120A alternator upgrade

[steveG]

I've finally ordered a decent 120A alternator to replace the original 40A one on my County.
Obviously there's a need to upgrade the output wire from the alternator, but I'm confused as to what size cable to use. I've tried 3 different online cable size calculators but if I put in 12v, 120A and 1m length they all return different recommendations - from ~8mm2 to ~25mm2. :confused:
What size should I use?

I'm thinking I should also install some sort of fuse at the alternator end. An ANL style fuse in a holder seems to be the best option I've found so far.
What size should the fuse be - 120A? Or does it need to be slightly above that so it doesn't blow when the alternator is at full output?

Anything else I need to be wary of?

Steve

[Homestar]

I would go as big as is practical for the size stud on the back of the alternator. I would imagine 16 mm would fit ok, 25 may be getting a bit big, but if it fits, then I would use it. If you really feel a need to fit a fuse, I would use a 120 amp fuse. If your 120 amp alternator is at full load, then you may well have a problem somewhere - that's a lot of power.

[steveG]

I would go as big as is practical for the size stud on the back of the alternator. I would imagine 16 mm would fit ok, 25 may be getting a bit big, but if it fits, then I would use it. If you really feel a need to fit a fuse, I would use a 120 amp fuse. If your 120 amp alternator is at full load, then you may well have a problem somewhere - that's a lot of power.

Thanks - I'll have a look at what will physically fit when it arrives. I'd prefer to go heavier gauge rather than too light.

To be honest, I'm not 100% sold on the fuse idea. From what I've read online, a fuse can protect against shorted diodes in the alternator itself - but I don't know how real that risk is.
I've got a winch fitted, so when that's in use I'd expect to be using the full output of the alternator.

Steve

[bee utey]

The trouble with fuses under a load near their rating is that there is a considerable voltage loss across it (could be around 0.5-1V). I've had fuses fail with vibration at near their rated current as the fusible bit is heated and slightly soft. I wouldn't run a fuse under say 160A in that application as its prime duty is to protect the battery cables if the alternator shorts internally. With a 16mm2 or 25mm2 cable you could easily run a 200A fuse and it would protect the cable. Perhaps two 100A maxi fuses side by side, a wreckers should have dozens of them at hand. Or if you prefer run a circuit breaker, at least you can reset it.

[steveG]

I like the idea of 2 maxi fuses as it would also mean I could run 2 smaller diameter cables which would be easier to route than one large one.
I've seen a few melted maxi fuse holders on 12v compressors, and with much smaller fuses than 110A. Whats the best source for decent quality holders?

Steve

[mike_ie]

I've finally ordered a decent 120A alternator to replace the original 40A one on my County.
Obviously there's a need to upgrade the output wire from the alternator, but I'm confused as to what size cable to use. I've tried 3 different online cable size calculators but if I put in 12v, 120A and 1m length they all return different recommendations - from ~8mm2 to ~25mm2. :confused:
What size should I use?


For a 2m run from alternator, 8mm2 cable is way too light - it'll vapourize at full load. Alternator that size, I'd be running #4 gauge cable - interwebs tells me that it is around 20mm2 in local currency. That'll happily take close to 200A without problem. You can of course go heavier, but as you're no doubt aware, you're going to hit the physical limitations of what size terminals or rings you can use to connect it to the alternator.



I'm thinking I should also install some sort of fuse at the alternator end. An ANL style fuse in a holder seems to be the best option I've found so far.
What size should the fuse be - 120A? Or does it need to be slightly above that so it doesn't blow when the alternator is at full output?


Go 120A or slightly lower - the fuse is there only to melt before anything else does if there's a huge draw on the alternator. Something high enough to not get fried by the alternator, low enough to not melt before the wire does. The ANL fuse holders are tidy, so why not.

Where'd you find a 120A alternator with a vacuum pump, by the way?


misread the initial values in the OP - updated accordingly

[steveG]

Thanks Mike.
Alternator was through ebay, but its decent quality with 2 year local warranty rather than a $150 Chinese special.

Steve

[mike_ie]

Thanks Mike.
Alternator was through ebay, but its decent quality with 2 year local warranty rather than a $150 Chinese special.

Steve
Any chance of a link for future reference???

[drivesafe]

Hi Steve and because you are running a winch, go for the thickest cable you can fit.

