Alternator Output

[ORCoaster]

On my 1981 Rabbit I have the 55 amp alternator.  I was wondering if the output is correct for the amps.  It seems to charge about 3-4 at idle but once the engine picks up to 1100 RPM or so it drops to 1 or 1.5.  Is this normal? 

I was in the process of swapping out the voltage regulator as the voltage was jumping around a lot and I got a good deal on the replacement.  But my amp meter went south on me just about the time I wanted to check the new equipment.  The gauge in the car shows near 30 with the heater on, the high beams on and I just couldn't verify if that was correct with my hand held.

The low values were with nothing on but the engine. 

I need another 3 of those 3 volt batteries to get real numbers and that will take me a week to get to the store and pick them up.  Have to travel for work this week, no time to play.

Let me know if this is standard behavior or not.  Thanks.

[Rabbit79]

I can't say that I've ever seen any numbers on what the amperage draw should be with just the engine running, but I'd suspect they'd be pretty low. The only things drawing amps would be the shutoff solenoid in the pump, maybe a couple gauges like fuel and temp, a clock if you have one, and the memory for your stereo. I was thinking that my battery charger only charges at 2 amps when I have it on trickle charge, so that number might not be too far off the mark.

[mtrans]

It seems to charge about 3-4 at idle but once the engine picks up to 1100 RPM or so it drops to 1 or 1.5.  Is this normal? 

No it`s not mr. Gremlin,ON START engine you must have 13.8 v and many A.
Change regulator but bad Accu can be factor also.
See that,because bad acu get fast V up and than you have A down.
Even that,bad accu can fail good alternator.

[fatmobile]

Most of the time I use a voltmeter to check my charging system.
 I understand you want to know how much load it's under and an ammeter is great for that.
 A voltmeter connected at the same time will let you know how well the alternator is handling the load. Also lets you know what the voltage regulator is thinking.
 A MK2 90 amp alternator setup is a good upgrade for a MK1.

[ORCoaster]

I decided to go surfing on alternator performance while I waited for sage or not so sage advice here.

Seems that at idle the amount of amps the alternator will develop is based on need.  So if I am not running lights, heater, glow plugs, radio or rear defroster the reading I should see is only what the battery needs to get back what I just pulled out of it to start the car.

Now once it is running if I kick on lights, heater, anything else the amperes will go up accordingly.  That is until I max out the 55 amp rating at 6000 rpms.  Remember the pulley on that is smaller and turns faster because of it.

So, what I needed to do and couldn't because of a lack of 9 volts in the meter was to watch the change from idle to say 1500 RPM's and see if voltage is in the 13.8-14.0 mark.  Then watch the amps as I flip on one after the other accessory.  I did see the internal gauges reflect the steady voltage and increased amperage but when you have a 0-60 amp gauge it is kind of coarse measurement.  About all I know is that it seemed to hit the 35 amp mark with all lights, heater fan on high and defrost on.

I will update when I can run another better measure but I don't think the alternator is bad at all so now I have to find the drain on the system as the battery is new and the cranking power is becoming weak.  Only 3 mth's on battery.  Volts on start-up near 9 or less. 

[TylerDurden]

While you're away from the car, I trust the battery is on a charger or the neg cable is disconnected? Not so good to leave a lead battery discharged for long.

[Rabbit79]

