Low voltage on leisure batteries while running alternator?

[Tasemu]

Hello again!

 

We have had a few rainy days here and not much sun, as such our leisure batteries have dropped to about 12.4v. I decided to run the engine for the first time to try and put some power back into the bank. I ran the motor on medium rpm for about 25 minutes and checked the battery bank and it still says 12.4v and finally jumped to 12.5 after 30 minutes or so.

 

We are running our alternator through the starter battery and a split charge relay to the leisure batteries. I checked online on how to test if the alternator is charging and the most common advice I see is to check the terminals on the leisure batteries with a multimeter and it should be at 14+ volts while the alternator is running.

 

I checked my terminals with my multimeter while the engine was running and the voltage displayed was 12.5v (the same as the display on my solar charge controller).

 

I checked online for a wiring diagram for a split charge relay and found this which appears to match how mine is wired up.

 

 

Could anyone give me some advice as to whether my alternator is broken, i'm doing it wrong, or i'm doing it fine and just need to leave it running.

 

Thanks in advance for any and all advice.

[nicknorman]

All with engine running: 

 

Check the voltage on the starter battery. If it is up above 14v then the split charge relay is likely goosed or not being operated.

Check for any voltage difference between the two fat leads of the split charge relay. Should be zero. If it is 2v or so, this confirms the split charge relay isn’t connecting.

 

If the voltage on the starter battery was 12.5v or so, check the voltage between the alternator case and the fat wire coming from the alternator. If it isn’t up around 14v or more, the alternator is faulty.

 

Also worth observing what happens as you start the engine. Turn on ignition - there should be an alternator warning light illuminated. Now start the engine. Warning light should go out.

 

Many alternators require the small current through the alternator warning light in order to kickstart the alternator. If the bulb is blown or disconnected, the alternator won’t start charging.

 

 

Edited May 29, 2019 by nicknorman

[WotEver]

In addition to Nick’s observations it’s also possible that you could have a relatively small alternator which is running flat out and until it’s got a bit more charge into the batteries the voltage will simply be depressed. 

 

This is where a multimeter with a clamp DC ammeter can help. If the current from the alternator is high then it’s probably just being held down by the battery load. 

7 minutes ago, nicknorman said:

If the voltage on the starter battery was 12.5v or so, check the voltage between the alternator case and the fat wire coming from the alternator. If it isn’t up around 14v or more, the alternator is faulty.

Or it’s simply running flat out and hasn’t got sufficient charge into the batteries yet for the voltage to rise. 

[nicknorman]

1 minute ago, WotEver said:

In addition to Nick’s observations it’s also possible that you could have a relatively small alternator which is running flat out and until it’s got a bit more charge into the batteries the voltage will simply be depressed. 

 

This is where a multimeter with a clamp DC ammeter can help. If the current from the alternator is high then it’s probably just being held down by the battery load. 

I’d say if it only takes the voltage up by between 0 and 0.1v (as reported) then it is either a child’s bicycle dynamo, or something is wrong!

[WotEver]

One more thing to note. Once you’ve established the situation and are happy that everything is working correctly, move the alternator feed to the domestic bank. And if the VSR isn’t dual sensing (most are these days) swap the two battery connections on it too. This will avoid the much higher charging currents demanded by the domestic bank from having to pass through the relay contacts. 

3 minutes ago, nicknorman said:

I’d say if it only takes the voltage up by between 0 and 0.1v (as reported) then it is either a child’s bicycle dynamo, or something is wrong!

Yup. It’s far more likely that the VSR or alternator is faulty, I agree. 

 

 

Edited May 29, 2019 by WotEver
Clarity

[Tasemu]

I've checked the voltage of the starter battery while the engine is off and it reads 12.7v, i'll run through all your advice and report back with details.

[WotEver]

1 minute ago, Tasemu said:

I've checked the voltage of the starter battery while the engine is off and it reads 12.7v, i'll run through all your advice and report back with details.

Sounds like the VSR then. Or its wiring. Or one of the fuses has blown. 

