Engine charging light and alarm coming on intermittently at low revs

Jungle Jim

Jungle Jim

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Please excuse the lack of technical detail, it's not my boat so I don't know all the make/models of the bits. I'm sort of honorary engineer as the skipper knows less than I do about it.

It's a Bav 36, 2005 model I believe. Friday night we moored up, put on the shore electrics although didn't realise the battery charger was not switched on. Next morning on booting up the instruments the auto pilot screen showed an error, which I believe signified low battery voltage. The Bav has two voltmeters, one registered 13.2 volts, the other 11.2 (don't ask me which battery is connected where). We realised the charger was off so turned it on for the remaining 10 minutes we were moored. The instruments then powered up normally.

On starting the engine after a few seconds at idle we got an intermittent engine alarm and charging light flickering on the engine panel. If we rev the engine it stops, and is fine when motoring.

The fan belt is a toothed belt and seems fine. All alternator connections that I can see are fine. After 2 days normal sailing, motoring and mooring on electric pontoons both batteries now show normal charged levels but we still have the engine issue.

I won't be back on the boat for two weeks and the skipper will probably pay a professional to sort it before then but for my own knowledge I woukd like to understand what is happening. Anyone got any ideas? Is it likely the low battery voltage had something to do with it?
 
When you allowed the battery voltage to drop so low, the demand for charging current will have been large. That will have placed the alternator under quite a load, which will in turn have required more power to be delivered to it by the belt. Any looseness in the belt will have led to a degree of slipping, so the alternator will not be turning fast enough to generate enough current to stop the charging alarm at tickover speed. That will have led to a degree of belt wear, hence the problem remaining despite charged batteries.

The light/alarm goes off when the engine is running at normal motoring speeds, as the belt slippage is overcome by the higher engine speed.

Check the tension of the alternator belt: if it is at all loose, tighten it. If it showing signs of significant wear, replace it.

If that fails to cure the problem, then get an engine person to check it over.
 
The alternator on my new engine started doing this, chirping and flickering at low revs after about 30 hours from new. A few days later it it died completely.
The manufacturers replaced it as a good will offer, thank you Beta.
A possible explanation from the electrician who tested it was that one of my batteries had been deeply disharged or faulty, and the alternator flogged itself to death trying to catch up.
That's probably technically wrong but it's in terms that I can understand.
 
I am not sure I agree with either answer. I think more likely is that the slip rings have become dirty or brushes lost some tension (pressure onto the slip rings) because of wear. This means you need to remove the alternator for bench check or at least remove it so you can check the slip rings.
Alternators often are a bit doubtful in performance at idle revs. Much depending on the pulley ratios. More engine revs give more capability to generate power. At idle revs it will not produce much power hence much less load on the belt so I doubt slipping is the problem. good luck olewill
 
A 2005 alternator should still be good.
How old is the belt?
Is the engine bay clear of any black dust (the sign of prolonged belt slippage)?
Assuming the batteries are good, my best bet is the belt with an outside chance of a corroded connection.
 
Mine did this, my engine will tickover happily at very low revs and the light starts to flicker just because the alternator is running too slowly. Basically my alternator doesn't produce full output until the engine is at 900 revs, alternator about 3000 and I think you'll find this the case with yours. Cure could be a smaller alternator pulley, not good as it cripples the belts. Mermaid sent me a small electrical component to fit across the charging light terminals, it keeps the alternator circuit open (or whatever) at low revs.
 
Get the batteries charged fully again, and then see what it does. If it's still flickering, then the alternator is either knackered or your diode splitter is failing.
 
Bobc said:
Get the batteries charged fully again, and then see what it does. If it's still flickering, then the alternator is either knackered or your diode splitter is failing.
Click to expand...

If you really suspect the alternator fit an ammeter, mine tells me exactly what's going on. I toyed with fitting a bigger one after having the same issues as you. Mine is 'only' 40 amp, and I have plenty of kit switched on all day. The ammeter tells me that my maximum discharge is 32 amps (inc bilge pumps and every light), and fitting a bigger alternator would not help, it won't cut in any sooner, it won't give max output below 3000 revs. If I'm ticking over for a long time with everything switched on then I start to lose, I have to crank the engine up to 900 revs to get parity.
 
