New 100 Amp Alternator from Battery Shack

Just ordered a 100 amp alternator to swap with my stock (51amp) on my 01 X-star. Questions:
Do I need to upgrade the wiring to the fuse and to the battery? Do I need to replace the fuse if the alternator and battery feed are both on the load side of the fuse?

Any install tips would be greatly appreciated. I plan on tackling this job this weekend. Thanks guys!

Gary: Here is a link to my website, http://svtsupercharged.com/gallery/alternator Where you will find How to photos and answers to your wire upgrade questions. I did the same thing from Battery shack.

in short you will need a 100 amp breaker and a upgraded wire from the back of your alt to your bat's

Hey Grant, I looked at your site too when I did my alternator upgrade, but there's some wiring difference(s) between the way your PCM electrical system in your Nautique is done, and the way my Indmar is done. I think Gary's gonna see this with his Mastercraft as well. I attached a simple graphic as to how I eventually handled the wiring on my Indmar. I essentially ran 'new' wiring along the same loop that was in the boat from the factory, and also added a main breaker with sufficient amperage to handle the new alternator. Just thought this info might also be helpful to Gary.

If you are going to install a circuit breaker on the alternator circuit you may want to stop and think about what it is you are protecting and make sure the the circuit breaker is sized appropriately.

If you are trying to protect your boat from an electrical fire then you want the circuit breaker mounted as close to the battery as possible (the battery is the only source of enough current to melt a #4 wire). If you are making the connection at the starter then the circuit breaker will be mounted on or near the engine and thus has to deal with the engine heat. Be very careful that the circuit breaker is not subjected to high heat due to the proximity of exhaust manifolds.

What you will be protecting in this case is the run of wire between the circuit breaker and the alternator. If this wire should become disconnected from the alternator, or you dropped a wrench onto the alternator terminal, the circuit breaker will save your boat. The circuit breaker won't do anything for you if wire between the starter and the circuit breaker shorts out.

The size of the breaker needs to be selected so that it protects the wire. Do NOT attempt to protect the alternator with the circuit breaker. Anchor wire, a common source of marine wire, rates their Size 4 AWG wire, used in the engine compartment (high heat) to be capable of handling 136 amps. The circuit breaker MUST be capable of handing the full alternator output, with appropriate derating for the heat. I would recommend a breaker between 120 and 135 amps.

Do NOT put a 100 amp breaker on a 100 amp alternator, you will be just asking for trouble and likely blow your alternator. The alternator can survive an overload for a considerable amount of time. What it won't survive is having the circuit breaker open while it is pumping out 100 amps.

If you are thinking that the circuit breaker will protect your expensive alternator, then don't install it, as it will likey do exactly the opposite.

Rod McInnis

You guys all amaze me. What a wealth of information. Very cool!

Hey Rod I have some questions.

I have a 165 Amp Alternator.
I have a 150 Amp breaker located about 3 feet from the Alternator. The 4 gauge goes form the output of the alternator to the breaker. From the 150 Amp breaker I have a 4 gauge wire that runs up to the battery's

I havent had any problems BUT I dont want to either.
The way I see it is you want the breaker as close to the alternator as possable, That way if somthing goes wrong you arent melting wire all the way the length of your boat where your battery's are located. I also ran a slightley smaller breaker than the Full output of the Alternator that way if things were going wrong the circut breaker would pop early before the Alternator went fully crazy.

What I have heard the most common problem is. When a alternator blows up it goes dead no Voltage in rare cases It over charges and burns up your system. I installed a breaker for that reason The over charging. They guy from the battery shack (bill) said not to even worry about a breaker that it was over kill and that in his 25 years of building HO marine alternators he has only seen a overcharge melt down 2 time from some one eles alternators, I would rather be safe than #3.
What do you have in your Boat?

Mike why did you run the output of your alternator into your starter?. I know thats the way it's hooked up stock. Did you not want to intsall a stand alone wire from the alternator to your battery's. I just dont know why you would want all the current to run threw your starter??

Rod... As Grant kinda eluded to, the breaker location in my boat was there stock (50amp). I simply upped the amperage on a new breaker and installed it in the stock location. The breaker I'm using (with my 100amp alternator) is a Blue Sea 135amp unit which was sized speciafically for the 4awg wire I'm using, and adjusted for heat.

Would I be better off wiring my setup like Grant did, and placing a breaker instead on a lead between the hot leg on the alternator and the positive battery terminal connection? If so, can you sight benefits to doing it that way. I'm always open to ideas.