The thinest cable you can safely use with a 120 amp alternator is 4B&S ( 20mm2 )

The thicker you go, the less voltage drop you will have while winching.

As far as the size of the stud on the alternator goes, you can use any size cable eye terminal and if there is a restriction because of a limited clearance area on the back of the alternator, you can always file the edges of a larger terminal down till it fits.

The size of the fuse will be governed by the thickness of the cable, for example, 4B&S ( 20mm2 ) = 120 amp fuse, 3B&S ( 25mm2 ) 150 amp fuse, and 2B&S ( 32mm2 ) = 200 amp fuse.

Don't forget that you will probably need to up size your earth cables.

[steveG]

Any chance of a link for future reference???

Recent thread here with links:
http://www.aulro.com/afvb/isuzu-landy-enthusiasts-section/65276-4bd1-alternator-3.html

In case the ebay listing disappears, the one I got was the "12V 120amp Isuzu N Series Holden Rodeo Jackaroo Diesel Vacuum Pump Alternator" from seller "fantassie".

Steve

[steveG]

Hi Steve and because you are running a winch, go for the thickest cable you can fit.

The thinest cable you can safely use with a 120 amp alternator is 4B&S ( 20mm2 )

The thicker you go, the less voltage drop you will have while winching.

As far as the size of the stud on the alternator goes, you can use any size cable eye terminal and if there is a restriction because of a limited clearance area on the back of the alternator, you can always file the edges of a larger terminal down till it fits.

The size of the fuse will be governed by the thickness of the cable, for example, 4B&S ( 20mm2 ) = 120 amp fuse, 3B&S ( 25mm2 ) 150 amp fuse, and 2B&S ( 32mm2 ) = 200 amp fuse.

Don't forget that you will probably need to up size your earth cables.

Good point about the earth cable. I upgraded it from the tiny one that was there from the original V8 setup - pretty sure its now 50mm2 but will double check.

I've got 70mm2 cable from the battery to the winch, and the solenoid is in the engine bay just behind the headlight. I'm going to connect the alternator output to that as it seems to make sense to a) connect it back to the battery via large existing cable, and b) have it feeding as directly as possible to the winch.

I still need to sketch it all out and make sure I'm not doing something stupid circuit-wise but seems to make sense so far.

Steve

[steveG]

I fitted the new alternator today.

Here's a couple of notes that might be useful to others:


Pulley offset is different to the original alternator.
Original alternator is 44.2mm from inside of forward alternator mounting foot to centerline of belt
Replacement is 48.8mm so pulley sits further forward.
I had to remove ~4.5mm from the rear of the pulley boss and put an extra thick washer/spacer under the pulley nut to compensate for the removed material. Pulley is cast iron FWIW.

The battery light on the dash won't light up unless you have the alternator output connected to your battery supply. Its probably the same for every alternator, but I only found it out today (after wasting a heap of time wondering why it wasn't working).

The adjustment lug on the new unit is a bit further around, so you may have to modify the adjustment arm. Mine was a bit of a mis-match anyway having an original Stage1 alternator, and a truck mount bracket to put the alternator in a higher position - so needed modifying.


Steve

[flagg]

[LIST=1]
Pulley offset is different to the original alternator.
Original alternator is 44.2mm from inside of forward alternator mounting foot to centerline of belt
Replacement is 48.8mm so pulley sits further forward.
I had to remove ~4.5mm from the rear of the pulley boss and put an extra thick washer/spacer under the pulley nut to compensate for the removed material. Pulley is cast iron FWIW.


Hey Steve thinking I may go down this route. I can't quite picture what you mean about re-aligning the pulley.. will it be obvious once I have it in front of me?



The thinest cable you can safely use with a 120 amp alternator is 4B&S ( 20mm2 )

The thicker you go, the less voltage drop you will have while winching.

The cable I have from the alt is about 5-6mm across. I"m not sure how to identify what is 4B&S or 20mm2... is what I have enough?

[steveG]

Hey Steve thinking I may go down this route. I can't quite picture what you mean about re-aligning the pulley.. will it be obvious once I have it in front of me?



The cable I have from the alt is about 5-6mm across. I"m not sure how to identify what is 4B&S or 20mm2... is what I have enough?

