Alternator giving low voltage

[Roberto Conigliaro]

Hi everybody, I am planning on changing my battery pack since the 3 x 230ah AGM seem to don't hold the charge anymore (they are 7 years old) - however I am testing the domestic alternator with a voltmeter direct to it, when the engine is running, it only gives me 13v max, the one below it, for the starter battery reads 14.2v. The voltmeter display for the leisure battery don't go higher than 12.8v when the engine is on.

Are the old batteries compromising the efficiency of the alternator or vice versa? What's the best way to check where the fault might be?

Can I swap the alternators feeds (above one to the starter battery / below to the leisure batteries) and see if I have the same problem?

Do I necessarily need two alternators? 

 

(Third alter. in the photo is the starter battery one)

 

 

[MtB]

If you happen to be charging a big bank of really flat batteries, I think they could easily hold the alternator voltage down to 13 for quite a while. 

 

To go with that voltage, we really need to know the charging current at the same time before passing any worthwhile comment. 

 

Others will know more about it than me tho! 

 

 

[BEngo]

There is an awful lot of dust round the nearest alternator in the lower picture

  Check the tension in that v belt. Insufficient  tension can cause belt slip and low output.

  The belf wrap on the two alternators on the v belt in the lower picture is poor.   The one witth the 6PK1123 belt looks much better. 

 

How well charged are the AGM batteries?.  If they are low they will hold the alternator voltage down, but should be taking quite a lot of current.  What is the charging current?

 

Yes you can swap the alternator outputs around, provided there is a common negative.  Only swap leads when the engine is stopped or you will blow an alternator.  Make sure any and all connections that are  swapped  are properly secure.

 

N

 

[Roberto Conigliaro]

19 minutes ago, BEngo said:

There is an awful lot of dust round the nearest alternator in the lower picture

  Check the tension in that v belt. Insufficient  tension can cause belt slip and low output.

  The belf wrap on the two alternators on the v belt in the lower picture is poor.   The one witth the 6PK1123 belt looks much better. 

 

How well charged are the AGM batteries?.  If they are low they will hold the alternator voltage down, but should be taking quite a lot of current.  What is the charging current?

 

Yes you can swap the alternator outputs around, provided there is a common negative.  Only swap leads when the engine is stopped or you will blow an alternator.  Make sure any and all connections that are  swapped  are properly secure.

 

N

 

The AGM stopped to get well charged since a couple of weeks now - they read up to 12.8v when engine is on, but as soon as I switch it off they drop to 12.2v and then very quickly lower than that. 

 

Sorry for the basic question - how do I check the charging current?

 

Would be okay if I swap the feeds behind the isolators? (photo below) That should give the feed of the start alternator to the leisure batteries and vice versa right? 

 

 

 

[MtB]

4 minutes ago, Roberto Conigliaro said:

Sorry for the basic question - how do I check the charging current?

 

 

There should be (needs to be) an ammeter to tell you! But if your boat doesn't have one, use a clamp meter like this. You just clamp it around a wire and it tells you the current that wire is carrying:

 

https://www.ebay.co.uk/itm/334340577064?

 

[Roberto Conigliaro]

4 minutes ago, MtB said:

 

 

There should be (needs to be) an ammeter to tell you! But if your boat doesn't have one, use a clamp meter like this. You just clamp it around a wire and it tells you the current that wire is carrying:

 

https://www.ebay.co.uk/itm/334340577064?

 

oh yes - as I mentioned the domestic alternator with the voltmeter reads 12.8v or 13.v max when revving the engine harder, the starter battery alternator reads 14.2v instead 

[MtB]

15 minutes ago, Roberto Conigliaro said:

oh yes - as I mentioned the domestic alternator with the voltmeter reads 12.8v or 13.v max when revving the engine harder, the starter battery alternator reads 14.2v instead 

 

Ammeter = "Amp meter". Amps are current, or the flow rate of the electricity. 

 

Your Volt meter is measuring Volts, which are a measure of the electrical 'pressure'. Both readings are interrelated and both are usually needed to come to any worthwhile or useful conclusions.

[Roberto Conigliaro]

7 minutes ago, MtB said:

 

Ammeter = "Amp meter". Amps are current, or the flow rate of the electricity. 

 

Your Volt meter is measuring Volts, which are a measure of the electrical 'pressure'. Both readings are interrelated and both are usually needed to come to any worthwhile or useful conclusions.

`I see! sorry.. can I do it with this voltmeter? or can I measure amps only to 10A with it? 

[nicknorman]

No that isn’t suitable for measuring any significant current, and you have to disconnect wires and insert the probes in series. A good way to blow the meter, or at least its fuse! The clamp meter MtB mentioned is the easiest way to measure current from alternators etc, you just clip it over the wire and hey presto! Or you can still get the slightly cheaper Uni-t ut203.

 

As I routinely say on here, electricity is invisible and so without adequate test equipment, we and you are just guessing. Lots of time is spent on guessing and false actions, when the solution is a £40 meter! Does not compute!