Grant... I don't really have a good answer for ya as to why I wired my boat the way I did, other than because that's the way it was wired by Indmar. I'm not adversed to trying something new, as I noted in my previous post to Rod, I just need to know specifically what it might effect in my wiring and if I'd have any other issues to address. So far, the way I've got her wired up right now hasn't caused any issues, but if there's a better way, I'm all ears.

Mike...I've done the exact thing you have done. I also have an Indmar and wired with 4g to the starter and left the stock wiring from the starter to the batteries.

Grant...Are you saying to bypass the wiring to the starter and go straight to the batteries, then have the stock wiring still in place going to the starter?

Im not saying my set up is better or do it my way type of thing. I was wondering myself, I ran the 4g from my breaker to my battery because thats the way I was told to do it, from Bill @ the Bat shack said. I to wonderd about running my wire just like Mike did, but didnt want to have a problem and have someone say I told you so.

I wanted to hear from Rod and have him explane more. The way I see it power comes from the Alternator. Thats where the 165 amp's that can cause dammage can start and end. I also see a need for a inline fuse from you battery's to the battery selector switch so there is protection on both end's. From what I undersatnd about batterys and alternators is this how it was explained to me.

Your system cant run from the alternator alone. the power need to go from your alternator to you battery's befor you can tap into it. So it very hard to fully use all 165 amps my alternator can put out. I was told when your system is dead you are pulling from power that is beeing shot up to your battery's and your ststem sucks threw your batterys, like a straw. So you cant ever pull directley from your alternator. Example you run all your batterys dead. You start you boat up and your alternator sees all your batterys are dead, It starts working overtime full juice to what ever your batterys will take. I have 4 Blue top Optims they will only take a 20 amp charge each so if all my batterys were dead the most my alternator would/could be pushing would be 80 amps, Even if I have a 165 amp alternator the alternator will only feed what it can to what is ever on the other side, Its like trying to fill a watter bottle with a 5 gallon bucket. You might be able to push all that amp's but your batterys can only accept a sertan charge. So the way I saw it was that the 4 Gauge wire running thew the 150 amp breaker would only ever see 80 -100 amps at most.

This is how it was explained to me from both the battery shack and "Balimar" maker of High output Marine alternators, I spent about 1 hour on the phone with them they were so cool and broke it down for me. I had one way I thought it worked and couldnt understand it. They explained it over and over till I got it and am explaining it to you. You might ask whell what about if all your batterys are dead and you are still running your lights and your stereo and your heater and every other accesory you can get a hold of your saying how does everything work if it cant run directley of the alternator, I guess the only way to find out is disconnect your batterys and try it your self, "No thanks" I trust you!

Grant:

You have a circuit that has an alternator at one end, a battery at the other end, and somewhere in the middle it branches out and goes to all the circuits in the boat. Current can come from either end.

The alternator can pump out the 100/150/165 or whatever amps. That's all it can put it out. You could short the output of the alternator and it might put a out a little more than what it was rated for but not by a whole lot.

The battery, at the other end of this wire, can put out thousands of amps into a dead short. A typical starting battery is rated in "cold cranking current", which is a measure of how much current it can supply to the starter at a voltage of 10 volts or more. The optima battery has a CCA rating of 750 amps. If it can put out 750 amps to the starter it could put much more into a dead short.

A blue top Optima battery has an "internal resistance" of about 0.0028 ohms (from their web site). This is what ultimately limits the current, either charge or short. If you short the battery directly across its terminals with a big enough wire (like a big crescent wrench), you have ~12 volts limited by a resistance of 0.0028 ohms, which would allow 12/0.0028= 4286 amps to flow. That's a lot. That will melt a crescent wrench.

When you are charging the battery, the internal resistance also limits how much current it will take. If the alternator is putting out 14.5 volts (the maximum it should put out) then you have a difference of the "rest voltage" of the battery and the charging voltage limited by the internal resistance. If the battery is nearly dead then its rest voltage might be 11 volts. 14.5 - 11 is 3.5 volts. 3.5/0.0028= 1250 amps.

I would say that if you connected your alternator up to a completely dead optima battery that it would take the entire output. It wouldn't take long, however, before the rest voltage rose to the point that the battery wouldn't take many amps.

Your system could run from the alternator alone, but it is a very bad idea to try and do so! Normally, when the engine is off any electrical demand flows from the battery. Once the engine starts the alternator begins to produce power. At idle speeds, especially with a small alternator, the load from the stereo, lights, etc. could draw more than the alternator is producing. In such a case the alternator will provide what it can and the rest comes from the battery.

