Alternator not outputting enough amps

Aztec8484

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Jun 23, 2017
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Does anyone know if it is possible for an alternator to partly fail? My 100 amp alternator used to put out around 80a when the batteries were at 65% reducing to around 30a when charged to about 90% then down to 12a to 100%. Now, the most I can get is 30a reducing as the charge builds. Output voltage looks normal.
Any thoughts?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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welcome aboard.

alternator output is directly tied to the reference voltage. if your battery voltage is low, the current output from the alternator is high. if the battery voltage is high, the output current will be low.

if you doubt your alternator output, have it tested.
 

Mischief Managed

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The alternator can only determine the output voltage, it's the load (in this case the batteries) that determines the amperage draw (that is until you reach the max current of the alternator, then the voltage will go down due to the load).

Could be an issue with the batteries not being able to accept a full charge. Alternatively, the problem could be with whatever device reports the charge percentage on the batteries.
 

GA_Boater

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Welcome aboard, Aztec.

How are you measuring the amperage while charging?

To eliminate stuff, clean the battery posts and connectors. Corrosion build up can weaken the connections and cause charging issues.

If you suspect the alternator, take it to an auto part store for testing. Drag the batteries along and have them tested too.
 

dennis461

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Aug 11, 2011
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Does anyone know if it is possible for an alternator to partly fail? ....


Short answer is yes.

Most alternators (you did not post make/model) have three internal windings to produce AC power (this is why they are called alternators not generators).
The three phase power is rectified internally by either 3 or 6 diodes, creating DC.
If one diode fails, output power is reduced.
The voltage regulator still functions to provide voltage control.

Another possibility is a loose drive belt, it may be slipping without making noise, which reduces mechanical power input, thus limiting electrical power output.

As others posted, get it tested.
 

bruceb58

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Out of curiosity, how are you measuring your output current?
 

Aztec8484

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Jun 23, 2017
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Thanks for comments. Suspect its a dide/ rectifier problem. Will get it checked out. Had not realised that you could have a partial failure.
 

gm280

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Jun 26, 2011
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Aztec, yes the usual alternator has six diodes to rectify the three phases it usually is wire for. And if any one or more diodes burn open, then you still have some capability, but not the full capability. And the voltage regulator can still be good as well.

If you understand how it works and own a multi-meter, you can test the diode pack to verify which one(s) are bad. And you could even replace the diode pack and make it 100% again. All the parts are available at a lot of places for them. Most just let a shop rebuilt them or buy new. But if you are adventurous, you can do it as well. It really is up to you about that. JMHO
 
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