine battery and the other for the house. If you have a 'both' setting on a manual switch, then both batteries are connected and only one circuit is needed, but if you forget to set this, you can destroy one bank of batteries from sulfation. Some marine chargers have more than two charging circuits. Most importantly, a marine charger isolates the 12 volt DC ground for the boat from the ground in the 115 volt AC power cord. This prevents electrolysis that can cause severe corrosion of metal parts of the boat. Automotive battery chargers usually have the two grounds tied together because it is a little cheaper to make the charger this way.
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Old 11-03-2014, 13:58   #9
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Re: Battery charger

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Originally Posted by sailorboy1 View Post

Assuming your 8Ds are flooded acid they have around 850 amp-hours of capacity.

Usually an 8D offers about 250 Ah, give or take, so that should be 500 Ah (or so) for a bank of two.

Lifeline, for example, says 255, and others should be similar-ish. Lifeline Batteries - Marine & RV Deep Cycle Batteries

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Old 11-03-2014, 17:29   #10
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Re: Battery charger

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Usually an 8D offers about 250 Ah, give or take, so that should be 500 Ah (or so) for a bank of two.

Lifeline, for example, says 255, and others should be similar-ish. Lifeline Batteries - Marine & RV Deep Cycle Batteries

-Chris
When I stated 200 amp-hours for the 8D that was from memory of a setup I did on a friend's RV so I am sure 255 amp-hour is closer. Anyway, on that RV we setup a 20 watt solar panel and regulator to keep the house battery and engine battery topped off. This worked very nicely for a RV situated in California. The batteries were always at peak capacity. If the sailboat is going to be on the hard for 8 months, you will need a solar panel to keep the batteries from self discharging, sulfating, and going bad needing replacement the first year.

You might want to consider using two of the 6 volt golf cart batteries (GC2) in series. The length of two of these is the same as the length on one 8D, the height is the same, and the width is 7.11 inches while the 8D is 10.95 inches wide. For Interstate brand 8D, the amp-hour rating is 270 and130 pounds while one GC2 is 62 pounds and 232 amp-hour so much easier to get in and out of the sailboat. However, one 8D deep cycle is $269 and two GC2 are $304.

The above panel is just to float the batteries. If you want to go solar to take the place of a generator, then you need to look carefully at the power demands for refrigeration, lighting, computers, autopilot etc. Note that with solar panels, you need to divide the number of watts by 5 to get 24 hour output because it is dark some of the time, and the sun angle is not always direct onto the panels
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Old 12-03-2014, 05:39   #11
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Re: Battery charger

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When I stated 200 amp-hours for the 8D that was from memory of a setup I did on a friend's RV so I am sure 255 amp-hour is closer.

I was just surprised that sailorboy said 850

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Old 12-03-2014, 05:42   #12
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Re: Battery charger

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I was just surprised that sailorboy said 850

-Chris

so I was wrong about that, take away my birthday
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Old 12-03-2014, 05:47   #13
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Re: Battery charger

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so I was wrong about that, take away my birthday

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Old 12-03-2014, 21:10   #14
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Re: Battery charger

the 2 8ds are for the house? and you have a starter too? or is one 8D a starter?

I would look into charger / inverters. they aren't much more then a big charger. plus inverters are usefull.

IE the magnum 2800w / 120a charger or 2000w / 100a charger
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Old 13-03-2014, 12:52   #15
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Re: Battery charger

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the 2 8ds are for the house? and you have a starter too? or is one 8D a starter?

I would look into charger / inverters. they aren't much more then a big charger. plus inverters are usefull.

IE the magnum 2800w / 120a charger or 2000w / 100a charger
I really like everything about the Magnum inverter/charger except for one very important thing: You can only use shore-power of a specific voltage. I'd really prefer something with universal, worldwide voltage input so that I could charge my batteries at any marina in the world I happen to find myself. That's the only hit against them. That's really the only thing making me go with a standalone battery charger and a separate inverter. There are some standalone chargers with universal voltage inputs. BTW, no, I can't afford the weight or space of a transformer.
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