When the engine RPM increases the output of the alternator may also increase. Once the alternator is able to produce more than the load is consuming the excess can flow into the batteries. As the voltage increases the voltage regulator backs off the alternator output to keep things under control.

Now, let's say you started your engine and accidently turned your battery switch to OFF (BAD BAD BAD!!!!)

For the moment, let's assume that the battery was fully charged when you did this an there wasn't much current flowing to the battery, otherwise this would be a very short discussion.

When you shut off the battery you still have the alternator providing the current to run the boat, just like it was before. You have actually reduced the load on the alternator slightly because even when fully charged the battery would still draw some. Everything is fine and dandy.

Now let's say you are running along, playing the stereo and drawing 50 amps from the alternator. You are still okay, as long as the alternator can provide 50 amps. You throttle back, however, and suddenly the alternator is not able to provide the 50 amps at idle speed. The voltage drops. Fortunately, the stereo gets upset that it doesn't have enough voltage and shuts off, dropping the load and keeping the engine from dying. You get pissed at your stereo for shutting off.

Now let's say you are running along at speed and it is getting dark. You decide to turn on your tower lights. CLICK. Suddenly, the alternator is being asked to supply a hundred amps to warm up cold lamps (inrush current to an incandescent bulb is incredible!). The alternator can't provide this current and the voltage drops. Your boat starts to miss badly because of the low voltage. Fortunately, the tower bulbs are able to warm up on the low voltage and don't draw as much any more. The voltage comes back up and the engine settles down. You are still okay. For now.

The fact that your boat just stuttered bothers you. You realize that this happened just as you turned your tower lights on, so you decide to try it again. Here is where troble starts.

You turn the lights off and CLICK, you just dropped 50 amps off the alternator. Sorry, the alternator can not react that fast. Just before you clicked the lights off it was producing, say, 55 amps. 50 went to the lights, 5 went everywhere else. Well, once you turned the lights off there is 55 amps that has to go someplace. If you had a battery in the circuit it would go there, but you turned the switch off so it has to go someplace else. POOF. All your dash lights get really bright for an instant and then they burn out.

You turn the tower lights back on again and the motor coughs even worse than before. You turn the tower lights off again and those 50 amps has to go someplace, except now you no longer have the dash lights to absorb it. It has to go someplace else.

Gentlemen, an alternator can produce any voltage it needs to in order to drive the amps it wants. If it can't drive the current someplace, the voltage increases to the point the diodes in the alternator fail and your alternator is destroyed.

The battery is an essential part of the circuit. It is what supplies extra current during peak demands and it is what absorbs excess output from the alternator.

This is why you never, ever turn the battery switch to OFF while the engine is running. One exception: you can have an arrangement where the battery switch has a field disconnect to the alternator. This turns off the field drive to the alternator, thus shutting it down a split second before the battery contact opens. Most boats do NOT have this feature, so unless you are sure you have it, don't assume you do.

Okay, now let's look at this circuit breaker you have. If the alternator can put out more current then the breaker can handle, you could pop the breaker. By definition the alternator is cranking out a lot of amps when this happens. The probability is that if the circuit breaker opens, your alternator is toast.

Rod

Rod... My God, I learned more in your last post than I have about any one topic in a long time. But, and there's usually always a "but", can you critique Grant's and my wiring setups and comment on the pluses and minuses of each. I still wanna know if there's something in my wiring setup that could be handled differently / better. I think that's what Grant is also after in his last post.

OK, I'm more confused than ever. I'm also looking at upgrading my alternator, but now I don't know to run the wire back through the starter or directly to the batteries. Also, should the wire from the starting battery to the starter contain an inline fuse at the battery?

Guys, great information, however, I am with Tim...somewhat confused... should I run the 4awg to the starter or directly to the batts. I am leaning towards Mike's install diagram-following the stock set-up and running 4awg from the output from the alternator to the new breaker (135amp) and then from the load side of the breaker, 4awg to the starter. If I do it that way, based on Rod's comments, I still need some type of inline fuse between the batts and the battery switch. What amp fuse should that be? If I ran the 4AWG all the way to the front of the boat where the batts are, what would I do with the existing wiring to the Starter? One more thing, what are the small wires (red and black) that attach to the batteries in the battery compartment? They have a 25 amp in-line fuse, must be for accessories?? Thanks guys, this has been a HUGE help!!!

