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Old 28-02-2019, 10:30   #1
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How large battery bank needed with a genset?

I'm interested in a 42 ft sailboat that has a single 4D house battery rated at 200 Ah (usable would be approx 75), and 2 Grp 27 starter batteries. It also has an Onan 5 KW genset. I haven't done an audit to figure daily electrical usage but with older electronics and autopilot I'd ballpark 150 Ah per day (frig is a Seafrost run off the engine). My first thought was that I would need to expand the battery bank (not easy with space being limited), but I'm now questioning that assumption. I'm obviously going to have to run the genset more frequently on a daily basis when I'm underway but other than the noise, is that an issue? Is there some rule of thumb for battery bank size if the boat has a genset? Any other gotchas with this current setup? I'd particularly like to hear from folks who have boats with gensets. TIA


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Old 28-02-2019, 13:04   #2
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Re: How large battery bank needed with a genset?

This is a question only you can answer.

Can you tolerate running the generator for several hours a day, two or three times per day?

Are you interested in solar or wind power to reduce your need from the generator?

Do you have a battery charger that is able to charge your battery quickly and effectively from the generator?
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Old 28-02-2019, 14:05   #3
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Re: How large battery bank needed with a genset?

Do you have any solar? Genset charging works best if you have a decently discharged battery when you start the genset. This allows the bulk charge current to be high - quicker charge and puts healthy load on genset. Then as the charge tails off you use solar to finish up. This means genset running in the early morning.
Gensets add noise as you mentioned. They also add heat which can be significant in the tropics, as well as maintenance. Their diesel consumption is usually pretty minimal, oil changes and filters can add up.
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Old 28-02-2019, 14:19   #4
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How large battery bank needed with a genset?

My overnight use is about 150 AH, and I have a 660AH bank.
I could get by with a 300AH bank, but being 660 will mean it will last years longer.
The shallower the discharge, the longer a bank will last, and itís not insignificant.
No way I know of to make this not be sideways
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Old 28-02-2019, 21:35   #5
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Re: How large battery bank needed with a genset?

things to think about. with a 5k gen. you could be pushing a 200a battery charger. which means if you are using 150ah a day. that is less then an hour a day of charging.

however your 200ah battery. if flooded would only take maybe 40a. maybe 80a if AGM so you currently have to run for ~3h a day with that small battery. (at different times durring the day) even if you had a huge battery charger.

this in one place bigger banks help. jamming power back in faster. (and agm's help)

your battery should do ~2 days. at 150ah a day that is 300 usable. and 600ah bank.

having a battery that can't even go 24h is useless.

around here common for a 40' boat to have ~800ah of house battery and ~200a of charging
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Old 28-02-2019, 21:48   #6
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Re: How large battery bank needed with a genset?

Thought I'd save this table and rotated it for you.
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Old 28-02-2019, 22:01   #7
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Re: How large battery bank needed with a genset?

You don't need a battery at all. Just run the genset all the time.


Seriously, you need to do some reading about boat electrical systems. Serious reading.


You have a 42 foot boat with one battery? How long have you owned the boat? Have you boated before?


Serious questions. If you are uninformed due to prior boating experiences, or lack thereof, we can help.
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Old 28-02-2019, 23:24   #8
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Re: How large battery bank needed with a genset?

Quote:
Originally Posted by Lepke View Post
Thought I'd save this table and rotated it for you.

You beat me to it!

There are a lot of ways but this time I copied the image to Word then dragged the image around 90į with the mouse pointer on one of the "handles".
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Old 28-02-2019, 23:29   #9
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Re: How large battery bank needed with a genset?

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Originally Posted by Stu Jackson View Post
You don't need a battery at all. Just run the genset all the time.


Seriously, you need to do some reading about boat electrical systems. Serious reading.


You have a 42 foot boat with one battery? How long have you owned the boat? Have you boated before?


Serious questions. If you are uninformed due to prior boating experiences, or lack thereof, we can help.

I think that may be a bit harsh.


Many power boats with generators are not intended to be run off battery power for any significant length of time -- generators go on before shore power is disconnected and run until the cruise is over. For that one battery is enough, and perhaps the OP's boat was designed for such a use case.


If he wants to shut down the generator and run off battery power, he just needs to decide how long he needs to be able to go. If you have a heavy duty generator, then battery capacity for a full 24 hours of normal use is fairly luxurious. If they are lead batts, then the practical capacity is about 40% of nominal, so if you need 200 amp/hours of realistic capacity, then the bank needs to be 200/40% = 500 amp/hours nominal. When new.


