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Old 02-15-2010, 08:37 PM   #1
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Year: 1981
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Engine: Ford 370 Propane
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Alternator rating

How would I find out the output rating of the alternator on my bus? It is a 1981 Ford B600 chassis with the Ford 370 engine running on propane. (Although I seem to think the previous owner may have swapped it to a 460 engine, but I don't know how to confirm that either!) I ask because right now a local shop has a sale on a 60 amp battery isolator for $28. But if the alternator puts out more than that, I need to get the 120 amp one, or maybe even bigger?

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Old 02-15-2010, 09:22 PM   #2
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Re: Alternator rating

You batteries are what are going to dictate the rate of charge, not the alternator. It's true that an old school 40 amp alternator isn't going to be able to supply 60 amps if the batteries demand it, but if the batteries only demand 40 amps a 120 watt alternator isn't going to piss pound them. As a general rule you shouldn't see batteries being charge/accepting a charge greater than 25% of their amp hour capacity. What kind of a battery bank are you looking at running? For $28 I might be willing to take the chance...
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Old 02-15-2010, 09:27 PM   #3
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Re: Alternator rating

I was planning on running two common RV deepcycle/starting batteries for the chassis, and either 4 group 31 deep cycle batteries or 6 golf cart batteries. My plans are not set in stone yet, as I haven't bought the coach batteries, but the bus currently has the two RV batteries in it that I will wire up for the chassis.
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Old 02-18-2010, 10:13 AM   #4
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Re: Alternator rating

i have had several bus alternators rebuilt, and in almost every case they were between 100 and 120 amps stock. I always upgraded alternators when i had them rebuilt as i generally use them to run a 2000 watt inverter. I tried a battery isolator once, but my "new" alternator gave up the ghost soon after installing the isolator. A far superior system is to use 2 alternators if money permits. The stock alternator can do what it does now, and the 2nd alternator can run just the "coach" systems. If either alternator fails, you can easily connect to the good alternator and continue your journey until it is convenient to replace the broken one.
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Old 02-18-2010, 10:15 AM   #5
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Re: Alternator rating

Interesting idea. That would eliminate the need for an isolator completely. How much power does it take to turn an alternator? I don't exactly have the most powerful engine as it is.
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Old 02-18-2010, 10:45 AM   #6
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Re: Alternator rating

your alternator case should have a tag or be stamped with it's rated volts and amp ratings, the power draw on the engine is directly related to howmuch power the alternator is making , 1.56 hp per kw of electricity, for chassis/battery charging&use the first 5 minuites after starting and then when funning with lights on are the high use times, if you want to charge house batteries or run an inverter while traveling add an auxillary alternator or run a genset if your power requirements are high (110v AC or houshold appliances, microwave,coffeepot space heater etc
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Old 02-18-2010, 11:58 AM   #7
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Re: Alternator rating

If you're going to go the second alternator route I would look at getting a 24 volt alternator and inverter combo. The amperage will be half of what it would be at 12 volts to deliver the same wattage which means way less heat so you can get more output from a given form factor.
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Old 02-18-2010, 01:08 PM   #8
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Re: Alternator rating

I am going to do a little searching at the local auto recycling yards and see if I can find a dual alternator setup. I would just need the brackets and the second alternator wouldn't I? There is definately enough room under the hood for it. In fact there is what appears to be an emmissions air pump that is not hooked to anything, just a belt, sitting there taking up room and stealing a little bit of power. Maybe I could just replace that with the second alternator and call it good!
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Old 02-18-2010, 10:51 PM   #9
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Re: Alternator rating

But...your 24 volt battery bank is going to consist of either two 12 volt batteries in series or four 6 volt batteries in series, correct? In the even of primary alternator failure you could tie your 12 volt mains into either a single 12 volt in the system or a pair of the 6 volts. It wouldn't be an efficient way to go about things, but it most certainly would 100% functional in maintaining 12 volts in the system and recharging the batteries.

My engine doesn't need electricity once it's running so I guess I've never worried about it.
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Old 02-20-2010, 10:29 AM   #10
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Re: Alternator rating

Quote:
Originally Posted by the_experience03
If you're going to go the second alternator route I would look at getting a 24 volt alternator and inverter combo. The amperage will be half of what it would be at 12 volts to deliver the same wattage which means way less heat so you can get more output from a given form factor.
Agreed! I have done exactly that in a bus. 24 volt alternator/inverter for the coach, and the stock 12 volt alternator doing what it did from the factory. And the above poster is correct that you can run 12 volt appliances from either of the 12 volt batteries you have wired in series. In a perfect world, you could connect lots of 12 volt items to the batteries, with approximately half the load form one battery, and half the load from the other.
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Old 07-13-2010, 11:54 AM   #11
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Re: Alternator sizes

Alternator size is determined by the unit output in voltage and amperage. But another factor is to be considered. The charging factor to be considered is the charging time. Most high output alternators are expensive, large and difficult to remove and replace. Usually a 125 amp alternator will do a great job of keeping a battery bank charged and powering a AC unit. In my Crown bus, there is a single prestolite 125 amp alternator and several 12/120 inverters. My battery bank is just a pair of Walmart 950 cca batteries. A charging system with a 125 amp alternator will recharge any battery bank easily and as well as a 200 amp alternator, but takes a little more time. Mostly the time factor is not important. Usually large frame alternators have large bearings and last longer.
The most important factor in any wiring is wire size and good firm connections. I prefer using a 100% duty cycle relay to manipulate electrical power. There are many good relay available. Frank in Idaho
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