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Brett Proctor

Well-known member
Out of curiosity I did a search for the highest output alternator for a VW 1600 bug. Most were in the 75 amp range but I found a 90 amp one.
The lowest Amp Electric AC compressor is listed at 40 amps

OK with that known fact I asked the question "How much HP does a 90 amp alternator need to produce 40 amps"
The RPM's that the alternator would have to be turning at to produce that output was not listed.

The Answer That the web came up with was

10 HP​

To produce 40 amps, a 90 amp alternator would need about 10 HP from an engine1. The output of the alternator is approximately 3.75 HP2.

So then I asked the same question this time using a 75 amp alternator

About 3.75 HP​

To produce 40 amps, a 75 amp alternator would need about 3.75 HP2. The overall efficiency of the alternator is around 20%1.

Don't ask me why the 75 amp alternator takes less hp to do the same job as a 90 amp alternator. Maybe the 90 amp alternator charges faster and puts more of a load on the engine.
Again the RPM's to put out those amps were not listed. I guess you would need to find a chart that shows the amp output at a given RPM. Maybe in stop and go traffic the alternator wont put out the needed amps for the AC compressor to work.

And I don't see how having a dual battery system would help. You still have only 1 alternator charging the system. The storage capacity is the only thing 2 batteries change. If you got more amps going out than going in your just prolonging the inevitable.

Interesting topic.
 
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farfegnubbin

Site Owner
Staff member
Thanks for digging for some actual numbers. Those actually jibe quite nicely with our educated guesses.

Yeah, I don’t know what the heck some people on FB were thinking today. I actually rarely post there, but when somebody confidently said that 1) an electric ac compressor doesn’t rob energy from the engine (!?!!!?) and that 2) having two batteries will solve the problem of needing a high output alternator (!?!) I felt a strong need to not let those statements go uncorrected.

And everyone ignored my summary. So… yeah. There’s that.

I need to remember that I don’t need to be the one to enforce the laws of thermodynamics; they will enforce themselves quite nicely.
 
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Brett Proctor

Well-known member
Something else that may have been overlooked is how many amps the car itself is pulling to stay running. (not including the drain from starting) (10 amps +-)
the amps the blower motor is pulling (25 amps +-)
And if you need to turn on the headlights (10 amps +-)
These could easily reach 40-50 amps alone.
Then if you have a high draw stereo system.

So if your determine to install AC, a compressor running off the motor is looking like a better choice. You may be losing some hp but it won't be a big drain on the electrical system.

Something that would be interesting to know would be what style compressor takes the least hp to operate
 
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Brett Proctor

Well-known member
Did a little research on blower motors. (mainly the one I'm using)

The blower motor that I'm using is on a 15 amp circuit while the main blower motor thats in the same car and controls the main HVAC unit for the car is on a 60 amp circuit.

The Bug came with a 2 speed fresh air blower motor but I couldn't find how many amp circuit it was on
 
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Brett Proctor

Well-known member
I was curious how long an alternator would last producing max amperage for long periods but couldn't find nothing.
I would think the life span would be shorter than one operating in a normal situation.
Alternator Overheating may be a problem.
 
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