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Old 15-05-2011, 04:40   #1
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Battery Bank / Alternator / Charger Size

I am refitting my ketch and installing a new electrical system. What is the rule to appropriately size the capacity of a battery bank and consequently the size of the shore power charger, the alternator to battery charger and finally the altrnetor itself?
Presently I have a 70 amp alternator on a new 65HP engine, i calculated an approx. 24hr need of 180-190 Amps.
Thank you for a rule of thumb advice.
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Old 15-05-2011, 05:32   #2
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Re: Battery Bank / Alternator / Charger Size

Hard to come up with a precise figure unless we know more.

However, assuming:

1. you're talking about actually cruising and not staying at the dock plugged into shorepower most of the time;

2. you're talking about flooded or gelled lead-acid batteries, not AGMs; and

3. for the moment, you're NOT talking about solar panels or wind generators or onboard generators....

then

- figure that only about 35-40% of the total rated battery capacity will actually be available for use (see below);

- a 70A alternator with a good external smart regulator will be your primary charging source when away from the dock; and

- occasionally you will be able to plug into shore power overnight.

Batteries should not be depleted below 50% capacity on a regular basis. Additionally, when cruising it's fairly easy to charge batteries back to about 80-85% full, but thereafter it takes a long time -- no matter the size of the alternator or charger. Hence, when cruising you'll be operating somewhere between 50% and, say, 85% capacity....so only about 1/3 of rated capacity will be actually available to you.

In this scenario you'll need a house bank of about 190AH x 3 or 570AH total capacity. Flooded lead-acid batteries can accept a charging current up to 25% or so of their rated capacity in the bulk mode. That's about twice your alternator output, so you could use a larger alternator. However, a 70A alternator properly regulated can do the job, though it will take longer.

Re: a battery charger, I'd get one rated at 50-80 amps output. This will be enough to charge the batteries fully overnite.

All batteries, particularly flooded lead-acid batteries, need to be kept charged as well as possible to avoid sulfation of the plates and reduction of their capacity over time to hold and delivery energy.

The above is really rule-of-thumb, but that's what you asked for :-)

Bill
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Old 15-05-2011, 07:35   #3
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Re: Battery Bank / Alternator / Charger Size

Quote:
Originally Posted by btrayfors View Post
Hard to come up with a precise figure unless we know more.

However, assuming:

1. you're talking about actually cruising and not staying at the dock plugged into shorepower most of the time;

2. you're talking about flooded or gelled lead-acid batteries, not AGMs; and

3. for the moment, you're NOT talking about solar panels or wind generators or onboard generators....

then

- figure that only about 35-40% of the total rated battery capacity will actually be available for use (see below);

- a 70A alternator with a good external smart regulator will be your primary charging source when away from the dock; and

- occasionally you will be able to plug into shore power overnight.

Batteries should not be depleted below 50% capacity on a regular basis. Additionally, when cruising it's fairly easy to charge batteries back to about 80-85% full, but thereafter it takes a long time -- no matter the size of the alternator or charger. Hence, when cruising you'll be operating somewhere between 50% and, say, 85% capacity....so only about 1/3 of rated capacity will be actually available to you.

In this scenario you'll need a house bank of about 190AH x 3 or 570AH total capacity. Flooded lead-acid batteries can accept a charging current up to 25% or so of their rated capacity in the bulk mode. That's about twice your alternator output, so you could use a larger alternator. However, a 70A alternator properly regulated can do the job, though it will take longer.

Re: a battery charger, I'd get one rated at 50-80 amps output. This will be enough to charge the batteries fully overnite.

All batteries, particularly flooded lead-acid batteries, need to be kept charged as well as possible to avoid sulfation of the plates and reduction of their capacity over time to hold and delivery energy.

The above is really rule-of-thumb, but that's what you asked for :-)

Bill
Yep..
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Old 15-05-2011, 11:19   #4
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Re: Battery Bank / Alternator / Charger Size

thank you very much, I will get a bigger alternator for now.
Ciao MW
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Old 15-05-2011, 12:25   #5
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Re: Battery Bank / Alternator / Charger Size

You can save a lot by cutting down on your usage. Unless you want to watch TV all day, I would say a 12m boat could get by on 60 amp hrs at anchor and 120 underway. This includes refrigeration (no freezer), autopilot, VHF, computer (netbook), led lighting, & fans.

