What alternator rating would you advise

Hi,

Did a search but did not find the answer I was looking for. Is a 100amp alternator sufficient or would you advise a higher rating.
 

Malcolm

Supporter
Racemettle Ltd made me an alternator that works well. It wasn't 100 amp, I think you are unlikeley to need one so highly rated. I think mine was rated at about 90 amp. It was also made to charge at exactly the right voltage for my red top battery, I think 14.7 volts. Red Top batteries are quite fussy on their charging. I sent Racemettle my alternator and they re wound it to what was needed. Hope this helps
 

Ian Anderson

Lifetime Supporter
Supporter
Nick
I would suggest you get out a piece of paper and start a list
Main beam,
Side lights
Spot Lights
Tail Lights
Number Lights
Dash / instrument Lights (They actually add a fair wattage count!)
One (or 2 if you are running EFI) fuel pumps
Ignition system
2 Rad Fans
Wipers
Aircon internal blower
Plus any other stuff you wish to have

Count up wattage and convert to amps (divide by 12) decide what you may have running together - perhaps everything except spots and add up your list and then find an alternator that can supply at least that number or you will be in a position where you may be draining your battery under load.

If your number starts looking too high for a reasonable sized alternator you may then want to look at HID headlights (about 1/2 current draw) LED brake, tail, side and instrument (1/10 wattage of bulbs)etv - a lot more expensive but it might pay in the long run.

And yes I have had everything on at the same time and my car ends up in a net drain situation - but that includes electric screen demisters

IAn
 

Howard Jones

Supporter
I wouldn't use less than a 100 amp unit. If it's a street car with AC and it gets hot at night (lights, fans, and AC on at the same time) where you live then a 100 won't be enough.

Total the loads. Fans about 30-40, MSD 10, Lights 30-40, AC clutch 10-15, dash lights/instruments 10-15, trans pump 10, pretty easy to get to a 100. If you need a computer to run EFI and engine management systems then you want the battery voltage to remain very stable and thus a lot of recharge capacity. 150 in a computer car with AC would be my recommendation.

A Texas, Florida or Arizona car could easily need a 150. My Calif GTD has a 100 and its marginal. My SLC track car is getting a 100 and I won't need fans when it moving or be using a AC system. Mainly MSD, brake lights, instruments, and trans pump.

More is better and less only saves a few dollars and a little weight.
 

Alan Watkins

Lifetime Supporter
More is better and less only saves a few dollars and a little weight.
There is also an upper limit provided by the drive mechanism; above a certain point (I think around 120-150A but I'm not sure) a single v-belt is insufficient and you have to add a second v-belt/pulley setup or switch to a serpentine belt. Of course you'll only run into that when and if you place that large a load on the alternator.... But in that range it would also be worthwhile to make sure you have enough wrap around the alternator pulley.

The other thing to consider is that the single-number rating of the alternator is its maximum output at a relatively high speed. You should also keep track of it's output at idle and what your expectations are under those conditions. Some alternators are better than others at maintaining a high idle-speed output relative to their full-speed output.

And alternator output is of course affected by the pulley ratios which you also have control over.

Also regarding expectations keep in mind that occasionally and temporarily draining the battery under operating conditions is not a problem per se, it becomes a problem when you drain it for so long that the engine quits working and/or won't start, or the battery gets fed up with being discharged all the time. To this end you can mitigate by installing a larger battery. So the right decision depends a lot on your own expected usage scenarios.
 
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From my experience with boats and their alternators, I don't think you need more than a 100 amp alternator. You might look at a West Marine catalog to see what amperage they advise using multiple v-belts; cruising boats often have big alternators and require multiple belts to drive them. I think it is around 100 amps. I think the loads cited above may be a bit high for some of them. The largest draw on a GT40 seems to be the multiple front lamps, if you don't have AC. Even with AC, the biggest load is on the blower, which isn't that much. Mind you, it will run all the time, since between the large windscreen and the engine right behind the driver, the heat load to the passenger space isn't small.

One way to do this would be to find out what ampere spec alternator SPF uses. For marine use, I have found that 65 amps seems to be plenty, and that runs multiple lights, the navigation systems, various pumps, toilet and treatment system, VHF radios, etc. Of course all that stuff is not working at the same time, and the battery bank depth is considerably more than in a GT40. But very few GT40s have their own toilet and treatment system....
 

Alan Watkins

Lifetime Supporter
... above a certain point (I think around 120-150A but I'm not sure) a single v-belt is insufficient and you have to add a second v-belt/pulley setup or switch to a serpentine belt. Of course you'll only run into that when and if you place that large a load on the alternator.... But in that range it would also be worthwhile to make sure you have enough wrap around the alternator pulley.
Just to confirm, Jim Allen in Jeep 4x4 Performance Handbook (p.205) "As a general rule, anything over 120 amps ought to have a serpentine belt or dual belts"
 
About batteries: engine starting batteries such as we use in cars or boats are not meant for deep cycling and will be ruined beyond salvage if they are cycled down more than a few times. The plates short out from sulfating and the battery is done for. If you anticipate having to drain down a starting battery that far, install a deep-cycle battery which has enough cranking amps to start your engine. Even a very high quality starting battery is not meant for deep discharges and it WILL fail in that kind of usage.

Good deep-cycle batteries are made by Odyssey, and Trojan as well. I have had good results from both of those. There is nothing wrong with a car battery in a starting application- just don't cycle it down to where it needs to be recharged to start the car, or you won't get much more life out of it.

My collector cars are kept on battery tenders; it costs some money to buy them and the cables, but not as much as replacing batteries all the time.

I have not tried Optima batteries; the reports are variable. Has anyone used them?
 
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