Slow alternator voltage creep

[Tasemu]

Hiya, sometimes when i run my alternator the voltage jumps to 13.x immediately and continues rising shortly after. Other times, the voltage barely jumps to 12.6 before it very slowly rises to expected charging voltage. I put the alternator on at around 12.4 resting battery voltage. Could someone explain to me why this happens?

 

Thanks in advance!

[BEngo]

The battery controls the current  the alternator regulator tries to control the voltage at a fixed level, usually about 14.4 volts these days.The regulator cannot always get the voltage up there when the battery is demanding a lot of current. So when you get a quick rise, the battery is allowing less current in.  It may be more charged than you think or it may be cold.  A slow rise is when the battery will accept high current and the alternator cannot provide that at anything more than 12.6v .  As the battery charges its voltage rises, amd the current falls, so the regultor can increase the alternator voltage.

 

N

[nicknorman]

I would say that 12.6 volts is very low for a charging voltage from even the smallest alternator, if the starting point is 12.4v. Something is wrong. As a starting point, could the drive belt be loose/slipping/contaminated with oil? Does it help if you rev the engine initially?

[Ianws]

As a very newbie to electrics, so big caveat here, is the alternator excited? I am ?

[Tony Brooks]

43 minutes ago, Tasemu said:

Hiya, sometimes when i run my alternator the voltage jumps to 13.x immediately and continues rising shortly after. Other times, the voltage barely jumps to 12.6 before it very slowly rises to expected charging voltage. I put the alternator on at around 12.4 resting battery voltage. Could someone explain to me why this happens?

 

Thanks in advance!

 

Inside the alternator there are a load of pulsing magnetic fields around the generating coils. These magnetic pulses induce a voltage into the coils min opposition to the generated voltage. The higher the charging current the larger the magnetic pulses and the greater the opposing voltage. So the alternator output voltage is the sum of the positive charging voltage and the negative opposing voltage.

 

When you start charging the charging current is high (the batteries are happy to accept all the current the alternator can supply) so the opposing voltage is high. This means the charging voltage is low - as you have observed. As the batteries start to charge they accept less and less current so the opposing voltage gradually falls. This allows the charging voltage to gradually rise.

 

Now, left yo its own devices the charging voltage would keep climbing - I have lit 240 volt light bulbs from an uncontrolled 12V alternator as a party trick - but that would wreck the batteries and all the electrical equipment connected to it so something has to be done. What is done is to fit a voltage regulator that starts to turn the alternator's magnet on and off very fast so it controls the charging voltage at a safe level. Typically these days at around 14.4 or more but its a gradual control so it takes a  long while to reach 14.4V.

 

That should answer your question but I agree with Nick 12.6 after just overnight use suggest to me that either the batteries were very discharged indeed or you have a fault. 12.6 is not impossible but  I would expect in excess of 13V after a single overnight discharge of a reasonably well specced battery bank/charging system/regeim.

[Ianws]

If you are at all in any doubt, listen to Tony and ignore me!

Edited May 18, 2020 by Ianws

[Tony Brooks]

10 minutes ago, Ianws said:

If you are at all in any doubt, listen to Tony and ignore me!

What Tasemu did not tell us was the voltage immediately before starting. My guess is that when he only gets 12.6 the rested voltage would be way below the 12.4V he states. If the voltage rises from 12.4 to 12.6 the alternator must be energised but remember we are relying on the accuracy of the question.

[Tasemu]

12 minutes ago, Tony Brooks said:

What Tasemu did not tell us was the voltage immediately before starting. My guess is that when he only gets 12.6 the rested voltage would be way below the 12.4V he states. If the voltage rises from 12.4 to 12.6 the alternator must be energised but remember we are relying on the accuracy of the question.

What you said above makes sense to me, thanks for the advice.

 

When i run the alternator the batteries are definitely at 12.3 - 12.4 resting voltage. Sometimes the voltage jumps healthily immediately, and sometimes it barely jumps at all. But it always climbs up to 13.9-14 volts fairly quickly, within a minute or two.

 

Perhaps the belt is slipping when i start the engine sometimes. The slow initial charging voltage i see only happens sometimes, other times there's no issue at all. Sorry if my information is a bit light, i'm just working on what I see from the battery monitor.  

[Tony Brooks]

20 minutes ago, Tasemu said:

What you said above makes sense to me, thanks for the advice.

 

When i run the alternator the batteries are definitely at 12.3 - 12.4 resting voltage. Sometimes the voltage jumps healthily immediately, and sometimes it barely jumps at all. But it always climbs up to 13.9-14 volts fairly quickly, within a minute or two.

 

Perhaps the belt is slipping when i start the engine sometimes. The slow initial charging voltage i see only happens sometimes, other times there's no issue at all. Sorry if my information is a bit light, i'm just working on what I see from the battery monitor.  

No problem with what you told us. Its when something seem no quiet right that we need extra info to make a reasonable diagnosis. definitely try tensioning the belt. If its an ordinary V belt make sure that you can see daylight between the bottom of the belt and pulley. If not its a badly worn belt, wrong belt or wrong pulley.

[Onewheeler]

We don't know if the engine speed is the same whenever starting. A slow tickover will give much less output than a fast, and indeed if very slow it might not even be kicking the alternator into operation.

[Tasemu]

7 minutes ago, Onewheeler said:

We don't know if the engine speed is the same whenever starting. A slow tickover will give much less output than a fast, and indeed if very slow it might not even be kicking the alternator into operation.

I usually start it just above idle for a few seconds, then give it a bit of a rev to excite the alternator, then bring it to a comfortable hum. Above idle, for 10 minutes, then put it into idle... If that helps at all haha.