I have 420 amp/hours @ 24v, equivalent to 840 amp/hours at 12v. It barely gets me through 24 hours of light consumption, and maybe 12 hours of more normal consumption, with the inverter being used. But my boat has a lot of electrical gear.


Just to give the OP a data point.


TWO days of consumption, as mentioned by someone else above, is real luxury. I've never been on a boat with that much battery capacity (or that little consumption).


How much battery capacity you need is a bit like how long is a piece of string. The more the better, but within practical limits of what can be installed in the available space, and avoiding massive paralleling of batteries.
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Old 01-03-2019, 01:30   #10
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Re: How large battery bank needed with a genset?

The genset and length of the boat is irrelevant in the question.

The real question should be, what loads do you want to run on battery and for how long before you need recharging.

With a 5kVA generator you can in theory produce a DC charge current of 300A at 14V after all the conversion losses, lead acid batteries usually should be charged at least with 1/10C, so in theory with sufficient chargers you could charge a battery bank of up to 3000Ah FLA easily in 10h or quick charge a LFP bank of 300Ah in one hour at 1C. In theory, because your charger very likely will not deliver 300A and you will probably use the generator for other loads while charging anyway - like watermaker and so on.
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Old 01-03-2019, 16:34   #11
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Re: How large battery bank needed with a genset?

Thanks all for your responses, as you've confirmed my original assumptions that I'll need a larger battery bank for this boat. I'm well aware of the general rule of thumb of a "3 to 1" battery bank size to daily Ah usage ratio (e.g. if daily usage is 200 Ah then battery bank size => 600 Ah). Or as Dockhead expressed it, 40% of nominal. My question was whether folks with a genset were adhering to that rule of thumb and seems like they are. For those who read my post carefully, I don't own this boat but I've just put an offer on her. However, one of the things I'll have to do is add significant capacity to the battery bank. Dockhead hit the nail on the head as the shipwright who built the boat must have built more power boats than sailboats and that explains his decision to install such a small battery bank. No older boat is perfect and this should make for an interesting upgrade project.
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Old 01-03-2019, 17:48   #12
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Re: How large battery bank needed with a genset?

My boat has 180 ah total and I run a fridge and small freezer. So if you have an engine drive fridge, the 4D should be fine. You gotta run that engine 1-2 hours? To cool the fridge and you got plenty of juice to start the engine each day!
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Old 01-03-2019, 20:25   #13
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Re: How large battery bank needed with a genset?

B.O.A.T means bring out another thousand!!!!!
just buy more batteries, better to have them and not need them than need them and not have them!!!!
and while you are at it get more solar panels and stop being cheap.
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Old 03-03-2019, 04:48   #14
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Re: How large battery bank needed with a genset?

Sportfishermen we have had always had three batteries, one for each motor and one for the generator.
The generator was started before the shorepower cord was unplugged and stopped after it was plugged back in, the batteries were for starting only.
Of course the boats were never cruised, they were fished, which most often meant that we were back in the Marina by late afternoon.
Going out for a few days at a time happened a couple times a year.

Intended use of a boat means everything
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Old 03-03-2019, 05:00   #15
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Re: How large battery bank needed with a genset?

Quote:
Originally Posted by jimp1234 View Post
I'm interested in a 42 ft sailboat that has a single 4D house battery rated at 200 Ah (usable would be approx 75), and 2 Grp 27 starter batteries. It also has an Onan 5 KW genset. I haven't done an audit to figure daily electrical usage but with older electronics and autopilot I'd ballpark 150 Ah per day (frig is a Seafrost run off the engine). My first thought was that I would need to expand the battery bank (not easy with space being limited), but I'm now questioning that assumption. I'm obviously going to have to run the genset more frequently on a daily basis when I'm underway but other than the noise, is that an issue? Is there some rule of thumb for battery bank size if the boat has a genset? Any other gotchas with this current setup? I'd particularly like to hear from folks who have boats with gensets. TIA


-Jim
I have the equalvet of 2 4d batteries and have electric refrigeration. In mornings I am down 60-90 ah. Had 290w solar and ran the generator about 40 minutes a day in morning (have 640w now and hope to not run the generator much anymore)

I think you will want to double your battery capacity or you are going to a slave to running the generator. Plus efficiency wise itís crazy to run a generator to charge 1 4D battery
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