Alternator sizing

Bill Roosa · Dec 15, 2014

The batteries control the amount of current that the alternator can put out. Bigger bank will take more current for a given state of charge. So with 130 Ah of battery bank you will be able to push 90 amps into DEEPLY discharged bank for a minute or two before the banks build up internal resistance and the amps drop off significantly. 2 minutes at 90 amps is a wapping 3 Ah!!!! Probably not worth the money to up grade unless you expect to go bigger on the bank size.

centerline · Dec 15, 2014

Bill Roosa said:
The batteries control the amount of current that the alternator can put out. Bigger bank will take more current for a given state of charge. So with 130 Ah of battery bank you will be able to push 90 amps into DEEPLY discharged bank for a minute or two before the banks build up internal resistance and the amps drop off significantly. 2 minutes at 90 amps is a wapping 3 Ah!!!! Probably not worth the money to up grade unless you expect to go bigger on the bank size.

I agree.... too many people spend big dollars on a high output alternator without having a battery bank that can use it... this is due to a lack of understanding in how it all works together.

a high powered alternator isnt going to slam a bunch of power to the batteries and charge them up any faster than the batteries can absorb the charge. it will be the batteries themselves that tell the alternator how much they can take, and thats all they WILL take....

with 130ah bank, about a 65-75amp alternator is all you could ever need.... until you get a bigger battery bank.

IStream · Dec 16, 2014

Just to chime in here, there is one advantage to a larger alternator even with a small bank and that's if you have a lot of electrical loads (e.g. refer, coffee maker, etc.) you want/need to service while you're charging your batteries. Not necessarily a huge consideration but I have a buddy with a use pattern that fit on his Catalina 320 and he made good use of his upgraded alternator.

centerline · Dec 16, 2014

IStream said:
Just to chime in here, there is one advantage to a larger alternator even with a small bank and that's if you have a lot of electrical loads (e.g. refer, coffee maker, etc.) you want/need to service while you're charging your batteries. Not necessarily a huge consideration but I have a buddy with a use pattern that fit on his Catalina 320 and he made good use of his upgraded alternator.

It will help a little, but only a little....
assumimg you are using 12v appliances, the coffee maker pulling 15amps for 10 minutes while its brewing is only 1.5 amps... the refer, even if its a power hungry model is about 5 amps while its running...

Using an inverter to power 120v appliances is inefficient, but sometimes there is enough bank/power in the batteries that make it a reasonable trade off... but even so, unless you are using high loads for a long time with a small bank of batteries, an alternator that is sized for the bank is going to be more than adaquate....
and if a person IS using high loads for extended periods from their small bank, they need to enlarge/rethink it if they want good life from the batts....

dlochner · Dec 16, 2014

marchem said:
Kind of a funny story: '82 Catalina 27 with a 5411 diesel. I was removing the alternator to get at the oil filter and had opened the shut off switch (Blue Seas 285 80A) I had installed on the alternator output wire (thanks to Maine Sail and others for the advice), but was listening to music so when I pulled it out I made contact with the hot lead on the starter and blew the 200A MRBF fuse at the battery. This also seemed to blow the diodes on the alternator. Next time I will leave the battery switch on off!

So I'm looking at either repairing or replacing it. If I do replace it, is there much advantage of going with a larger output unit, like the Leece Neville 8MR2069TA 90A Maine Sail has a write up on (http://www.pbase.com/mainecruising/catalina_36_alternator)? I've got 2 65 AH size 24 batteries and don't think I will be getting anything much larger. Mostly day sailing with access to AC at the dock. Minimal DC needs, no A/C or reefer.

No need to replace the alternator. Take it to an automotive electrical shop and they will replace the regulator and service the alternator. When it comes back it will as good as new for a lot less than a new alternator. A few years ago it cost about $100 to have my alternator refurbished.