Yes - it will be obvious once you have the alternator in front of you. Essentially you need to move the pulley rearwards to align it with the crank and water pump pulleys, so machine some off the back of the pulley boss, and put a thick washer under the nut to compensate for the removed material (so the nut doesn't bind on the shaft).

You need cable that's got about 6mm diam copper in it. If the 5-6mm you've got includes the insulation its too thin.

Steve

[MLD]

I've got 70mm2 cable from the battery to the winch, and the solenoid is in the engine bay just behind the headlight. I'm going to connect the alternator output to that as it seems to make sense to a) connect it back to the battery via large existing cable, and b) have it feeding as directly as possible to the winch.

Steve

I'm no auto electrician but i'm sure i read somewhere that it was not wise to connect the winch to the alternator without passing through the battery. The rationale applies to all loads.

I'm curious of the thoughts from Drivesafe and any other auto-sparky out there to that set up. To my lay mind it stands to reason otherwise the auto makers would run loads off the alternator without passing through the battery.

cheers MLD

[steveG]

I'm no auto electrician but i'm sure i read somewhere that it was not wise to connect the winch to the alternator without passing through the battery. The rationale applies to all loads.

I'm curious of the thoughts from Drivesafe and any other auto-sparky out there to that set up. To my lay mind it stands to reason otherwise the auto makers would run loads off the alternator without passing through the battery.

cheers MLD

You can't pass current through the battery to a winch - the battery effectively hangs off the side of the circuit like a reservoir in parallel with the alternator.
Your alternator will never supply a winch by itself. The battery(s) will be assisting as the winch draws more current than the alternator can supply - therefore the voltage drops below battery voltage and the battery feeds back into the circuit.

Maybe what you've read is that it isn't a good idea to hook up a separate alternator straight to a winch with no battery in the circuit. I could see that not being a good thing.

Steve

[swivelrat]

G'day,

my 95 130 tdi alt decided to throw in the towel the other day, a MM 127-65 which had a good innings all considering. I read quite a few blogs on replacement options and it looked like I would be best tracking down a 100 amper off a disco. I rang a few joints, couldn't find a bosch model to suit but found a mob doing Merelli Magnetti's. A 65amp was $127 and the 120amp was a whopping $485. So I did the sums to work out an average load required and worst case. With everything going flat out, plus fridges etc, 65 amp chits it in. So I'll the $350 for the starter when it goes.

On a different note, Im more interested in battery preservation. Say if dropped the batteries to a very low voltage, my understanding is that its important to charge the battery steadily and not to subject it to shock treatment so to speak with 120amps up front, but Im keen to be proven wrong here????

[steveG]

G'day,

my 95 130 tdi alt decided to throw in the towel the other day, a MM 127-65 which had a good innings all considering. I read quite a few blogs on replacement options and it looked like I would be best tracking down a 100 amper off a disco. I rang a few joints, couldn't find a bosch model to suit but found a mob doing Merelli Magnetti's. A 65amp was $127 and the 120amp was a whopping $485. So I did the sums to work out an average load required and worst case. With everything going flat out, plus fridges etc, 65 amp chits it in. So I'll the $350 for the starter when it goes.

On a different note, Im more interested in battery preservation. Say if dropped the batteries to a very low voltage, my understanding is that its important to charge the battery steadily and not to subject it to shock treatment so to speak with 120amps up front, but Im keen to be proven wrong here????

If you've got multiple batteries and are draining them I think you might need to take another look at your sums if you think 65A will handle it. How much have you allowed for battery charging in your calcs? What type of batteries?
Best thing for batteries is not to drain them down too low.
Have a look at the specs for most deep cycle or AGM batteries (you don't usually find any charts for purely crank batteries). Draining to 80% remaining they will do LOTS of cycles, 50% its significantly less and if you are draining them to 10% you might be down to less than 100 cycles.
While some batteries don't like high charge rates, the likes of AGM are happy with it.
If its going to take you 20hours to recharge a deeply discharged battery then you won't be fully charged the next night so you have less capacity and will draw it down even further.

IMO, best thing is plenty of battery capacity so you don't discharge them too deeply, batteries that will take high charge rates, and an alternator with plenty of capacity to provide the charging. If you're going to camp for extended periods without running the engine (ie more than a night or two) you probably need to consider another source of charging (portable charger or solar).

Steve