Faulty alternator?

[Mark R]

Hi all,

 

I wonder if any of you could offer advice regarding alternators.

 

I have a 1.8 BMC engine with alternator charging a single battery. I believe I may have an issue with the alternator. Using a multimeter I measured the voltage at the battery with the battery isolator isolated and no engine running, recording a voltage of 12.32V. Turning the key on the isolator and with the engine started the voltage remains around 12.32V. The battery charge light is lit on the control panel. I have checked that all connections are secure, tested continuity between battery and alternator and also tested for bad ground connections, all seems ok. This makes me think I have a faulty alternator as I’d be expecting an increase of voltage.

 

What is puzzling me is that if I was to accidentally leave the key in the ‘on’ position of the isolator, engine not running, my battery would flatten within a matter of a few hours. However, with the key in the ‘on’ position of the isolator and engine running the battery doesn’t flatten battery. Why would this be? Might it be that the alternator is running to some extend?

 

I had previously got to a stage where I was having to jump start the engine using a boost pack. Having run, even for a couple of hours, there would not be enough charge in the battery to turn over the engine without having to use a boost pack again.

 

At this stage I took the battery home and put it on a trickle charge, after refitting the battery (over three months ago) I have probably travelled 14 miles and have started the engine well in excess of 20 times, some of those in the cold weather we have been experiencing, with the voltage still being around the 12.3V mark. 

 

While typing this post it occurred to me that maybe I should measure the voltage from the alternator with the engine running and battery disconnected. Would this prove the it was outputting some voltage?

 

Any thoughts would be appreciated!

 

Mark

[nicknorman]

It is inadvisable to run the engine and then disconnect the battery. Alternator damage may ensue.

 

from your description it does sound as though the alternator is faulty. The voltage should rise to around 14v straight away, assuming you rev the engine a bit.
If the alternator warning light is on, this means something is wrong and it is not charging. Probably a diode issue.

 

If your battery is going flat with the isolator in the on position, but not in the off position, one explanation could be failed diodes in the alternator that allow the battery to discharge back into the alternator. You can check for this by turning on the isolator, not starting the engine, and then after 15 mins or so feel the alternator. If it has become slightly warm to the touch, it definitely points to failed diodes.

[Mark R]

1 minute ago, nicknorman said:

It is inadvisable to run the engine and then disconnect the battery. Alternator damage may ensue.

 

from your description it does sound as though the alternator is faulty. The voltage should rise to around 14v straight away, assuming you rev the engine a bit.
If the alternator warning light is on, this means something is wrong and it is not charging. Probably a diode issue.

 

If your battery is going flat with the isolator in the on position, but not in the off position, one explanation could be failed diodes in the alternator that allow the battery to discharge back into the alternator. You can check for this by turning on the isolator, not starting the engine, and then after 15 mins or so feel the alternator. If it has become slightly warm to the touch, it definitely points to failed diodes.

 

Thanks for that insight, I wasn't sure what could be draining the battery when nothing was being used. Interestingly this draining occurred from the day I bought the boat (June 2023), I'll test your hypothesis and update the post!

 

Thanks for your help!

[ditchcrawler]

Also with the engine running and all isolators closed measure the voltage on the alternator terminal just in case you have a problem between there and the batteries. Also a faulty diode can result in the alternator getting warm when the engine isn't running .

[matty40s]

13 hours ago, nicknorman said:

 The voltage should rise to around 14v straight away, assuming you rev the engine a bit.

We are assuming the OP has actually tried to make the red light go out....

Starting engine 20 times and travelling 14 miles suggests someone who isnt cruising any distance, and may rarely, or never rev the engine to excite the alternator.

[blackrose]

Going back to basics, are your fanbelts tensioned correctly? If a belt is slipping around an alternator pulley you won't get much voltage out of it. Also are all connections to the batteries clean and tight?

Edited January 29 by blackrose

[LEO]

An alternator charging only one battery.............sounds like the old argument again - alternator only charges for a short while after starting, and needing a -stimulator to keep them charging.