IStream · Dec 16, 2014

centerline said:
It will help a little, but only a little....
assumimg you are using 12v appliances, the coffee maker pulling 15amps for 10 minutes while its brewing is only 1.5 amps... the refer, even if its a power hungry model is about 5 amps while its running...

Using an inverter to power 120v appliances is inefficient, but sometimes there is enough bank/power in the batteries that make it a reasonable trade off... but even so, unless you are using high loads for a long time with a small bank of batteries, an alternator that is sized for the bank is going to be more than adaquate....
and if a person IS using high loads for extended periods from their small bank, they need to enlarge/rethink it if they want good life from the batts....

I don't disagree in general but this particular situation involved the use of a lot of 120 VAC appliances via his inverter and a desire to keep the engine charging run limited to about an hour. I don't think it's terribly unusual for a lot of coastal cruisers.

10 cup home coffee maker: 1000W at 120VAC => ~90A x 10 min = 9AH. Toaster oven for breakfast: 1500W at 120VAC => ~130A x 10 min =13AH. Wife's hair dryer: 1500W at 120VAC => ~130A x 10 min =13AH.
Wife's curling iron: 1000W at 120VAC => ~90A x 10 min = 9AH.
Hot water heater: 500W at 120VAC => ~45A x 30 min = 23AH.

The last item was to speed the heating of the hot water heater, which couldn't get up to full temp on engine heat alone during a one hour engine run at fast idle. That's about 65 amp hours used, ending with a full breakfast with coffee for a family of five, a happy wife, charged batteries, and plenty of hot water for the rest of the day.

Maine Sail · Dec 16, 2014

marchem said:
Kind of a funny story: '82 Catalina 27 with a 5411 diesel. I was removing the alternator to get at the oil filter and had opened the shut off switch (Blue Seas 285 80A) I had installed on the alternator output wire (thanks to Maine Sail and others for the advice), but was listening to music so when I pulled it out I made contact with the hot lead on the starter and blew the 200A MRBF fuse at the battery. This also seemed to blow the diodes on the alternator. Next time I will leave the battery switch on off!

So I'm looking at either repairing or replacing it. If I do replace it, is there much advantage of going with a larger output unit, like the Leece Neville 8MR2069TA 90A Maine Sail has a write up on (http://www.pbase.com/mainecruising/catalina_36_alternator)? I've got 2 65 AH size 24 batteries and don't think I will be getting anything much larger. Mostly day sailing with access to AC at the dock. Minimal DC needs, no A/C or reefer.

Your engine has a 55A alt as stock and your bank is pretty small. You won't gain much by going to the 90A unless you had AGM batteries than could take more current in bulk..

You can realize charging performance gains by addressing the piss poor Universal factory wiring and making sure you have little to no voltage drop between the alternator and the batteries. Your hindrances are not in how many amps you have but what your voltage is at the battery terminals. It should be nearly the same as at the alt when driving near full output. Addressing voltage drop in the factory system will lead to bigger performance gains for you than adding more amperage to a poorly wired system....

gettinthere · Dec 16, 2014

We do quite well with a 100 amp alt for our 900 amp/hr house bank.

wufibugs · Jan 27, 2015

Kind of strange that Rob's 27 comes with a 55 amp alternator and my H-31 with the Yanmar 2G20F only comes with a 35. From what Maine says, this might be under powered for my 3 group 31 AGMs (and yes, I already all wiring involving the Batts to 0 or 2 awg). I was thinking of going up to the 85 amp aftermarket unit.

Ron20324 · Jan 27, 2015

wufibugs said:
...H-31 with the Yanmar 2G20F only comes with a 35. From what Maine says, this might be under powered for my 3 group 31 AGMs ... I was thinking of going up to the 85 amp aftermarket unit.

You're sucking alot of horsepower from that engine to run an alternator. I certainly would not go with a bigger alt. Go solar. I'm sure someone on this forum can tell us how much h.p. an alternator draws down???