A lot of people mis-understand what amps actually are, but what they do is measure the current flow in a circuit. I run into the thought process a lot that says 'well, I have a 55 amp alternator so I should be reading 55 amps all the time".... Not the case. The 55 amp number is the maximum amperage the alternator can put out. You can roughly compare electricity to the flow of water in a hose, with voltage converting to pressure (psi) and amps converting to a flow measurement (such as gallons per minute ie; gpm). So say you have a hose that has 12 psi on it, but that hose is capped off at the end. The 12 psi/volts has the potential to push the water through, but with a cap on it has nowhere to go so you have no flow. With no water flowing your gpm/amps will be zero. Now put a sprinkler on that hose and your gpm/amps will go up, as you now have water flowing through the hose. That is comparable to say, turning on your stereo. Put 10 sprinklers on that hose and your gpm/amps continue to go up as you add sprinklers, comparable to turning on your lights, heater fan etc etc.
I can't say that I know how much alternator output is devoted to charging the battery, but it's probably not a whole lot. As I said above with just the engine running and nothing turned on, there are not many things drawing amps, so you're reading should be pretty low.
Usually what I do if I suspect I'm getting a battery drain is check each wire coming off the positive terminal of the battery one by one to see which one is drawing amps, that narrows it down to a particular circuit, then I trace it out on down the line. The wire that goes to the 'H' terminal on the fuse box may be drawing a small amount of amperage if you have things like a clock or digital stereo that draw power when the car is off, but that should be less than one amp.

[mtrans]

Right on,accu is JUST for start,after that all juise is from alternator.
for 9 V on start,it `s all about temp.

[ORCoaster]

Rabbit79,  I like the garden hose analogy here.  Most folks will get it.  I was not sure what the flow of current should be into the battery at idle but I knew it would be low.  When the meter was working for me I did measure 0.02 amps trailing out of the battery and the only thing that was drawing power that I know of was the clock for the radio.  I might try measuring the radio on and key on for different things.  Just for grins and giggles.

As for the battery condition at this time. It is a steady 11.5 volts and I was thinking that for times that I do not use the car that a small solar powered trickle charger would be in order here.  They generally plug into the power outlet or a cigarette lighter, do cars even come with those anymore?  A 40 dollar option? 

[Rabbit79]

It sounds to me like your charging system is ok. My battery usually hangs out right in that 11.5 volt range as well, and I rarely have trouble cranking unless it's really cold outside and I haven't plugged it in. Was that 9 volt reading while you were cranking? I can't say that I know how much voltage drop you would normally get while cranking, so I just went out and checked on mine to see what it is and my volt gauge dropped down to 10 while I was cranking on it, so 9 might be a little low. My battery is about 3 years old so yours should be in better shape than mine. With a pretty new battery in there I wonder if your starter is beginning to think about failing.

[ORCoaster]

A nine volt reading is low for a new battery so I did pull the starter to check out the operation outside the housing.  Spun up real nice, looked in the housing to see what the bushing looked like and it was not showing signs of wear either.  What I did notice was that it was sort of hard to get out, like it was torqued in there or something.  So maybe alignment was off.  I was trying to correct the shakes out of it a month ago and perhaps raising and lowering the front mount did something to starter.

I have to think that getting the charger on the battery will bring it back to full strength and I can go from there.  I might need to advance the timing a tad as it starts less than good when it is in that temperature zone where the GPs don't come on and the engine isn't warmed all the way.  An over-ride switch has been mentioned here lately and I might go that route as well.  I could confuse the heck out of folks with a big GP button glowing on the dash.  Push for luke warm start?   We don't get cold weather here unless you count 30 F as cold.

[RunninWild]

9v is a dead battery. a fully charged battery should hold pretty close to 12.6v. There are 6 cells in series that should hold 2.1v each in a 12v battery. If I remember right from school .04A or .06A is considered an acceptable current draw on a vehicle that is turned off.

[ORCoaster]

Clarification to 9 volt number,  That was when cranking the 11.5 or so was static charge with no draw other than radio clock. 

[burn_your_money]

11.5v is basically a dead battery

State of Charge   Voltage
        
100%      12.7   
75%               12.4   
50%               12.2   
25%               12.0   
Discharged      11.9   

[RunninWild]

if your alternator is outputting 11.5v with no load (and doesnt increase with rpm) then you want a new alty. Mine was putting out like 12.6v for the past 6 months or so. 2 weeks ago I barely made it home on a 2 hour drive, I had to turn every electronic off on the car other then the headlights (even had to unplug 1 headlight). I was lucky it was still making enough charge to power 1 headlight and the distributor (this is my gasoline vehicle), I did however have to bump start it when I stopped for dinner and to get gas.

New one outputs 13.8v.