Gary

On my set up, the only other wires that directly attach to my batteries are for the bilge pump.

Tim

Hey all... This thread got me so interested in this issue that I decided to take some time at lunch today and call Indmar's tech support to see what they'd tell me. Here's what I learned from my phone call.

Indmar has a 70 and a 90amp alternator upgrade kit available for purchase through your local dealer. The kits are pricey at around $300+. With either kit, you get the alternator, a new set of mounting bolts, and here's the kicker, a new 'harness'. The new harness is essentially a single larger diameter wire (he thought 8awg) with ring terminals on both ends. Indmar's instructions that come with the kit instruct the installer to:

1-Remove the existing power wire (orange) connected to the output leg of the alternator, the exiter wire (purple) and ground wire (black).
2-Remove the existing alternator and replace it with the new one using the supplied hardware.
3-Reconnect the ground (black) and the exciter (purple) wires to the new alternator. Wrap back the original power wire (ornage) as it will not be used.
4-Take the 'new' wiring harness provided with the kit (remember, a single larger gauge wire) and connect it to the output leg of the alternator. Then run it around the rear of the motor and 'past' (i.e. not through) the existing main breaker on the back of the motor, and connect the free end to the back of the starter.

Of course the instructions assume the 'new' alternator is a three wire setup. My new alternator is internally grounded, so I didn't have to use the black ground wire that was connected to the original alternator. If your new alternator is self-exciting, you won't use that wire (purple) either.

According to the tech rep, no breaker between the alternator and starter is necessary. And as to the issue of running a larger amount of amperage across the back of the starter, he said that the starter draws approximately 125amps from the battery at startup, so my 100amp alternator was not going to do it any harm. Given this wiring setup, there will be no breaker on the main power loop (i.e. from the alternator, to the starter, to the positive terminal on the battery), which Indmar feels is ok. He also mentioned that the stock 50amp breaker in my boat was actually wired downline of the main power loop we were discussing, and as such should absolutely be left in place as is. This 50amp breaker only protects the main wiring harness from an overload, nothing else.

I further asked whether there'd be any advantages to installing the new 135amp breaker I have between the alternator and starter to protect the starter and/or battery from damage if the alternator went bad. The Indmar rep really didn't feel there was any advantage to installing it, but it would not hurt anything if I did. He said the likelihood of the regulator in an alternator failing in such a way as to allow an unregulated flow of current to the starter/battery is highly unlikely. It would also be immediately evident throught the charge meter on the dash. According to him there have been virtually no cases of this happening.

The result of my conversation today was that I have some issues to address in my wiring setup before I go boating again. Namely, I need to put back the original 50amp breaker at the rear of the motor, and to make up a 'new' 4awg wring harness to run from my alternator to the starter. And yes, because I have it (and I'm hopelessly anal) I'm going to install my new 135amp breaker in my 'new' harness between the alternator and starter. It can't hurt to have all the prtoection you can.

Anyway, I hope this helps further explain what needs to be done to install a new high output alternator. I know I learned a lot today.

Wow Rod, that was a very informative post. At first I was uneasy with the statement (not made by you) that a engine can't run without a battery connected, but after your explanation I can understand why an engine should not be run without a battery.

Mike:

I basically got the same information as you after talking to our department mechanic today. He too confirmed that running the extra amperage across the back of the starter would be perfectly acceptable.

Tim

Mike, that info was great. Makes the weekend job very simple. Thanks for you effort with Indmar...sounds like we all learned a lot. Thanks again everyone. The new alternator goes in this weekend.

Glad I could be of help. And like you, I'll be doing a little re-wiring myself tomorrow evening after work, so I can hit the water this weekend with no worries.

I'm hoping for clarification on one item. Can an alternator power your system, or is it just there for charging the batteries?

Grant said that it cannot add power to your system, and that it is actually only the batteries that can power your system. He mentions that the alternator can only charge at about 20 amps per battery. This would lead me to believe that a 100 amp alternator running at full power is only being fully utilized if you have five batteries.

Rod on the other hand says that a 100 amp alternator can be used for power a system and the batteries only get a charge if there is left offer amps available after the system is powered.

Who is correct here? Thanks!

I found an article on the West Marine Advisor page that leads me to believe that alternators can actually run DC loands (stereo) and are not just actually for charging the batteries as Grant (per Battery Shack) had implied. Here are some excerpts from the article and the full link below:

From West Marine...Alternators use rotational energy from your boat's engine or genset to provide electrical power that can be used to charge batteries and run DC loads on board. They are generally the most potent source of charging current on board, producing up to 200 amperes of current.