[blackrose]

4 minutes ago, LEO said:

An alternator charging only one battery.............sounds like the old argument again - alternator only charges for a short while after starting, and needing a -stimulator to keep them charging.

 

One of my alternators only charges one battery (start), the other alternator charges 3. I thought that was a fairly standard setup? I never see less than 14v on the start battery, maybe reducing to 13.6v.

[jonathanA]

10 minutes ago, LEO said:

An alternator charging only one battery.............sounds like the old argument again - alternator only charges for a short while after starting, and needing a -stimulator to keep them charging.

yep agree with blackrose, can't quite see what you are getting at there Leo.  if the alternator voltage regulator isn't working properly (in some way) then the alternator is faulty.  Remember that for lead acids its the batteries that decide what charge they will take, although they if they are knackered winding up the charge voltage under controlled conditions might get them to start taking some charge (equalisation charging) but thats usually a function of mains battery charger not the alternator. 

[Tony Brooks]

2 hours ago, LEO said:

An alternator charging only one battery.............sounds like the old argument again - alternator only charges for a short while after starting, and needing a -stimulator to keep them charging.

 

That sounds like a faulty alternator to me - one or more phases out. With LA batteries it should keep charging, at a reducing rate, for many hours.

[Tracy D'arth]

2 hours ago, LEO said:

An alternator charging only one battery.............sounds like the old argument again - alternator only charges for a short while after starting, and needing a -stimulator to keep them charging.

Bunkum I reckon. He is not getting any sensible charge into the battery.

[Tony Brooks]

1 minute ago, Tracy D'arth said:

Bunkum I reckon. He is not getting any sensible charge into the battery.

 

Or even the battery has internal shorts so has a massive self discharge that robs a high proportion of the charging currant. But then the self flattening with the master switch off does tend to point to diode problems.

[Mark R]

Thanks for all your input. Just to add to my original post (and possibly answer some questions raised), the boat did a journey of around 2 and a half hours a week or so ago, it was certainly revved during this journey. I went to the boat today and checked two things, firstly I started the engine, let it run for a good 5 minutes with some revs for good measure. I then isolated the battery and measured voltage across the negative terminal on the battery and the positive of the alternator and got nothing. I also tried leaving the isolator off with the engine not running. I didn't detect any heat in the alternator but did notice a reduction in voltage across the battery terminals afterwards.

 

The belt is tensioned correctly to the best of my knowledge, all connections are definetly clean and tight!

 

I'm at the boat again tomorrow, is it worth me giving it one more go with even more revs for longer?

 

Mark

Edited January 29 by Mark R

[magnetman]

Have you tried turning the alternator fan by hand (engine not running obviously) as @blackrose suggested earlier that the belt may be slipping? 

 

If it will turn easily then the belt may be oily or not tight enough. 

 

That might be the problem so worth checking I think. 

 

 

 

 

It could happen that the belt is tight enough to run the water pump but not the alternator. 

Edited January 29 by magnetman

[nicknorman]

20 minutes ago, Mark R said:

Thanks for all your input. Just to add to my original post (and possibly answer some questions raised), the boat did a journey of around 2 and a half hours a week or so ago, it was certainly revved during this journey. I went to the boat today and checked two things, firstly I started the engine, let it run for a good 5 minutes with some revs for good measure. I then isolated the battery and measured voltage across the negative terminal on the battery and the positive of the alternator and got nothing. I also tried leaving the isolator off with the engine not running. I didn't detect any heat in the alternator but did notice a reduction in voltage across the battery terminals afterwards.

 

I'm at the boat again tomorrow, is it worth me giving it one more go with even more revs for longer?

 

Mark

I wasn’t quite clear from the above whether the engine was completely cold when you turned on the isolator and checked for alternator warming. The test would be to start off with a completely cold engine, since the alternator is only going to get slightly warm from back flowing current. If it is already warm,  you probably aren’t going to notice the difference.

