Dual Alts or one big one?

[Spargman]

I'm in the process of designing my electrical system and was wondering what is "better", dual alts or one big one?

Some things I'm thinking about:
-Will two alternators running the same combined amperage as one big (i.e. 220amp+) alternator cause more or less strain on the engine? More strain equals less mpg
-If I run two alts, I can run two completely separate electrical systems and isolate the truck from all the accessories. This saves money on a dual battery kit and provides a nice backup should something fail
- If one alt fails in the dual setup, I can easily find another one of equal power pretty easily...if the big one fails, that's a mail order job (but I would have a smaller backup with me)

Out of the above, my biggest concern is the extra strain on the engine and reduction in mpg. What do you guys think/recommend?

 

[locrwln]

I will say this, when GM and Ford want to add juice to their electrical system for Ambulance/Severe Duty applications, they use two alternators. I would say that if they feel that two is better than one big one, then maybe it is so. I can't imagine for the manufacturers that building a two alternator system is cheaper than one big one, so they must not trust or expect too much out of one big alternator.

What kind of draw are you expecting to need? I had the regular old 65 amp single alternator on my 80 with a dual battery system and it was pretty much worry free. Mine was set up with a heavy duty main and a smaller back up battery. My current Duramax is a single alt/ two battery system as well. I have ran my Waeco all night long, ham radio, camper lights, etc. and got up on 20 degree mornings and hot mornings and the trucks have started with no problems.

I think unless your vehicle is set up for it, dual batteries will be easier to install than dual alternators.

Jack

 

[Spargman]

Jack,
Thank you for that information.

I will have two batteries on this rig and one on the trailer...but my 140amp is having a problem keep up and I have not yet installed the offroad lights. As a side note, I will be testing the alternator's output next week b/c I have a feeling it's not putting out 140amps anymore.

My draw:
Electric Fan: 30-50amps
Sounds System: up to 50amps
HID Lights: 5amps
Offroad Lights (front): 5-10amps
Offroad Lights (bar): up to 20amps
Offroad lights (rear): 5amps
Fridge (not sure)
Trailer (not sure)
Draw from OEM accessories (unknown)

I also have my misc gear similiar to yours...GPS, Radio, Vehicle Monitoring equip, etc

 

[Scoutn79]

Your worry about MPG is correct but likely not as much as you think. Since most times you aren't going to be requiring full loads on you system the alternators will be free wheeling except for just maintaining what the vehicle uses for operation. It takes a certain amount of hp to produce a given amperage whether using one alt or two, the difference is the drag from the second alts belt and bearings. So I think from that stand point the difference is negligible. Redundancy is nice to have so from that standpoint two are better.
May I offer up what I did. I run a single 150 amp alt for charging the batteries and a second that has been modified as a stick welder BUT I keep all of the removed electronics from the welding alt so I can reinstall them if need be if the main alt fails. The two alts have the same basic case and mounting so if I need to swap the welder into a charging position it will just bolt right in.
Darrell

 

[Scoutn79]

WOW are you using that many amps all the time? In this case two alts might be your best bet. You may want to use smaller pulleys on you alts also to bring up the speed and thus the output. Looks like you need about 100 amps at idle and a single alt won't do that, if you can get two to put out 90 amps or so at idle you might get what you need. IE my single CS144 will put out 45 amps at 650 engine rpm but two would provide 90 amps at idle. You might consider using an engine driven fan and getting rid of the amp hungry electric ones except as a back up if your engine has a tendency to overheat. Then you can take out there demand from your needs.
Darrell

 

[Spargman]

Thanks Darrell.

For clarification, are both of your alts mounted to the engine at that same time? (it sounds like they are, just checking)

What about the drag caused by the modified angle of the belt due to the second alt...I'm not as strong in alts and the hp they require, so bear with me

 

[Spargman]

WOW are you using that many amps all the time? In this case two alts might be your best bet. You may want to use smaller pulleys on you alts also to bring up the speed and thus the output. Looks like you need about 100 amps at idle and a single alt won't do that, if you can get two to put out 90 amps or so at idle you might get what you need. IE my single CS144 will put out 45 amps at 650 engine rpm but two would provide 90 amps at idle. You might consider using an engine driven fan and getting rid of the amp hungry electric ones except as a back up if your engine has a tendency to overheat. Then you can take out there demand from your needs.
Darrell

No no, not all the time...this is my max load and it drives me nuts when voltage drops b/c of the potential damage to my electric components. I actually am not a big proponent of smaller pulleys b/c of how hard the alt will be working on a more regular basis. I'd rather have a bigger alt working @ 50% capacity then a smaller alt working @ 90% capacity.