From West Marine... Your alternator should be roughly 25-40% as large as your battery capacity for effective charging. If you regularly operate large DC loads underway, it could be even larger. Small-frame alternators range from 75 to 150 amperes, while large-frame alternators produce 130 to 275 amps.

From West Marine...It's a waste of money to buy a large alternator to charge a small battery bank, unless you have large DC loads operating. Batteries need to have sufficient acceptance to utilize the alternator's capabilities.

Full Article: http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorDisplayView?storeId=10001&langId=-1&catalogId=10001&advisor=selectingalternators.h tm

An alternator has a limit on the number of amps it can put out. One limit is the rate at which the alternator is turning, so if the engine is idling you simply won't get as much out of the alternator as when the engine is turning at cruising speed.

The second limit is a function of how the alternator is built. If the windings are made with larger wire and more current is pumped through the "field" the alternator can have a greater output. This, of course, requires more torque on the shaft, which may require additional belts.

The alternator has a voltage regulator that is used to cut back the amount of current the alternator will produce when the voltage gets too high.

The alternator doesn't know and couldn't care less where the current goes. If it puts out more amps than the load is using any excess will flow into the battery. If it puts out less than the load is using then the battery will supply the difference, at least until the battery goes dead.

The standard voltage regulator creates a voltage/current curve. As the voltage increases, the amount of current that the alternator puts out will decrease. With this type of regulator you will only get full output from the alternator if the batteries are very low or if the load is demanding all that the alternator can provide.

If you measure the voltage you can get a pretty good idea of what is going on. Here is a quick table of voltages and what it means: (all assuming that the engine RPMs are 2000 or above)

14.2 volts: Your battery is fully charged, there isn't much load on and the alternator should be loafing

13.8 volts: Battery is fully charged and the alternator is providing a modest amount to the system

13.2 volts: The alternator should be putting out its full capacity. The battery is getting a significant amount of charge and may be fully charged.

13.0 volts: alternator is having difficulty, the system is demanding more than it can provide. The battery is getting some charge, however, so there is not a problem.

12.8 volts: Full output on the alternator, most of which would be going to the load and very little of it is being used to charge the battery.

12.4 volts: The alternator is not quite keeping up, the battery is either somewhat discharged and/or is providing some of the current to the load.

12.0 volts: The alternator is definately not keeping up. The battery is not getting charged, and may be discharging at a significant rate.

Less than 12.0 volts: you have serious problem and you had better cut the load or fix the alternator real soon or you will end up with a dead battery.

I once started a dodge dart then disconnected the battery and pulled it out of the car. I then drove it to the junk yard(without a battery). Im guessing the cars electrical load was not too high that at idle the voltage dropped below what the ignition needed to run and somehow did not cause the alternator to run away with the voltage and damage the diodes (maybe since it was an external regulator it was not succeptible to this type of failure).

You can get away with disconnecting the battery (or switching it to OFF) if the current going into the battery was really low. If the battery was pretty close to fully charged and the engine was at idle then there probably wasnt' much current flowing.

In my younger days I used to do a lot of off roading. I was down in Baja once and driving the back roads at night. I had three sets of lights: standard headlights, bottom fog lights and roof mounted super brights.

I was driving along and a car was coming the other way so I turned off the top lights. I noticed a flash of light from the dash when I did this, and then all the insturment lights were out. When the car passed I turned the top lights on, and the engine just about died when I did.

I thought that was odd, so I turned the top lights off again. Another flash of light, and I could tell that I had lost either the fog lights or the regular headlamps. I turn the top lights back on and the engine cuts out for several seconds.

I should have been smarter than this but it was late and I was tired. I turned the top lights off again. Another flash, and I was in total darkness so I turned the top lights on again.

The engine cut out, and I almost came to a stop before it started to fire again. The top lights started off really dim and came up to the expect brightness as the engine picked up.

I left the top lights on and didn't shut the engine off until I got to where I was going that night. Pull into camp, turn the key off, then back on: nothing.

One of the battery cables had come loose. Everytime I turned the top lights OFF there was about 20 amps of current that had to go someplace else, and it went into the remaining lights. They didn't like that and would burn out. When I turned the lights back on there was 20 amps not enough and it pulled the voltage down to the point that the ignition wouldn't work.

Once I got the battery cable hooked up again everything was fine, except that I had blown every light in the car except the top lights and front turn signals.