As usual we are up against the fact that electricity is invisible! Without proper test /measuring equipment it is very difficult to now what is going on.  And yet adequate test equipment is so cheap! I would recommend getting a multimeter that includes a DC current clamp-meter (be careful, lots only do ac current). The Uni-t UT203 is the old favourite from ebay, used to be £30, now seems to be £40 (that’s inflation for you!) eg https://www.ebay.co.uk/itm/334520176779

 

You can clip the meter around a cable, eg the battery to alternator cable, and see if there is any current flowing. Or clip it on elsewhere in the hunt for where the current is going. Takes seconds! It is not super accurate as it works on magnetic field, you need 1/2 amp or so at least. But in the context of boat electrics, it is very useful.

 

 

[Mark R]

21 minutes ago, magnetman said:

Have you tried turning the alternator fan by hand (engine not running obviously) as @blackrose suggested earlier that the belt may be slipping? 

 

If it will turn easily then the belt may be oily or not tight enough. 

 

That might be the problem so worth checking I think. 

 

 

 

 

It could happen that the belt is tight enough to run the water pump but not the alternator. 

 

@magnetman The fan moves freely by hand, I can see the fan blades are turning when the engine is running, I could measure the rpm using a tachometer to get an idea of numbers, but it definitely is spinning around.

 

@nicknorman I did try to measure the heat in the alternator with the engine cold, didn't notice a change but maybe I didn't do it for long enough! I'll try a clamp meter on the positive cable between the battery and alternator, I'm assuming that if functioning correctly that I'd be measuring in excess of 35 amps (dependant on alternator output)?

 

Is there a RPM that I need to get the engine running at to ensure that the alternator kicks in?

 

A few months ago I was travelling the Aire and Calder at a 'good speed' for a fair number of hours, had to jump the engine before the journey and next time I turned it over again, so it's not through lack of motoring!

[Tony Brooks]

58 minutes ago, Mark R said:

Thanks for all your input. Just to add to my original post (and possibly answer some questions raised), the boat did a journey of around 2 and a half hours a week or so ago, it was certainly revved during this journey. I went to the boat today and checked two things, firstly I started the engine, let it run for a good 5 minutes with some revs for good measure. I then isolated the battery and measured voltage across the negative terminal on the battery and the positive of the alternator and got nothing. I also tried leaving the isolator off with the engine not running. I didn't detect any heat in the alternator but did notice a reduction in voltage across the battery terminals afterwards.

 

The belt is tensioned correctly to the best of my knowledge, all connections are definetly clean and tight!

 

I'm at the boat again tomorrow, is it worth me giving it one more go with even more revs for longer?

 

Mark

 

1. if you isolated the battery with the engine running, you may well have goosed the alternator now.

 

2. Isolator off, so battery not connected to the alternator positive (unless wired contrary to the BSS) or if switched in the negative, then the negative is not connected to the engine. In the first case, why would you expect voltage between alternator B+ and the battery negative - it is an open circuit.

 

3. As Nick says, you need to measure the current flowing out of the battery with the isolator closed and the engine stationary with the ignition off. This would allow you to see if anything else is drawing current by disconnecting the alternator. If the suggestion of diode is right, you will find current flow and when you disconnect the alternator B+ it should drop to very close to zero.

 

4. You have not told us anything about the history of the battery.

 

 

[nicknorman]

19 minutes ago, Mark R said:

 

@magnetman The fan moves freely by hand, I can see the fan blades are turning when the engine is running, I could measure the rpm using a tachometer to get an idea of numbers, but it definitely is spinning around.

 

@nicknorman I did try to measure the heat in the alternator with the engine cold, didn't notice a change but maybe I didn't do it for long enough! I'll try a clamp meter on the positive cable between the battery and alternator, I'm assuming that if functioning correctly that I'd be measuring in excess of 35 amps (dependant on alternator output)?

 

Is there a RPM that I need to get the engine running at to ensure that the alternator kicks in?

 

A few months ago I was travelling the Aire and Calder at a 'good speed' for a fair number of hours, had to jump the engine before the journey and next time I turned it over again, so it's not through lack of motoring!