[MtB]

5 minutes ago, Roberto Conigliaro said:

`I see! sorry.. can I do it with this voltmeter? or can I measure amps only to 10A with it? 

 

 

No need to apologise! But you really do need to get yourself a basic understanding of DC (direct current) electricity. FInd out about what Volts, Amps and Ohms are, and the relationship between the three. Youtube is probably a good place to have a search

[nicknorman]

I’m a bit confused about your setup as the photos aren’t great. I think there are 2 alternators run by one V belt? Is one for the leisure batteries and one for the engine battery? And there is a polyvee belt in the picture, what is that for?

 

Anyway, I would say that the leisure alternator is most likely knackered BUT as was mentioned, there is a lot of black dust which is indicative of belt slip. Can you rotate the leisure battery alternator pulley without the engine turning? If so it is slipping too much and probably the belt is worn to the point it’s not gripping due to being bottomed out in the pulley.

 

It is worth bearing in mind that the propensity for the belt to slip depends on the electrical load on the alternator. Starting the engine uses probably less than 1 amp hour which is rapidly replaced and hence the load and propensity to slip on the engine alternator is minimal. By contrast low leisure batteries can take the full output of the alternator, maximum electrical and hence maximum mechanical load and hence could just be permanently slipping.

[Roberto Conigliaro]

11 minutes ago, MtB said:

 

No need to apologise! But you really do need to get yourself a basic understanding of DC (direct current) electricity. FInd out about what Volts, Amps and Ohms are, and the relationship between the three. Youtube is probably a good place to have a search

 

sure thing, it doesn't seem to have one so gonna order an ammeter right now and do some reading, thanks for the advice - meantime can I run the test I mentioned earlier to check if the alternator is underperforming cause of the battery condition, ie but swapping the alternators : Would be okay if I swap the feeds behind the isolators? (photo below) That should give the feed of the start alternator to the leisure batteries and vice versa, right? Could this cause any problem? 

[nicknorman]

It would be an interesting test and shouldn’t cause a problem, but don’t disconnect anything with the engine running.

But see my post about belt slip, it could just cause the other alternator to slip on the worn belt when it is loaded up by the flat domestic batteries. Only one way to find out!

 

Edit: actually I’m not clear what you would be swapping. Are you going to end up swapping which alternator feeds which battery by this swap? Or just swapping the alternator and battery combination between domestic and leisure supplies? If the former, good test. If the latter, not sure what it would achieve.

Edited January 31 by nicknorman

[Roberto Conigliaro]

4 minutes ago, nicknorman said:

I’m a bit confused about your setup as the photos aren’t great. I think there are 2 alternators run by one V belt? Is one for the leisure batteries and one for the engine battery? And there is a polyvee belt in the picture, what is that for?

 

Anyway, I would say that the leisure alternator is most likely knackered BUT as was mentioned, there is a lot of black dust which is indicative of belt slip. Can you rotate the leisure battery alternator pulley without the engine turning? If so it is slipping too much and probably the belt is worn to the point it’s not gripping due to being bottomed out in the pulley.

 

It is worth bearing in mind that the propensity for the belt to slip depends on the electrical load on the alternator. Starting the engine uses probably less than 1 amp hour which is rapidly replaced and hence the load and propensity to slip on the engine alternator is minimal. By contrast low leisure batteries can take the full output of the alternator, maximum electrical and hence maximum mechanical load and hence could just be permanently slipping.

Hi, yes correct - the 2 alternator (starter and leisure) are run by one belt, the other belt is part of a third alternator which doesn't have any purpose anymore as it was giving to a travel pack not in use anymore. 

 

Yes also correct, the belt of the 2 alternators seems quite loose - I guess it's worth tightening it? or just replacing it? It is still an option that this might simply be the problem right? 

[Tony Brooks]

7 minutes ago, Roberto Conigliaro said:

 

sure thing, it doesn't seem to have one so gonna order an ammeter right now and do some reading, thanks for the advice - meantime can I run the test I mentioned earlier to check if the alternator is underperforming cause of the battery condition, ie but swapping the alternators : Would be okay if I swap the feeds behind the isolators? (photo below) That should give the feed of the start alternator to the leisure batteries and vice versa, right? Could this cause any problem? 

 

Unfortunately, we don't know how your boat is wired and you have not indicated which wire you describe as "feeds". I can't see any that look thick enough to be the output cables from the alternators. That just leaves the main battery cables that may or may not be acting as alternator feeds. I can understand the for many engine battery circuits, but find it more difficult to understand for the domestic system. As long as you are careful I agree with Nick, you are unlikely to do any damage, but if the charge does not switch to the domestic bank it may not tell you much.

 

If you what to try to get a handle on 12V DC electrics, then study the electrical course notes on my website and feel free to ask questions. tb-training.co.uk

[nicknorman]

2 minutes ago, Roberto Conigliaro said:

Hi, yes correct - the 2 alternator (starter and leisure) are run by one belt, the other belt is part of a third alternator which doesn't have any purpose anymore as it was giving to a travel pack not in use anymore. 

