Alternator changover

easy most alternators are a fixed voltage (once excited) device.

providing you're spinning them hard enough to make enough power they try to make a fixed voltage (13.8 usually for a 12v system)

if the battery is dead flat the alternator pumps out whatevers needed to make 13.8v if that means pushing 60 amps out of 100 thats all they push the battery or load is the deciding factor. (an alternator will run at upto about 120% of its nominal amps if its setup correctly but it wont last long doing it)

if the alternators not spinning hard enough then it will make whatever voltage it can at as many amps as it can and the battery will slowly come up then once the regulated voltage is reached the amps will drop off as the battery tops off.

as a very rough rule of thumb whenever I install aftermarket alternators, unless a specific reason (like welding or stationary charging of a bulk battery unit on a solar rig or camper) I use 5% of CCA + the predicted nominal load to get the amps rating of an uprated alternator. so in real world numbers for an isuzu which has a 700CCA you start at a 35 amp alternator to charge the battery and then add your nominal electrical load say 10A for normal lighting, 5A for a radio, 2 way and the aircon/fan then 5 more for a fridge/phone charger and GPS and you get a about 55A alternator as near enough to the right size.

go too small and like Relay says you risk cooking the alternator too big and theres a chance you can kill the harness.

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Originally Posted by Blknight.aus

easy most alternators are a fixed voltage (once excited) device.

providing you're spinning them hard enough to make enough power they try to make a fixed voltage (13.8 usually for a 12v system)

if the battery is dead flat the alternator pumps out whatevers needed to make 13.8v if that means pushing 60 amps out of 100 thats all they push the battery or load is the deciding factor. (an alternator will run at upto about 120% of its nominal amps if its setup correctly but it wont last long doing it)

if the alternators not spinning hard enough then it will make whatever voltage it can at as many amps as it can and the battery will slowly come up then once the regulated voltage is reached the amps will drop off as the battery tops off.

as a very rough rule of thumb whenever I install aftermarket alternators, unless a specific reason (like welding or stationary charging of a bulk battery unit on a solar rig or camper) I use 5% of CCA + the predicted nominal load to get the amps rating of an uprated alternator. so in real world numbers for an isuzu which has a 700CCA you start at a 35 amp alternator to charge the battery and then add your nominal electrical load say 10A for normal lighting, 5A for a radio, 2 way and the aircon/fan then 5 more for a fridge/phone charger and GPS and you get a about 55A alternator as near enough to the right size.

go too small and like Relay says you risk cooking the alternator too big and theres a chance you can kill the harness.

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Tanks, that has cleared things up, and I know nothing is wrong.
What you say is what i thought in laymans.

so in the real world, I don't need the bigger ALT, (money saver)
as I still have 40-70 amps to play with.
both batteries have a volt gauge starting battery jumps to 14 vlts,
then you can see when the dual battery switch turns on,
as the 2nd battrey go to 14 vlts. (13.8)
only thing I don't have is a winch

I recently got a Baxters NAB 900 from a wreckers, it comes with a universal mounting bracket, hope this helps, I put it in the wrong way yesterday arvo, hence the 2nd pic

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Originally Posted by hook

I now wonder about bigger ALT.
I have a amp gauge fitted.
The gauge sit at around 30 amps,
only ever seen it go up to 60 amps when the 2nd battery was flat.
even with eveything running,
never seen it over 60 amp, this is out of a 100amp ALT?????????????????????

any of you sparky shed light on this.

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When you start talking about currents of this magnitude, it does not take much resistance to give a voltage drop enough to make the alternator thing that battery voltage is higher than it really is, cutting back on the charge accordingly.

How serious the effect is will depend on the size of wiring and the number and quality of the connections between the alternator and battery, and where the alternator senses the voltage. The fact that you have an ammeter may be part of the problem - assuming this is not one with a remote shunt, it means you have a couple of extra metres of wire plus probably four extra connections between the alternator and battery. There is a good reason why ammeters faded off the scene as alternators came on the scene.

John

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Originally Posted by Blknight.aus

be wary of you're charging rig if you go a massively uprated alternator and for those of you with diesels + aircon + belt drive power steering pumps be wary of the potential extra load your putting onto the belt and balancer.

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Would you have any hesitation fitting the 120A Hitachi alt onto an '86 Isuzu county?

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Originally Posted by flagg

Would you have any hesitation fitting the 120A Hitachi alt onto an '86 Isuzu county?

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no, it is the one i have. 1 belt replaced in 60,000km. had a reg problem, unfortunately discovered none were available for that model anymore and had to replace the whole alt:( ok now though:)

jc

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Originally Posted by flagg

Would you have any hesitation fitting the 120A Hitachi alt onto an '86 Isuzu county?

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Me personally I would fit a bigger power cable from the alternator to the battery, make sure the drive belt has a good grip on the alternator and you shouldn't have a problem. I have fitted a couple of 120 amp alts to early model cars this way and they aren't a risk.

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Originally Posted by flagg

Would you have any hesitation fitting the 120A Hitachi alt onto an '86 Isuzu county?

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yes, the wiring isnt up to it...

run a better cable from the ALT to the hot leg on the starter solenoid and make sure the drive belts and idlers are all ok and no.

Quote:

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Originally Posted by Blknight.aus

yes, the wiring isnt up to it...

run a better cable from the ALT to the hot leg on the starter solenoid and make sure the drive belts and idlers are all ok and no.

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X2:)

I brought a brand new alternator for my 93 3.5 V8 Disco 3 years ago, its done roughly 16,000km since being fitted and sadly never been off road. it is a 65-70 NAB 900 alternator running dual batteries and a totally standard car except a pair of 55 watt driving lights. It has had a fridge that max draw is 2.2 amps running in it for a couple of trips.
A few days ago the charge light came on for around a minute then went off and ever since then the alternator hasn't been charging. I am amazed that battery kept it running without charge for two days driving to work, its only 12km.

my question is no i have to buy a new one was this alternator too small could that have caused it to fail with so few KM's?