When you say “the fan moves freely by hand” do you mean with the belt taken off, or as it is now? You should not be able to move the fan if the belt is on, otherwise it is far too loose. Just because you can see the fan rotating doesn’t mean the belt isn’t slipping madly and the fan turning at 1/4 speed.

Edited January 29 by nicknorman

[Mark R]

9 minutes ago, Tony Brooks said:

 

1. if you isolated the battery with the engine running, you may well have goosed the alternator now.

 

2. Isolator off, so battery not connected to the alternator positive (unless wired contrary to the BSS) or if switched in the negative, then the negative is not connected to the engine. In the first case, why would you expect voltage between alternator B+ and the battery negative - it is an open circuit.

 

3. As Nick says, you need to measure the current flowing out of the battery with the isolator closed and the engine stationary with the ignition off. This would allow you to see if anything else is drawing current by disconnecting the alternator. If the suggestion of diode is right, you will find current flow and when you disconnect the alternator B+ it should drop to very close to zero.

 

4. You have not told us anything about the history of the battery.

 

 

@Tony Brooks: If I've goosed it then it's definitely time for a new one. As much as I want to understand point 2 it twists my head in a big knot! I'm afraid I don't know the history of the battery if I'm honest with you.

 

Isolator closed and isolator off, are they the same thing of different?

 

[ditchcrawler]

6 minutes ago, Mark R said:

@Tony Brooks: If I've goosed it then it's definitely time for a new one. As much as I want to understand point 2 it twists my head in a big knot! I'm afraid I don't know the history of the battery if I'm honest with you.

 

Isolator closed and isolator off, are they the same thing of different?

 

Closed = On Open =Off

[Tony Brooks]

3 minutes ago, Mark R said:

@Tony Brooks: If I've goosed it then it's definitely time for a new one. As much as I want to understand point 2 it twists my head in a big knot! I'm afraid I don't know the history of the battery if I'm honest with you.

 

Isolator closed and isolator off, are they the same thing of different?

 

 

Sorry no - isolator open = isolator off, Isolator closed = isolator on.

 

For the battery to discharge, you need a continuous path of cables and components to allow the electricity to flow through. If you break that path with a switch etc. then no current can flow.

 

If you do not have an ammeter, could you rig up a 12V test light using a low wattage bulb. If so, take a battery terminal off and connect the lamp into the gap produced. If there is enough current flow to discharge the battery as you suggest, then the bulb is likely to at the least glow, if not light up.

 

Although the talk about isolators on and off does not accord with an internally shorting battery, if we ignore that for now the battery could be on its last legs with internal short circuits.

 

[Mark R]

16 minutes ago, Tony Brooks said:

 

Sorry no - isolator open = isolator off, Isolator closed = isolator on.

 

For the battery to discharge, you need a continuous path of cables and components to allow the electricity to flow through. If you break that path with a switch etc. then no current can flow.

 

If you do not have an ammeter, could you rig up a 12V test light using a low wattage bulb. If so, take a battery terminal off and connect the lamp into the gap produced. If there is enough current flow to discharge the battery as you suggest, then the bulb is likely to at the least glow, if not light up.

 

Although the talk about isolators on and off does not accord with an internally shorting battery, if we ignore that for now the battery could be on its last legs with internal short circuits.

 

 

Sorry Tony, 

 

I'm getting lost here. Am I now focussing on whether I have a dodgy battery, as there seems to be a shift away from the alternator discussion. Given that I might have damaged the alternator do I just but a new one? 

 

 

[magnetman]

48 minutes ago, nicknorman said:

When you say “the fan moves freely by hand” do you mean with the belt taken off, or as it is now? You should not be able to move the fan if the belt is on, otherwise it is far too loose. Just because you can see the fan rotating doesn’t mean the belt isn’t slipping madly and the fan turning at 1/4 speed.

maybe he took the belt off

[Mark R]

With the belt removed the fan moves freely, I wouldn't be able to turn it whilst the belt is on. 

Edited January 29 by Mark R

[magnetman]

OK. I did mean with the belt on. Worth checking with belt on rather than assuming it won't turn.