I suspect that if I had turned those top lights off one more time the alternator would have been destroyed.

Rod

So how do cart, indy and drag cars work if they don't even have a battery or a starter, or do those cars have a battery, and no starter.

(Remote removable starters)

I know top fuel dragsters use magnitos for their ignition, so I assume the alt discussion would not apply to them.

Rod, thatnks for the info. I currently have a 65 amp alternator and a stereo that can draw over 150 amps. I have two 6V batteris run in series for the house bank and 12V for the starter bank. The digital volt meter on my distribution blocks for my stereo reads under 12.0 when my stereo is really cranking. I have been having problems with my batteries not getting charged and I now know that it's becauase my alternator is too small. I was considering getting a 130 amp alternator, but only want to use one belt. Is this possible, or would I need to stick with a 100 amp alternator. Thanks for your help.

Mike

Robert:

A race car could either use small battery to provide 12 volts or run a magneto system. It actually takes a fair amount of horsepower to drive a belt, but it doesn't take much to run a magneto system off the flywheel.

Porter:

The more power you draw from the alternator, the more power you have to transfer from the engine to the alternator shaft. There are two primary factors that determine the amount of force you can transfer:

1) how tight the belt is

2) how many degrees of wrap the belt gets

You can obviously tighten the belt all you want, within the limits of how much the belt will stand before it breaks. There is a danger in doing this, however; it can put a tremendous side load on the other pulleys it goes around. Generally not a problem for the main engine pulley but it can cause the water pump to fail very prematurely.

The amount of wrap the belt gets is a function of the overall engine/belt design. A typical V belt arangment has the belt go around the water pump and then over to the alternator, providing about a 90 degree wrap around the small alternator pulley.

You can get more wrap by installing an "idler pulley" to bring the belt more around the alternator pulley. You will, of course, need a longer belt.

You can also get more wrap by using two belts. If you have a second belt that drives your raw water pump then you could use one belt for the water circulation pump and alternator and then a second belt that drives the raw water pump and alternator.

You could also add another pulley or change the pulley on the engine and provide a dedicated belt to the alternator. This would allow you to have more wrap and you could tighten the belt more without fear of hurting the water pump.

Balmar, who makes large marine alternators, does not recommend going over 100 amps from a single belt. I am not sure what adjustments need to be made if you have one of these newer "serpentine" belts.

Rod

Hey Rod. Do you have a HO alternator? If so what kind and how many batterys do you run?

No, I have the standard alternator. I don't play the stereo that loud! :-)

I was just wondering. I have never seen anything higher than 13.5 volts out of my system. Thats when.'
A. My batterys are fully charged
B. The system has no load on it
C. Its just off Idle like 900 RPM's

My system seems to like to push 12.5 volts.
SO I was thinking it doesnt matter how many volts your system is showing, its the amps your pushing to the batterys that charges them. Because if you go by what you posted above I have some serious problems 14.2 volts is a feet that not many systems will ever see. Some cars that run custom SPL (Sound Pressure level) systems have 2 or 3 alternators running off there engine to see that kind of voltage, Im not discountion you or calling BS I would like to understand more, Thats why I asked you what your were running. You seem to know alot I just wanted to know what applaction you were talking about in your post. And when I say Im seeing 12.5 thats from a Dakota Digital volt meter running right of the battery's, So I know its not lying like a stock voltmeter can.

Grant:

First off, the engine will need to be turning a lot faster than idle (like 2000-3000 RPM) before the alternator has a chance of producing full output.

If you are getting 13.5, and that's all, then it is probably because the regulator is throttling it down. 13.5 is a good number. 14.2 is the upper limit of what you want, and you really don't want that continuously as it will cook your battery.

On the other hand, 12.5 is too low. If that is all you are getting, then the alternator is not providing enough output.

The voltage regulator should be trying to maintain 13.2 volts. If the load is very light, the voltage might increase, and if the load is very heavy the voltage will decrease. If the load is light, and the engine RPMS are 2000 or higher and you are not getting 13.2 volts then either the voltage regulator or the alternator is bad.

Help!!
I just got my 150 amp alternator from battery shack and I am confused. I am installing it into my Air Nautique, and I was told that it would be a one wire unit that just goes to the battery now...It has the one battery screw on back, but there is also a small white wire coming out of the side as well, where does this wire go???

And for the battery wire is it better to go from the alternator to the breaker and then to the starter like stock or straight to the battery...I am truly confused now. Please help me sort this out.