[nicknorman]

1 minute ago, Tasemu said:

I've checked the voltage of the starter battery while the engine is off and it reads 12.7v, i'll run through all your advice and report back with details.

That suggests it is definitely the split charge relay or associated wiring at fault. However do the checks mentioned earlier to be sure.

[Tasemu]

I've checked the starter battery while the engine is running and it appears to be charging, it started at 12.5 (i accidently left the key in and had to jump start it) and rose to 12.7 in a few minutes. Does this sound right? I assume once it gets high enough it would begin charging the leisure batteries... Not quite sure, if that were the case then I probably wouldn't have had to make this thread in the first place haha.

[roland elsdon]

You should be around 14 volts very shortly after starting on the starter battery. By very shortly within a couple of mins of the alternator starting to charge.

check the battery in your multimeter too...

[Momac]

12 minutes ago, Tasemu said:

I've checked the starter battery while the engine is running and it appears to be charging, it started at 12.5 (i accidently left the key in and had to jump start it) and rose to 12.7 in a few minutes. Does this sound right? I assume once it gets high enough it would begin charging the leisure batteries... Not quite sure, if that were the case then I probably wouldn't have had to make this thread in the first place haha.

Maybe your starter battery is on it's way out? 

[hider]

If the alternator is working, it will get warm to hot after a short time.

[MtB]

1 hour ago, MartynG said:

Maybe your starter battery is on it's way out? 

 

Let's not get into the realms of the unlikely before checking the basics.

 

[nicknorman]

9 hours ago, Tasemu said:

I've checked the starter battery while the engine is running and it appears to be charging, it started at 12.5 (i accidently left the key in and had to jump start it) and rose to 12.7 in a few minutes. Does this sound right? I assume once it gets high enough it would begin charging the leisure batteries... Not quite sure, if that were the case then I probably wouldn't have had to make this thread in the first place haha.

No, that is much too low. Either the alternator isn’t connected to the batteries (wiring fault), the alternator is faulty, or not being activated due to failed warning light circuit.

 

Check the voltage between the alternator’s fat wire and its case. Also tell us about the alternator warning light behaviour.

 

 

[Tony Brooks]

I do not see where the OP says he has a VSR but he does say he has a split charge relay and teh diagram shows a simple split charge relay. However after a period of heavy domestic use with a newish start battery I had very similar symptoms (red face time I suppose).

 

To the best of my knowledge I had the solar charging the domestic bank and the alternator also charging the domestic bank with the VSR I fitted last year sensed from the domestic bank. Although all had appeared to work perfe3ctly for the last year suddenly the ammeter showed only a few amps charge within 10 minutes of start up, 14V+ engine battery voltage and about 12.2V on the domestic bank. My ammeter is wired in a non-standard way because I want to monitor the alternator out-put rather than the charge into the domestic bank.

 

I found that for some reason i must have had a brain fart when I fitted the VSR (the split charge relay it replaced worked find as it would being switched by the W/l supply) and had managed to get or left the alternator charging the engine bank but the VSR sensing the domestic bank. So with a low domestic bank the VSR never energised. That particular day was cloudy so the solar on the domestic bank had not managed to raise the domestic bank voltage high enough to operate the VSR. A quick terminal swap solved the problem.

 

So if its a VSR then check which bank is being sensed unless the VSR is duel sensing. If its a slit charge relay as shown in the diagram I would suspect its a relay fault but to be sure we need the actual diagram from the boat, not something off the net.

[nicknorman]

7 minutes ago, Tony Brooks said:

I do not see where the OP says he has a VSR but he does say he has a split charge relay and teh diagram shows a simple split charge relay. However after a period of heavy domestic use with a newish start battery I had very similar symptoms (red face time I suppose).

 

To the best of my knowledge I had the solar charging the domestic bank and the alternator also charging the domestic bank with the VSR I fitted last year sensed from the domestic bank. Although all had appeared to work perfe3ctly for the last year suddenly the ammeter showed only a few amps charge within 10 minutes of start up, 14V+ engine battery voltage and about 12.2V on the domestic bank. My ammeter is wired in a non-standard way because I want to monitor the alternator out-put rather than the charge into the domestic bank.

