alternator sizing

[bmcewen]

how many amps should i need? I'm only seeing 12.9 on the volt meter and my optima isn't getting charged i believe i only have a 63 amp since it was previously carbureted...do I need higher amps? how do i calculate what i need?

[trakkies]

how many amps should i need? I'm only seeing 12.9 on the volt meter and my optima isn't getting charged i believe i only have a 63 amp since it was previously carbureted...do I need higher amps? how do i calculate what i need?

What do you see with a fully charged battery? Should be 13.8v with the engine running. MS and ignition will only take something like 15 amps max. If you're not seeing at least 13.8 with a fully charged battery and just the engine running (no other large loads) it's likely the alternator is faulty. Also check the battery is showing around 12.5 volts after being charged and left to settle for an hour or so, while not connected.

[Teranfirbt]

If you're going to spend the time and money to upgrade the alternator, get one with the highest current output you can afford, can't hurt anything.

[trakkies]

If you're going to spend the time and money to upgrade the alternator, get one with the highest current output you can afford, can't hurt anything.

Within reason - yes. But there's no point in going mad. Work out the total maximum load and add about 20%. Anything larger won't charge a low battery any faster. And there will be a limit to how large you can go if driven by a V belt.

[bmcewen]

Well i had the alternator tested and it's good. the fully charged battery read 12.8, 12.2 after about an hour but still read 12.2 after 5 hours i started the car it was reading at 13.8....but after letting it run for 15-20 min it only read 13.3, its seem like the alternator is not charging as fast as it's draining in addition to the ms stuff(with 8 injectors) it also has dual electric cooling fans and an msd, the face plate for the stereo is taken off and it seems to charge less/ drain faster if i power the laptop from the accessory port. So do you think a higher amp alternator is in order? i was looking at a 100 amp possibly

[trakkies]

How many amps output has the existing alternator? Most standard ones are big enough to cope with all lights, heated screens etc. With those off it should still be ok with extra cooling fans.

Before paying out for a new alternator, check the basics. Ie voltage drop due to a bad connector, etc.

With a good DVM set to read a couple of volts or so, measure the voltage between battery positive and alternator main output. This should read less than 0.5v or so. Do the same between battery negative and alternator body. These tests will show if there is any excessive voltage drop in those circuits.

If that's ok, I'd try a new regulator in the alternator if you can source one at a good price - about $20. And check the brushes at the same time. That's the most common thing to cause problems with an alternator.

[Matt Cramer]

How many amps output has the existing alternator? Most standard ones are big enough to cope with all lights, heated screens etc. With those off it should still be ok with extra cooling fans.

Some carburted cars (I believe the OP is running MS on a '78 Camaro with the stock alternator) were way undersized. My Dodge Dart came with a 23 amp (!) alternator, with the "heavy duty" option being a 45 amp. Lots of older electrical systems were marginal to begin with, and if you add 20 amps for E-fans and 10 amps for the MS and fuel pump, you can push them over the edge pretty quickly.

[trakkies]

How many amps output has the existing alternator? Most standard ones are big enough to cope with all lights, heated screens etc. With those off it should still be ok with extra cooling fans.

Some carburted cars (I believe the OP is running MS on a '78 Camaro with the stock alternator) were way undersized. My Dodge Dart came with a 23 amp (!) alternator, with the "heavy duty" option being a 45 amp. Lots of older electrical systems were marginal to begin with, and if you add 20 amps for E-fans and 10 amps for the MS and fuel pump, you can push them over the edge pretty quickly.

Crikey. And I though Lucas mean. I don't think they ever made an alternator that small for car use. But in any case you can tell by looking at the voltage output with no load like headlamps etc. Even the smallest one should manage an MS. Disable the fans, if need be, first. If it still isn't giving 13.8v, it needs fixing.

[bmcewen]

The alternator is a 63 amp but I moved the battery to the trunk with a 10 inch ground wire (2 gauge) wire strait to the frame could this possibly cause ground issues? Voltage spikes ...tach on dash is off about a hundred rpm from Ms aswell as my lc1 gauge

[Six_Shooter]

Is this at idle? What happens when you rev the engine to 1000 RPM, 1200, 1600...?

What size charge lead is running from the alternator to the battery or junction point (at the starter?)?

What size ground is between the alternator and/or the engine and chassis? Ground carries just as much current as the positive lead does.