A 500 amp-hr bank and 100 amp-hr alternator should be sufficient, and 300 watts of solar should eliminate the need for the alternator at anchor.
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Old 17-05-2011, 11:34   #6
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Re: Battery Bank / Alternator / Charger Size

Quote:
Originally Posted by btrayfors View Post
Hard to come up with a precise figure unless we know more.

However, assuming:

1. you're talking about actually cruising and not staying at the dock plugged into shorepower most of the time;

2. you're talking about flooded or gelled lead-acid batteries, not AGMs; and

3. for the moment, you're NOT talking about solar panels or wind generators or onboard generators....

then

- figure that only about 35-40% of the total rated battery capacity will actually be available for use (see below);

- a 70A alternator with a good external smart regulator will be your primary charging source when away from the dock; and

- occasionally you will be able to plug into shore power overnight.

Batteries should not be depleted below 50% capacity on a regular basis. Additionally, when cruising it's fairly easy to charge batteries back to about 80-85% full, but thereafter it takes a long time -- no matter the size of the alternator or charger. Hence, when cruising you'll be operating somewhere between 50% and, say, 85% capacity....so only about 1/3 of rated capacity will be actually available to you.

In this scenario you'll need a house bank of about 190AH x 3 or 570AH total capacity. Flooded lead-acid batteries can accept a charging current up to 25% or so of their rated capacity in the bulk mode. That's about twice your alternator output, so you could use a larger alternator. However, a 70A alternator properly regulated can do the job, though it will take longer.

Re: a battery charger, I'd get one rated at 50-80 amps output. This will be enough to charge the batteries fully overnite.

All batteries, particularly flooded lead-acid batteries, need to be kept charged as well as possible to avoid sulfation of the plates and reduction of their capacity over time to hold and delivery energy.

The above is really rule-of-thumb, but that's what you asked for :-)

Bill
Hi Bill,
I re-read your well received rule of thumb. With a sterling alternator to battery charger won't I be able to charge over 90% of capacity.
MW
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Old 17-05-2011, 13:16   #7
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Re: Battery Bank / Alternator / Charger Size

Quote:
Originally Posted by wizz View Post
Hi Bill,
I re-read your well received rule of thumb. With a sterling alternator to battery charger won't I be able to charge over 90% of capacity.
MW
Depends how long you motor :-)

Charge capacity really matters during the bulk and, to a lesser extent, the acceptance or absorption stages.

When the batteries reach 80-85% capacity, their ability to accept a charge diminishes considerably, so much that charger capacity is no longer a factor. Even if you had a 200A alternator, the batteries in the final stages of charging will only accept so much, an no more...so long as you keep the charging voltage within recommended parameters.

So, reaching 90% or 100% state of charge (SOC) becomes almost purely a question of TIME. If you power for 6-10 hours or so, yes, you'll achieve something near a full charge. But if you're at anchor, you're surely not going to run your engine that long, and in the 1-2 hours of so of charging you'll not likely exceed 80-85% SOC.

Bill
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Old 17-05-2011, 13:59   #8
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Re: Battery Bank / Alternator / Charger Size

190 AH per day sounds high, and you could end up running your engine three hours per day with the system described above just to keep up. I wouldn't want to do such a thing to my engine.

I like to structure my system with the biggest alternator my engine can handle without diminished performance. In your case that would probably be somewhere in the 80-90 amp range. This would match up well to a two-battery bank, either 4D or 8D.

If your calculations for 190 AH per day is correct, you should really look into ways to make your electrical system more efficient. Start with LED cabin lights and a well-insulated fridge.
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Old 17-05-2011, 14:20   #9
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Re: Battery Bank / Alternator / Charger Size

Its nice that you calculated your usage, many posts don’t. which makes it tough to give advice, but 180 Ahrs a day is a lot.
If you want to generate this with your engine your 70a alternator will probably give less than 50a in real life which means an excessive 3-4 engine run to replace the power each day.
To replace the power with batteries charged on shore power you will need 360AHrs (to keep the batteries above 50%) each day away from dock.
In practice you will use a combination of alternator and accumulated battery power, but you will need a lot of batteries and / or a long engine run time to spend more than a few days away from shore power.
I agree with Bash the first step is to see if you can reduce your consumption.

How long do you want to spend away from shore power?
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Old 17-05-2011, 20:40   #10
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Re: Battery Bank / Alternator / Charger Size

Take a look at Electrical Planning http://newboatbuilders.com/pages/ele..._planning.html
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