 

I found that for some reason i must have had a brain fart when I fitted the VSR (the split charge relay it replaced worked find as it would being switched by the W/l supply) and had managed to get or left the alternator charging the engine bank but the VSR sensing the domestic bank. So with a low domestic bank the VSR never energised. That particular day was cloudy so the solar on the domestic bank had not managed to raise the domestic bank voltage high enough to operate the VSR. A quick terminal swap solved the problem.

 

So if its a VSR then check which bank is being sensed unless the VSR is duel sensing. If its a slit charge relay as shown in the diagram I would suspect its a relay fault but to be sure we need the actual diagram from the boat, not something off the net.

The OP reports that neither the domestic nor the engine battery are being significantly charged, therefore the problem isn’t with the split charge relay / VSR. 

8 hours ago, hider said:

If the alternator is working, it will get warm to hot after a short time.

This isn’t necessarily a good test because if the alternator has blown diodes it may get hot whilst putting out very little or no DC component.

[Tasemu]

I'll see how high the starter battery gets with the motor running, i'll also test the voltage between the case of the alternator and the fat red wire coming out. Will report back with details.

[Momac]

9 hours ago, Mike the Boilerman said:

 

Let's not get into the realms of the unlikely before checking the basics.

 

The domestics will not see a charge until the starter battery is charged to a certain level. If the starter battery is shot it will take a while to recharge  before the domestics see any charge. 

 

[MtB]

14 minutes ago, MartynG said:

The domestics will not see a charge until the starter battery is charged to a certain level. If the starter battery is shot it will take a while to recharge  before the domestics see any charge. 

 

 

 

I disagree, an old and sulphated (or 'shot' as you put it) starter battery will charge up very quickly to 14.4v (or whatever the alternator output is regulated at).

 

Perhaps you are being sloppy with your language and by 'shot' you mean a battery has a cell has shorted internally.  If this is what you mean its best to say so. In technical diagnosis discussions, loose terms with no technical definition are not really very helpful especially if the OP is seeking to understand the nature of the fault as well as achieve a solution.

 

 I agree a shorted internal cell could produce the stated symptoms, but I hold that shorted internal cells are unusual and it is better to approach remote diagnosis in a structured way than the scattergun approach of bunging in wild guesses with no explanation of how they would produce the observed symptoms until challenged.

 

 

[Tasemu]

Maybe i should just buy one of those clamp meters, and check to see if current is going through the relay into the domestics.

[Momac]

41 minutes ago, Mike the Boilerman said:

 

 

I disagree, an old and sulphated (or 'shot' as you put it) starter battery will charge up very quickly to 14.4v (or whatever the alternator output is regulated at).

 

Perhaps you are being sloppy with your language and by 'shot' you mean a battery has a cell has shorted internally.  If this is what you mean its best to say so. In technical diagnosis discussions, loose terms with no technical definition are not really very helpful especially if the OP is seeking to understand the nature of the fault as well as achieve a solution.

 

 I agree a shorted internal cell could produce the stated symptoms, but I hold that shorted internal cells are unusual and it is better to approach remote diagnosis in a structured way than the scattergun approach of bunging in wild guesses with no explanation of how they would produce the observed symptoms until challenged.

 

 

Seems logical to me.

Easily checked by swapping batteries around.

[MtB]

3 minutes ago, MartynG said:

Seems logical to me.

Easily checked by swapping batteries around.

 

Or even more easily with a DVM and clamp meter!

 

 

[Tasemu]

I got a clamp meter coming tomorrow from amazon and a spare starter battery in the boat which i'll try tonight. What voltage should my spare starter battery be at while the engine is running if I were to switch them out. Should it jump to 14v right away?

[Tasemu]

Also i've looked everywhere for a light for my alternator but have found none, only one big red light that comes on when i put the key in. Assuming that's just power on.

[MtB]

1 minute ago, Tasemu said:

Also i've looked everywhere for a light for my alternator but have found none, only one big red light that comes on when i put the key in. Assuming that's just power on.

 

No, that IS the light we are all asking about! What happens to it when you start the engine? Exactly?