[WotEver]

15 minutes ago, Tasemu said:

I usually start it just above idle for a few seconds, then give it a bit of a rev to excite the alternator, then bring it to a comfortable hum. Above idle, for 10 minutes, then put it into idle... If that helps at all haha.

Better to raise the engine revs while watching the charging current.  As you raise the revs you'll see the current increase.  When it's no longer increasing you can leave the engine at those revs for a while (it should be a fast tickover).  As the voltage rises and the current drops you will be able to slow the engine down bit by bit, eventually reaching tickover.

 

2 hours ago, nicknorman said:

I would say that 12.6 volts is very low for a charging voltage from even the smallest alternator, if the starting point is 12.4v. Something is wrong. As a starting point, could the drive belt be loose/slipping/contaminated with oil? Does it help if you rev the engine initially?

It could simply be that the engine revs are too low.

[Tony Brooks]

The Bukh needed to be in excess of 1200 RPM for maximum current at first start so  a bit more than  a fast idle.

[WotEver]

34 minutes ago, Tony Brooks said:

The Bukh needed to be in excess of 1200 RPM for maximum current at first start so  a bit more than  a fast idle.

A good point, well made

[Sir Nibble]

Since it does get to regulated voltage I don't think there is actually a problem. What the op is observing is the difference between bulk and absorption charging. That doesn't mean the system is optimised of course but I see no evidence it is actually faulty.

[Tony Brooks]

27 minutes ago, Sir Nibble said:

Since it does get to regulated voltage I don't think there is actually a problem. What the op is observing is the difference between bulk and absorption charging. That doesn't mean the system is optimised of course but I see no evidence it is actually faulty.

Its the apparent erratic initial charging voltage from what we are assured is an identical rested voltage that makes me think something may not be quiet right. Hence Nick and myself suggesting the OP checks his belt tension. If I were pushed to make any more of a diagnosis I would veer towards operator error. Either not giving accurate readings or a variable charging speed from day to day.

[Tasemu]

Thanks for all the feedback, i will check the tension today.

[WotEver]

39 minutes ago, Tasemu said:

Thanks for all the feedback, i will check the tension today.

And engine speed

[Tony Brooks]

1 hour ago, Tasemu said:

Thanks for all the feedback, i will check the tension today.

Just for clarity I did not intend to criticise you in any way. I know its hard to know just how accurate readings need to be etc. Especially if you aren't will up on the subject.

[Tasemu]

1 hour ago, Tony Brooks said:

Just for clarity I did not intend to criticise you in any way. I know its hard to know just how accurate readings need to be etc. Especially if you aren't will up on the subject.

All good mate, didn't take it like that. Thanks for the help.

[Sugar n spice]

The other thing to have a good look at is the quality of all the connections both on the earth side and the main charge output, most meters read the battery voltage but a poor connection on either path to the battery will cause problems, I have 4 plus years in the motor repair trade and alternators on cars are similar to boats just setup to charge multiple batteries rather than just one

42 years experience

42 years not 4

[Tony Brooks]

58 minutes ago, Sugar n spice said:

The other thing to have a good look at is the quality of all the connections both on the earth side and the main charge output, most meters read the battery voltage but a poor connection on either path to the battery will cause problems, I have 4 plus years in the motor repair trade and alternators on cars are similar to boats just setup to charge multiple batteries rather than just one

42 years experience

42 years not 4

 

To ensure you do not assume the boat systems are all like car systems. Few vehicles use battery sensed alternators that would in a large part overcome the problems you correctly identify they are a little more common on boats (but not much more). What is far more common are a variety of so called alternator controllers that  convert machine sensed alternators to battery sensed. It is important not to assume any specific system will be machine sensed when giving advice but its often impossible because we rarely get the full information about a problem.

[Sugar n spice]

Totally agree with you Tony, the voltage drop in either the earth/return track or the power+ was my main point, I presume boats are not using the ‘smart’ charge systems that control charge voltage in a very tight window to not damage electronic controllers as yet either, even checking voltage drops needs an accurate volt meter as you obviously know

[Tony Brooks]

43 minutes ago, Sugar n spice said:

Totally agree with you Tony, the voltage drop in either the earth/return track or the power+ was my main point, I presume boats are not using the ‘smart’ charge systems that control charge voltage in a very tight window to not damage electronic controllers as yet either, even checking voltage drops needs an accurate volt meter as you obviously know

 

A lot of boats use these alternator controller things because as you say they produce a higher charging voltage and probably the majority of boaters think a higher voltage equals better charging. In reality in far too many cases a few quid spent on the proper sized charging circuit cables, connectors, and battery terminal maintenance would do just as good a job as you alluding to in your first post because they are battery sensing. However with something lie a 14.5 or higher voltage regulator their benefit is questionable apart from the battery sensing covering up circuit resistance faults.

 

Then there are the mainly Sterling Alternator to Battery systems where the device forces the alternator to maximum output and the processes the high current, lower voltage alternator output to give higher voltage, lower current charging. Some combine the output of two or more alternators and then split it to charge two banks. You may have come across similar devices on things like ambulances with ECU controlled charging.

 

One thing to be aware of is the psychological effect in far too many cases of having purchased an expensive bit of extra equipment when assessing its effectiveness. I suspect far too many of "the best thing since sliced bread" type opinions are not backed up by careful measurement and observation.

[WotEver]

46 minutes ago, Tony Brooks said:

A lot of boats use these alternator controller things because as you say they produce a higher charging voltage and probably the majority of boaters think a higher voltage equals better charging.

Witness the post earlier in the thread that postulated 14.7 good, 14.4 bad.