 

[Scoutn79]

Yes both alts are mounted at the same time.One hp is 746 watts or about 57 ampos at 13 volts. So 150 amp alt would use about 3 hp from your engine. I am not sure what you mean by the modified belt angle?
Darrell

 

[Spargman]

The belt angle would change b/c instead of the belt going near vertical to pick up the OEM alt (with the blue front), it would now have swing out a bit wider for the attachment of the 2nd alt (the lower one)...I wonder if that change alone would suck up any hp.

(an example below, not mine)

 

[Scoutn79]

another thing to consider is the cooling of the alt...look at the alt in a semi and it has a HUGE case and fan heat will kill the electronics. So thinking of this two alts would have better cooling per amp ( twice the cooling) than one of equal size. My comment about a smaller pulley was just to get more amps at idle. Typically high amp alts don't put out many amps at low rpm.
Darrell

 

[94Discovery]

On an ambulance the second alternator is to provide electricity to all the outside lamps and electrical equipment inside the cabine related to life saving .
You can modifie your cs 144 to be more heavy duty and keep a steady 14.7 volts ,make sure that all your off road lights are converted to hid you will cut the power in half ,use the 35 watts .
Make sure that the cable that connects your alternator is big 4 to 2 awg ,do not use amplifier cables.
As for mpg 1 alternator is better less mechanicle drag ,2 has more drag but you have redundancy .
http://www.alternatorparts.com/

 

[Patman]

Packing more amperage output into the same size housing will always be less reliable (all other things being equal i.e. quality of components, etc)

Theoretically the draw will be the same weather you use two or one alternator at the same load. This does not take into account pulley drag, alternator mechanical advantage in the design, etc. (but that's just getting anal)

If you have the room, a second alt is a great way to build redundancy into the system. Weather you set it up as a completely separate system, or incorporate it into the current set up. I would still run dual batteries. To get it to work right, is by no means simple, but not too bad.

One thing to keep in mind, if you have them completely separate, and one fails. You loose all the items on that system. So make sure to design the system so that everything can run off the one battery/alt and be switched over simply in the field.

 

[Scoutn79]

One my Scout the alts are mounted in different locations using different belts. The nice thing about the Scout is that the factory air pump (not installed) uses a bracket that allows the installation of a Delco Si CS style alt and the pulley that drove the air pump now does the welder...Very simple conversion for us IH owners.
Darrell

 

[Buliwyf]

One alternator will be more fuel efficient.

Two alternators are used because two cheap alternators are cheaper than one high quality, high performance alternator. GM and Ford aren't allways concerned about doing things the right way. Many ambulances only have one alternator now. I haven't seen dual alternators on anything in a really long time.

You can order a Ford with twin 160a cheaper alternators or a more expensive single 200a alternator.

There are some very high quality alternators out there right now. That's the route I'd go, depending on your truck, sometimes you're better off with your stock alternator.

250 amps max /165 idle for my truck= $375.
http://www.qualitypowerauto.com/catalog.php?item=137&catid=45&ret=catalog.php?category=45

DB alternators has a good rep as well:
http://www.dbelectrical.com/
$267
http://www.dbelectrical.com/c-6907-250-amp.aspx

I get my stock alternator rebuilt and I keep it under my rear seat as a spare. Generally late model Ford and GM alternators are quite good. I'll likely find a core that matches my stock alternator in a Pick/pull yard and have it rebuilt. And my local rebuilder rebuilds alot of my alternators for heavy equipment and life safety fire pumps and generators.

He usually rebuilds them even better than stock. (based on experience, nothing scientific, just keeping said machines running when the battery cable falls off the battery, or the wall charger nukes and takes a cable with it, or the starter stays engaged and running whenever the machine is running, etc. etc.) But after disassembling a few of his alternator after my controller failed and nuked the bendix gear by trying to restart an engine allready running, i've found that his work inside a starter, like the big Bosches used on Semi-trucks, is as good as my own.

My point, if you don't need mondo amps: Just get a quality alternator rebuilt right and use a large 2g or 4g cable. Don't underestimate your local rebuilder.

Just watch out for undersize pulleys that overdrive the alternator. The last thing anyone needs is a serpintine belt slipping on thier water pump if their tensioner can't take up the slack. Haven't seen that yet, but weirder things have happened, just keep a close eye on stuff like that.

Battery isolator for multi battery systems:
http://www.dbelectrical.com/p-2271-120-amp-3-battery-isolator-marine-sure-power-1203.aspx

 

[1911]

Mean Green makes a good 200-amp alternator also:

http://www.mean-green.com/

I'm running one in one of my trucks and like it real well: