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Dual Alternators?

jlysaght3

Basic User
City
West Deptford
State
NJ
I wasn't sure if this should go under Electrical or Engine, so please forgive me if I placed this in the wrong forum.

I want to mount a second alternator on the engine to completely isolate my auxilary battery (for the winch, aux lights, cooling fans, radio/CB, 12V power ports) from my primary battery (ignition, lights, base OE electrical system). I would be going with a slightly more powerful alternator for the aux system (a 73-amp from a late-80's Ford, also 3-post and understand I would need a second voltage regulator).

I do have a battery isolator, but I won't be driving the Jeep enough to fully recharge the primary and secondary battery, especially if I end up using the winch off-roading (which I intend on doing) off of the miniscule 40 amps the standard alternator generates. I also see the redundancy value in two separate charging/battery systems, and plan on using a set of Anderson plugs and leads to provide an easy option to 'jump' the two systems together in the event of one system's failure.

Has anyone installed a second alternator on a Scrambler with power steering and no A/C? Does anyone have any information that may help me achieve this?
 

Ghostwave

Scrambler Junkie
Lifetime Member
City
Kirkland
State
Wa
Most of those dual alternators are false economy unless you have really invested in the end to end system.

I would go with a large frame single alternator of no more than 100 amps, while ensuring that the belt ratio did not overdrive it. More than 100 amps is too hard on both the alternator but more so the belts. If you want to maximize your charging, your money is better spent on more sophisticated charge controllers than second alternators and the associated drag and losses. Even if you size the alternator larger, your batteries will typically limit the charge current to no more than about 25% of their amp hour capacity anyway. For a typical group 31 this means you are stuck well under 50 amps max anyway. One way to increasse the charge rate is to increase the size of the battery, which is one thing that you actually achieve when your batteries are paralleled. Agm batteries also have an advantage over flooded lead batts on acceptance rate, so that's another option to help.

Adding a second alternator is as simple as cobbling the brackets and belts. If you do go that route, you will want to keep with your plan to isolate the batts. Two alternators trying to charge the same bank will compete with each other and actually not output the capacity you would think. The downside is that you are back to isolated and therefore smaller amp hour banks, thus your charge current limitation is the size of the batteries more than the size of the alternators. Again, synchronized charge controllers exist, but they are not cheap and you will wind up needing three of them, not two. One for alt1, one for alt2, and a center fielder to synchronize the alternator fields.

In a practical sense, it is hard to beat a single continuous rated alternator (large frame truck alternator), into dual banks with automatic isolation (combiner). External regulated with a good regulator, not a fifteen dollar schucks special. Good external regulators can set you back a couple bills, but if you want to maximize chage current, that's what you get. Also, high quality Agm or even spiral batts will take a quicker charge, but flooded deep cycle lead acids will work fine and can't be beat for economy. Cheap batts are just that. Either the batt was built with deep cycle plates or it wasn't. You get what you pay for.

All of this assuming you want things to be reliable when off road. If you don't mind frying things and buying junkyard replacements, you can get away without the investment, but you can't do so reliably. A two dollar switch and a rheostat from RadioShack will have your alternator putting out full current, bypassing a cheap voltage regulator but it won't stop you from boiling your battery. Charging is still best left to automation.
 

Breal

Scrambler Junkie
Lifetime Member
City
Jackson
State
WY
Most of those dual alternators are false economy unless you have really invested in the end to end system.

I would go with a large frame single alternator of no more than 100 amps, while ensuring that the belt ratio did not overdrive it. More than 100 amps is too hard on both the alternator but more so the belts. If you want to maximize your charging, your money is better spent on more sophisticated charge controllers than second alternators and the associated drag and losses. Even if you size the alternator larger, your batteries will typically limit the charge current to no more than about 25% of their amp hour capacity anyway. For a typical group 31 this means you are stuck well under 50 amps max anyway. One way to increasse the charge rate is to increase the size of the battery, which is one thing that you actually achieve when your batteries are paralleled. Agm batteries also have an advantage over flooded lead batts on acceptance rate, so that's another option to help.

Adding a second alternator is as simple as cobbling the brackets and belts. If you do go that route, you will want to keep with your plan to isolate the batts. Two alternators trying to charge the same bank will compete with each other and actually not output the capacity you would think. The downside is that you are back to isolated and therefore smaller amp hour banks, thus your charge current limitation is the size of the batteries more than the size of the alternators. Again, synchronized charge controllers exist, but they are not cheap and you will wind up needing three of them, not two. One for alt1, one for alt2, and a center fielder to synchronize the alternator fields.

In a practical sense, it is hard to beat a single continuous rated alternator (large frame truck alternator), into dual banks with automatic isolation (combiner). External regulated with a good regulator, not a fifteen dollar schucks special. Good external regulators can set you back a couple bills, but if you want to maximize chage current, that's what you get. Also, high quality Agm or even spiral batts will take a quicker charge, but flooded deep cycle lead acids will work fine and can't be beat for economy. Cheap batts are just that. Either the batt was built with deep cycle plates or it wasn't. You get what you pay for.

All of this assuming you want things to be reliable when off road. If you don't mind frying things and buying junkyard replacements, you can get away without the investment, but you can't do so reliably. A two dollar switch and a rheostat from RadioShack will have your alternator putting out full current, bypassing a cheap voltage regulator but it won't stop you from boiling your battery. Charging is still best left to automation.

http://stream1.gifsoup.com/view4/4370079/old-school-debate-o.gif

:)
 
Last edited:

Polarfire

Jeep Aficionado
Lifetime Member
City
Columbia
State
MO
Whoa.
f179b620d38df628bda04458b90fb33a-1.jpg
 

Ghostwave

Scrambler Junkie
Lifetime Member
City
Kirkland
State
Wa
Sorry guys. I kind of take it for granted, my main hobby I go to work for every day to support is cruising the inside passage on a boat. If you want to spend a month or two cruising you get very dependent upon your batteries and charging systems because without them you can't stay at anchor for a few days. It's also rather critical to know that you can always start your engines. Isolation is a good principal for any activity where you can't just call for help and need to be self reliant. Good idea for jeeps too.

On my boat, I have two 100 amp alternators feeding a single 1500 amp hour battery bank, with a fully isolated single starting battery for both engines.

My jeep has two group 31 batteries and a single 100 amp alternator. One battery is dedicated to the winch.
 

tower210

Scrambler Junkie
Lifetime Member
City
Olathe
State
KS
This thread should be made "sticky" under the topic of electrical charging / storage / discharge.

What does the external regulator do that is so superior to an internal regulator?
 

don87401

Original Owner
BENEFACTOR
Gold Member
Lifetime Member
SOA Member
City
Farmington
State
NM
I just use two Promariner chargers mounted on the fender, one for each battery and the 110v side wired to a flush mount plug in the fender, when the Jeep sits for more than a few day I just plug the extension cord in the fender plug and keep them charged.

I have a TJ fuel door covering the flush mount plug on the fender. You can see where it is mount in the upper picture by my name, it's the dot on the black stripe.
 
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