 

Yes also correct, the belt of the 2 alternators seems quite loose - I guess it's worth tightening it? or just replacing it? It is still an option that this might simply be the problem right? 

Yes it could just be a loose belt. You can try tightening but it may be worn. V belts are designed to contact the V shaped sides of the pulleys. If the belt or the pulleys get too worn, the bottom of the Vbelt can contact the bottom of the pulley and so there is no force on the V sides (which have a much better contact area). So even if you tighten it correctly, it can still slip. Have a close look at the bottom of the V on the pulley - if it is shiny, this indicates the belt has been bottoming out on it due to being too worn. New belt required.

 

This design of alternator drive with 2 alternators on 1 belt (and a water pump) is not a great design as the “wrap” (% of the pulley in contact with the belt) isn’t great and it can be a bit marginal at the best of times even with belt and pulley in good condition.

[Tony Brooks]

7 minutes ago, Roberto Conigliaro said:

Hi, yes correct - the 2 alternator (starter and leisure) are run by one belt, the other belt is part of a third alternator which doesn't have any purpose anymore as it was giving to a travel pack not in use anymore. 

 

Yes also correct, the belt of the 2 alternators seems quite loose - I guess it's worth tightening it? or just replacing it? It is still an option that this might simply be the problem right? 

 

I think that you would be better advised to get the domestic alternator driven by the poll-V belt and then get the wrap for the engine alternator on the ordinary V belt fuller than it is now.

[Tracy D'arth]

8 minutes ago, Tony Brooks said:

 

I think that you would be better advised to get the domestic alternator driven by the poll-V belt and then get the wrap for the engine alternator on the ordinary V belt fuller than it is now.

Agreed but the belt drives on the front of Barrus engines are very difficult to alter. I have had severe problems getting the single belt drive on these engines for two alternators to work properly and there is no way that I have found at a sensible price to change to two separate belts. They have a reputation for rapid belt wear and lack of proper drive to the alternators.

Edited January 31 by Tracy D'arth

[nicknorman]

The other option would be to restore the Travelpower and use it to run a battery charger.

[BEngo]

If the AGM batteries are at 12.2 Volts after standing for an hour with nothing switched on then they are roughly  half discharged.  Even knackered  half-discharged batteries will hold the voltage of the alternator down below its regulated voltage, so it is quite possible that  your  alternator is not duff. 

 

Swapping another alternator feed to the domestic batteries will not prove much if the batteries are half discharged.  They will simply hold down the voltage of the 'new' alternator. You need to look at the voltage of the alternator when feeding a battery at about 12.6 V rested.

 

Long term, if you no longer want TravelPower, the best solution may be to have the old travel power alternator swapped for a 12 V alternator that comes with the same frame and use this as the domestic alternator.  The poly V belt is a much better way to drive high power alternators.  You could then still use the existing starter alternator, with the dust cleaned up, and a new belt, while the current domestic alternator can be a mounted and ready to wire in emergency backup for both start and domestic.

 

N

  

[nicknorman]

23 minutes ago, BEngo said:

If the AGM batteries are at 12.2 Volts after standing for an hour with nothing switched on then they are roughly  half discharged.  Even knackered  half-discharged batteries will hold the voltage of the alternator down below its regulated voltage, so it is quite possible that  your  alternator is not duff. 

 

N

  

I disagree. OP reported 12.8v which is too low for a reasonable size of alternator. Yes of course low batteries will reduce the alternator voltage, but not by anything like that much. Mid to high 13s would be reasonable.

Edited January 31 by nicknorman

[MtB]

1 hour ago, nicknorman said:

I disagree. OP reported 12.8v which is too low for a reasonable size of alternator. Yes of course low batteries will reduce the alternator voltage, but not by anything like that much. Mid to high 13s would be reasonable.

 

 

Even with the raging belt-slip evidenced by insufficient belt wrap and black dust plastered all over everything? 

[jonathanA]

I would suggest a quick simple check might be to remove the inter-battey +ve links. measure the voltages across each battery, connect the one with the highest resting voltage back up, leaving the other two out of circuit, then see what happens.  its a big bank at nearly 700AH, and each AGM of 230 is nearly as big as some peoples whole bank 🙂

 

Edited February 1 by jonathanA
hit send too early !

[nicknorman]

10 hours ago, MtB said:

 

 

Even with the raging belt-slip evidenced by insufficient belt wrap and black dust plastered all over everything? 

No. My point was that the reported voltage was too low to be explained away by a fully functional alternator being pulled down by low batteries.
The reported voltage indicates something is wrong. Quite possibly belt slip.

[Martin Nicholas]

Looks like a standard engine with standard fanbelt and standard pulleys.

Adding a second alternator results in poor performance.

Conclusion - upgrade the belt and pulleys.