Alternator giving out high voltage but low current. Why??

[mascip]

My friend's alternator (Lucas 40A I think) is connected to one battery.
It does get excited and produces 14.4V, and it gave out about 15A for a few seconds, but then the current went down to 3A within one minute, which is too low.

What are possible causes of this behaviour?

 

[Marvo]

Why do you think 3A is too low? Is there any other load running on the battery or is it just the battery?
If the battery is fully charged but hasn't been float charged for a while and there's no other load then 15A for a few seconds and decreasing to 3A might be about right.

 

[amlu]

connect some bigger load or another battery in parallel. see if current goes up...

 

[mascip]

Thank you, I will try that: Using a bigger load and checking for increase in current.

I am pretty sure that I measured the same current when the battery was around 12.3v, which is not so full.

A friend said that the alternator's voltage regulator might have been fried by trying to charge near-empty batteries. Questions about this:

Q1a. Could a broken voltage regulator affect the output current? I thought that the regulator only affected the output voltage (which is 14.4v, so it's good). And I thought that for a given alternator and voltage the current depended only on the load.
Q1b. Are these two assumptions wrong?

Q2a. How could charging empty batteries damage the voltage regulator?
Q2b. Is there anything that could have been damaged by trying to charge empty batteries?

 

[telectrix]

Thank you, I will try that: Using a bigger load and checking for increase in current.

I am pretty sure that I measured the same current when the battery was around 12.3v, which is not so full.

A friend said that the alternator's voltage regulator might have been fried by trying to charge near-empty batteries. Questions about this:

Q1a. Could a broken voltage regulator affect the output current? I thought that the regulator only affected the output voltage (which is 14.4v, so it's good). And I thought that for a given alternator and voltage the current depended only on the load.
Q1b. Are these two assumptions wrong?

Q2a. How could charging empty batteries damage the voltage regulator? doubtful
Q2b. Is there anything that could have been damaged by trying to charge empty batteries?a knackered battery could do it.

see red.

 

[mascip]

Thank you.

What do you mean by "knackered battery could do it"?
A knackered battery...
- ...could damage the alternator?
- ...could cause what I observed? (high current 15A followed by low current 3A)

 

[telectrix]

no. a knackered battery ( as opposed top a flat one) could damage alternator by drawing excessive current. a flat battery will draw somewhere between 10A and 20A on charge. well within the capability of your average alternator.

 

[mascip]

Ok, I need to test the alternator with more load on the battery (5A enough?), to check whether the alternator is ok or not. If it still doesn't give out more than 3A, I can assume that it's damaged? What part of the alternator could have been damaged, which would cause this behavior?

Also, when I add a 5A load, I am guessing that the output current for the alternator should increase by about 5A?

 

[telectrix]

the alternator will not output any more current than that demanded by the load. try wiring a headlamp bulb to a battery and the alternator. when you rev the alternator up, the lamp should go brighter and the voltage should increase to around 14V.

 

[Marvo]

Is this alternator/regulator actually part of a vehicle? If the charging current is too low then the battery light will illuminate on the dashboard. There's also a rectifier involved in the mix which are prone to problems as well, especially if a battery was ever connected the wrong way around or a bad/faulty battery was connected or too many batteries were connected.

 

[mascip]

Part of a boat. I will try to add some load and see what happens. Probably tomorrow.

Thanks for your help

 

[Lucien Nunes]

The behaviour you describe is typical of a fully-charged battery in the 50-200Ah range, or one at the end of its life that has very little capacity left to store charge. If the alternator is holding a steady voltage and the current is low, there is absolutely no evidence of a problem with the alt. If the battery were already under suspicion for low capacity I would start out with a heavy current drop-test and/or a hydrometer test on that.

However, to answer your q's:

1a) Correct, inasmuch as varying the voltage will vary the current but the regulator's job is to hold the voltage constant. If it is doing that, it can't further influence the current, only the load can.
1b) No, they are correct under normal working conditions.
2a) It can't really. The regulator works on the field circuit, not the output, so none of the charging current passes through it (unlike a dynamo regulator). If the alternator is fully loaded to the point where its output voltage sags (because vehicle alternators are magnetically self-limiting) then all the regulator does is leave the field running at full strength. Arguably the regulator has less work to do.
2b) The alt windings and rectifier will run hot at full load and if the rectifier is doubtful or starved of cooling that can fail, completely or partly. The symptom you describe does not sound like a bad rectifier, that usually just reduces the maximum output permanently as one or more phases fail, or the whole thing goes short-circuit.

A proper test of an alternator involves running it at full output current to prove that all phases of windings and rectifier hold up. I'd put your machine on the Octopus and pump 40A out of it for 10 mins while it heats up, then make sure it can keep that up at rated voltage at the minimum speed specced for full output. Any less of a load is not a thorough test.

FWIW my boat has (at the moment) four 120Ah batteries and two 150Ah, all charged via relays from a 65A alternator. If the domestic batts are low when the engine is started the alt will run at a steady 65A for a few hours, get toasty hot and the belt can complain a little. After a long day's run its output will be only a few amps above the system load.

 

[mascip]

Wow, thank you SO much for all these answers and extra info! So useful and hard to come by.

Ok, it's time to test that battery properly then. I was thinking of getting a Durite 524-08 discharge tester, unless if you have a better contender to propose. In the past I tried a Silverline load tester, and a battery was tested as being "ok", whilst it was obviously dead (it was able to give 5A for only 30minutes when fully charged). But this Silverline tester only passed 50A for 10 seconds. I'm hoping that a better quality tester will do the job.

And/or I might run a 10A load on my battery for a while to see what happens, but I wanted to get a load tester anyway.

1a) your answer leads me to think that if I used two different alternators (say, one rated 40A and one rated 170A) on exactly the same battery bank, then the current would be exactly the same. The only difference is if the battery bank is very big and pretty low: in this case only the 170A alternator would be able to give more than 40A. Is this correct?

 

[mascip]

The culprit was the battery! When adding load, the current increases.
And loading the battery with 5A for 20min leads to it being empty. Whilst it was full initially: 24h on CTEK 7.0 charger, and 12.8v after 24h at rest.

Adding some load did increase the current. All is good

 

[mascip]

One more question, about the warning light bulb. To excite the alternator my friend must take the revs quite high (2,200 RPM). I noticed that by using a 10W bulb instead of his 6W bulb, the alternator got excited immediately, when the engine starts. Might it damage the alternator, to use a bulb with too high wattage? How would it damage it?