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Old 09-11-2015, 22:48   #1
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40amp alternator to charge two 150 amp hr house batteries

I have a newly acquired Pearson P-35 with a vetus m3.10. The highest output alternator I can find is 40amps. Is that enough to keep two 150 amp hour house batteries charged? I will also be using 300 watts of solar as well. Any input is greatly appreciated!
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Old 09-11-2015, 22:57   #2
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Re: 40amp alternator to charge two 150 amp hr house batteries

Depends on how much you draw per day out of your house batteries and how long your run the alternator.

300 Watts of solar should give you around 100 Ah per day. If you run the engine for a couple of hours when the batteries are depleted and before the solar kicks in, you may get close to 60-80 Ah of input.

So with 2 hours running per day, you are looking at getting back maybe 150 Ah per day which is the maximum you should plan on drawing from your 300 Ah bank if you want your batteries to last.
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Old 10-11-2015, 07:59   #3
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Re: 40amp alternator to charge two 150 amp hr house batteries

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... If you run the engine for a couple of hours when the batteries are depleted and before the solar kicks in, you may get close to 60-80 Ah of input ...
That's averaging 30 - 40 Amps output from an alternator rated 40A.
I'd expect, even into depleted batteries, to get no more than 20 - 30 Amps, which might equal as much as 50 A/Hr over 2 hours. Perhaps even less.
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Old 10-11-2015, 08:37   #4
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Re: 40amp alternator to charge two 150 amp hr house batteries

I would like to find a bigger alternator say 70-90 amp but in my search I'm not coming up with anything yet but I am not giving up!!!!
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Old 10-11-2015, 09:25   #5
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Re: 40amp alternator to charge two 150 amp hr house batteries

I am assuming your batteries are both 12v so you have a combined 300Ah at 12v, i.e. not only 150Ah from two 6v batteries. So if that is correct, then a 40A alternator is on the low side to really push enough amps to the batteries. There will be times you need to get the batteries charged more completely and if the solar is impaired you will not be able to do so without extended run times. GordMay is correct in that a 40A alternator will not actually put out the full rated juice, and it certainly will not do it after the beginning of the charge process - it will ramp off dramatically as the batteries fill.

The standard rule of thumb is the alternator should be rated (nominal not actual) at 25% of amps to Ah of the batteries, so that would be 75A for the alternator. 75A alternators are quite common and easily found but you may have a mounting problem with your particular engine. Usually all you will need is a different mount setup. But there are many base types of mounting for many alternators so you should be able to find one that works. Balmar is an example.
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Old 10-11-2015, 10:32   #6
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Re: 40amp alternator to charge two 150 amp hr house batteries

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Originally Posted by Mayhemdog75 View Post
I would like to find a bigger alternator say 70-90 amp but in my search I'm not coming up with anything yet but I am not giving up!!!!
Balmar doesn’t list “Vetus” in it’s engine application guide.
Engine applications database

However, their dual foot 3.15" separation Series 60 or 70 might work
http://www.marinewarehouse.net/image...or%20Guide.pdf
http://www.balmar.net/PDF/seven-series-alternators.pdf
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Old 10-11-2015, 14:36   #7
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Re: 40amp alternator to charge two 150 amp hr house batteries

I just looked at a web pic of a Vetus m3.10. The alternator foot looks like it is a one inch foot. I would confirm that by measuring the foot on the engine in the boat. I think it is a Mitsubishi engine underneath the marinization and it should use a standard foot of some sort. Take a look at the Balmar page below to see what they offer. Other alternator brands will have these too. Contact Vetus and ask if needed, or Mitsubishi. I'll be very surprised if one of these don't work.


6-Series Alternators and Charging Packages
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Old 10-11-2015, 14:57   #8
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Re: 40amp alternator to charge two 150 amp hr house batteries

The question we should be asking is how many amps are ya gonna be using !
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Old 10-11-2015, 17:18   #9
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Re: 40amp alternator to charge two 150 amp hr house batteries

The boat is in Washington North Carolina and I'm in the mid west finishing up a seismic job(GIS mapping) and will be getting back to the boat sometime in December and will be able to get measurements and a better idea of what may work. As far as amps we will be using..... Basic electronics, L.E.D. Nav and cabin lights and charging of laptop for opencpn. We won't be using refridgeration for now. I've only spent 1 week at the boat since buying her due to work but that's about to change. The input you folks are giving is awesome!!!!
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