As this happened after the battery had been deeply discharged it is reasonable to assume that this is causing the problem. I would start by charging off the mains overnight and see if you still have this problem in the morning. It might be worth investigating further though as the alternator should charge the battery regardless of its state of charge.
I doubt if a reasonably modern boat has a diode splitter these days.
It would help others with more electrical knowledge to know more about the setup.
 
ghostlymoron said:
As this happened after the battery had been deeply discharged it is reasonable to assume that this is causing the problem. I would start by charging off the mains overnight and see if you still have this problem in the morning. It might be worth investigating further though as the alternator should charge the battery regardless of its state of charge.
I doubt if a reasonably modern boat has a diode splitter these days.
It would help others with more electrical knowledge to know more about the setup.
Click to expand...
You are quite right, the issue arose after a discharged battery. Alternator worked hard to catch up, either belt worn and loose and/or brushes worn, showing up now because they had to pass a large amount of current to energise the alternator more. Classic symptoms of worn brushes by the way.
Either case, lady skipper, put your hand on the alternator pulley and try and turn it, if it turns within the belt then the belt is loose. Easy to tighten.
If the belt is tight, get an electrician to look at the brushes.
Stu
 
It does look like worn brushes or similar fault in the alternator.
The warning light circuit is mportant for the alternator to start working, so if the brushes are OK, a quick check of the alt light socket and wiring might be worthwhile.
 
I have a pair of TAMD40b's which have the flickering ignition issue, but both charge fine and have no charging issues.

I suspect a glitch in the splitting mechanism, but also suspect Alternator(s)

However, it works and I have lots of batteries. I'll look at it later. In life :encouragement:
 
No Regrets said:
I have a pair of TAMD40b's which have the flickering ignition issue, but both charge fine and have no charging issues.

I suspect a glitch in the splitting mechanism, but also suspect Alternator(s)

However, it works and I have lots of batteries. I'll look at it later. In life :encouragement:
Click to expand...

Is that only when both are running?
Two identical alternators not sharing the load happily...?
 
Thanks all for the replies. This started after forgetting to turn the charger on over Friday night. We were on overnight mains in Poole on Saturday, and had the motor on as well over the weekend for probably another 2 hours in total so everything was fully charged and it still happened.

I checked the belt, it was tight with no signs of wear. All the connections that I could see looked corrosion free. The suggestions that the brushes are worn could fit the bill, we must have drawn a lot of current on Saturday morning and I doubt that they have ever been changed. I know from working with dc motors that the first sign of worn brushes is intermittent operation and heavy load wears them very quickly. They could have been very worn to start and that extra load finished them off, we'll get that checked first.
 
Jungle Jim said:
Thanks all for the replies. This started after forgetting to turn the charger on over Friday night. We were on overnight mains in Poole on Saturday, and had the motor on as well over the weekend for probably another 2 hours in total so everything was fully charged and it still happened.

I checked the belt, it was tight with no signs of wear. All the connections that I could see looked corrosion free. The suggestions that the brushes are worn could fit the bill, we must have drawn a lot of current on Saturday morning and I doubt that they have ever been changed. I know from working with dc motors that the first sign of worn brushes is intermittent operation and heavy load wears them very quickly. They could have been very worn to start and that extra load finished them off, we'll get that checked first.
Click to expand...

The brushes of an alternator are very different to those of a DC motor. In the DC motor the brushes rub on a commutator which has copper segments perhaps 16 or so each insulated from the next and the brushes carry all the motor current as it skips across the segments. So a lot of wear.
By contrast the alternator brushes rub on a smooth copper ring carrying continuous current (although regulated) of the order of 1 or 2 amps to provide field current. so should and do last a lot longer than a DC motor but still eventually thy wear and get poor connection.
Usually on the back of the alternator there will be a small plate or often on cars a regulator plastic box screwed on the back which carries the brushes. Remove the screws and the plate or reg will come out with brushes attached. You will then be able to see the slip rings which can be cleaned with fine wet and dry sand paper or brasso. olewill
 
I'm basing my view on what I think I know, but don't pretend to be an expert.

That said ... I think that what you are seeing is perfectly normal. The alarm simply says whether there is more power being spent than being generated. At low revs, you will be generating very little - perhaps near to nothing.

If you have a low battery sucking power, and perhaps your instruments and fridge are on, then up pops the alarm. Nothing wrong, everything working, it just needs a few more revs to turn the equation around.
 
Update for those interested.

All fixed now :encouragement:

It was indeed worn brushes, they were worn down to nubs and would have been about 15mm longer when new. Thanks all for the suggestions.
 
Yes, only when both are running! A quick flick on one throttle and it goes away, perhaps all day. It only does it when it feels like it, and not very often, but has done it for a few years. I suspect a split diode, but you know what electrics are like...
 
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