What is your pulley ratio?

I would consider the 63 amp alternator to be undersized, even in perfect working condition. Any EFI swaps I do, I like to use a minimum of 105 amp.

[trakkies]

65 amp was the standard fitting to my car which had factory EFI. Went up to 75 amp with factory AC fitted.
100 amp may be too much for a V belt drive - depending on the belts wrap etc - and you'd likely need to enlarge the cable size too.

[bmcewen]

My grounds are all over sized i believe... 3/4 inch braided ground strap to firewall and 2awg welding cable to frame. I did find a 108 amp alternator out of an 85 buick park ave that was vbelt driven, however i found that my optima red top had a dead cell which hopefully was the problem i 'll find out next week when i get the new battery

[.boB]

With the stereo off and the headlamps off, running the engine at low rpm's shouldn't take more than about 45 amps or so. And that's a performance engine with a CD ignition system. You can temporarily install an amp gauge and see what your engine is drawing.

An alternater will only produce as many amps as you need at the time. If you only need 20A, that's all it will make regardless of what it's rated at. Installing a bigger alernater costs money, adds weight, and adds drag. If you don't need it, don't do it.

[bmcewen]

Well I would want the alternator to handle everything being ON obviously the auto manufacturers felt that cars with fuel injection and power everything needed larger (105amp) alternators, I'm not running a drag only car

[trakkies]

Well I would want the alternator to handle everything being ON obviously the auto manufacturers felt that cars with fuel injection and power everything needed larger (105amp) alternators, I'm not running a drag only car

Why not work out the total load? It's not rocket science.

Don't just go by fuse ratings as these are only to protect the wiring. The average current draw in that circuit will be very much less. Things like heated screens are the real current draw in a modern car. And perhaps the fan if it is the only means of cooling. So often that modern car has a larger alternator than really needed to give an adequate output at low revs.

If you have a good DVM, you can measure whether it's charging or discharging by setting it to millivolts and wiring between battery ground terminal and chassis. To measure the voltage drop in that cable. You'll get a positive reading on charge, and a negative reading on discharge.
Calibrate by noting the reading with a known discharge with the engine stopped - say headlights.

[bmcewen]

Thnx trakkies ill have to try that

[Six_Shooter]

65 amp was the standard fitting to my car which had factory EFI. Went up to 75 amp with factory AC fitted.
100 amp may be too much for a V belt drive - depending on the belts wrap etc - and you'd likely need to enlarge the cable size too.

I have a 160 amp alternator in my Datsun, run by a V-belt, no issues, and I even have undersized pulleys. I really should have a 100 or amp alternator but I had this one sitting on the shelf already.

If you have a good DVM, you can measure whether it's charging or discharging by setting it to millivolts and wiring between battery ground terminal and chassis. To measure the voltage drop in that cable. You'll get a positive reading on charge, and a negative reading on discharge.
Calibrate by noting the reading with a known discharge with the engine stopped - say headlights.

Uhh, you'll want to use the current reading. There will always be "discharge" on a power supply wire to one or many components.

An alternator is one of those things that's perfectly fine being oversized and not hurting anything, until you get to extremes. My car being one example, I have my idle set to 900 RPM, because that's the lowest RPM where this built alternator has acceptable (to me) charge, also keeps my oil pressure at a point I'm ok with. If I used a stock alternator of the same type I'm sure I could lower my idle RPM. There are a lot of 100 AMP or so alternators that have great idle charge, and if there are any added components, like amplifiers for audio systems, extra fans, etc, this will only help. The only time I ever try to run the smallest possible alternator is in a race car, where I might shave a few ounces.

[trakkies]

I'd suggest you try the millivolt trick across the battery ground lead before dismissing it. Every cable will introduce some voltage drop and that's what you're measuring. And the reading will change depending on the charge from the alternator or not. FWIW, it's how an ammeter works.

[floppy]

You might consider using the voltage sensing pole of your GM alternator to make sure that the alternator knows to put out more voltage under load. Attach it to the device with a large load that is relevant to your application like the electric fan or headlights. Then you should meter closer to 14.2 volts as intended. A common error is to use a "one-wire" set-up or loop the sensing wire to the hot pole and then get a larger amp alternator to try and keep up with the load. This is especially important if your battery is in a remote location. Try different locations and watch your voltage datalogs.