Alternator drawing from the battery

[BWM]

I have discovered that the alternator on my son's boat is emptying the battery once the engine is stopped, an odd fault that i haven't come across before. Someone on our travels suggested that a defective diode was the likely cause, is this a straightforward fix or best left to a professional? It appears to charge perfectly, especially considering the low rpm of the engine. 

  I have attached a picture of the connections, it appears that the previous owner has used two wires for both positive and negative feeds with the fifth cable going from the central spade connector to the live terminal of the battery. 

[GUMPY]

Assuming that the fifth cable is what would normally be connected through the warning light / ign switch this could be draining the battery.

Try disconnecting it (fit a switch in line) when not in use and see if that cures the problem. 

 

ETA it may be that the alternator will self excite with it disconnected, see ifi it charges without it connected, so you may not need it at all

 

Edited December 3, 2018 by Loddon

[BEngo]

That is a Lucas A127.Available from ECUs Direct for £50 or so next day delivered. It may be  a diode or the way it is fitted- see below - but for that money why worry?

The wiring looks fine.  The black wire is the field and should have a switch and a 2 W light in it between the alternator and the battery.  If it does not then the battery will discharge through the regulator when the engine is not running.   Charge the battery and then disconnect this wire. If the battery still goes flat the alternator is at fault.  If not sort the field wiring before buying a new alternator.

N

[MoominPapa]

I predict that if you disconnect the central spade terminal, the battery drain will stop.

 

That terminal should be switched by the ignition switch, a for preference it should have a warning light the the circuit.

 

MP.

 

[Sir Nibble]

42 minutes ago, MoominPapa said:

I predict that if you disconnect the central spade terminal, the battery drain will stop.

 

That terminal should be switched by the ignition switch, a for preference it should have a warning light the the circuit.

 

MP.

 

It should certainly have a warning light in circuit. That's your problem, wrongly wired.

[ditchcrawler]

13 minutes ago, Sir Nibble said:

It should certainly have a warning light in circuit. That's your problem, wrongly wired.

Would the alternator get warm while not running if its wired direct to the batteries ?

[Tony Brooks]

7 minutes ago, ditchcrawler said:

Would the alternator get warm while not running if its wired direct to the batteries ?

Yes but not as hot as when its running and charging. Its only the rotor that will get warm and that has typically a draw of about 3 to 4 amps energised and stationary.

[dmr]

7 minutes ago, ditchcrawler said:

Would the alternator get warm while not running if its wired direct to the batteries ?

I had some sort of diode failure on one of mine a while ago and  the alternator got very noticeably warm.

 

..............Dave

[Tony Brooks]

1 minute ago, dmr said:

I had some sort of diode failure on one of mine a while ago and  the alternator got very noticeably warm.

 

..............Dave

It would do, that  gives pretty much a dead short through the failed diode apart from the voltdrop across the second., in the OP's case it is likely to be a bit cooler than yours.

[BWM]

I only checked the temperature when running, but the battery would be entirely flat by morning unless the alternator was disconnected. 

Thanks for the feedback. 

The electrics are generally quite shaky, i have improved and simplified all standard connections but know little about the correct way of wiring an alternator - not guilty of the current set up!

Edited December 3, 2018 by BWM

[WotEver]

13 minutes ago, BWM said:

The electrics are generally quite shaky, i have improved and simplified all standard connections but know little about the correct way of wiring an alternator

Take a look at the schematic, 7th image on this page. It should help. 

http://www.tb-training.co.uk/MarineE08.html#CHARGING SYSTEM – ALTERNATOR

[dmr]

Do you have an "ignition" or charge light? or maybe there is an alternator on-off switch that you have to remember to turn off at the end of your engine run (I have seen this done).

 

If you have an ignition light (or equivalent resistor) I would not expect the battery to go flat in a day unless its in poor condition. This is where one of those clamp on ammeters might be a useful tool to own.

 

...........Dave

[BWM]

4 hours ago, WotEver said:

Take a look at the schematic, 7th image on this page. It should help. 

http://www.tb-training.co.uk/MarineE08.html#CHARGING SYSTEM – ALTERNATOR

Thanks 

4 hours ago, dmr said:

Do you have an "ignition" or charge light? or maybe there is an alternator on-off switch that you have to remember to turn off at the end of your engine run (I have seen this done).

 

If you have an ignition light (or equivalent resistor) I would not expect the battery to go flat in a day unless its in poor condition. This is where one of those clamp on ammeters might be a useful tool to own.

 

...........Dave

No switch or indicator lamp, just a wire from terminal to battery. I didn't like the look of it, and my multi meter shows the equivalent battery voltage when put on the alternator terminals with engine off.

[dmr]

There could be a switch somewhere, but is this a recently purchased or repaired boat?  Those wires look very new so maybe somebody has done a temporary (or incompetent) rewire of the alternator.

 

.............Dave

[Tony Brooks]

The lack of a warning lamp or far less commonly an "energisation switch" means the battery will be discharged and unfortunately the constant discharge means the batteries have been partially or fully discharged for a while so have probably badly sulphated with little storage capacity.

 

The simplest way to sort it is to take a wire from the "ign. on" terminal on the ignition switch to a 2 watt plus bulb and from the bulb to the push on blade on the alternator.

 

If you don’t have an ignition switch put a push button switch that will not latch on in that black wire so it goes battery - new switch - alternator but I would also like a bulb in that wire. Once the engine is running push the switch to energise the alternator. But a proper warning lamp is better, gives visual indication if the alternator has problems (sometimes) and avoids other rarer problems.

 

 

[Sir Nibble]

It seems lost upon those who carry out wiring like this that the alternator is trying to charge the battery through those little 8A diodes.

[Sea Dog]

19 hours ago, BWM said:

....but the battery would be entirely flat by morning unless the alternator was disconnected. 

If that's been happening, I suspect little will change there even after a correctly wired, properly functioning alternator is achieved.  Add a new battery to the budget!

 

[BWM]

16 hours ago, dmr said:

There could be a switch somewhere, but is this a recently purchased or repaired boat?  Those wires look very new so maybe somebody has done a temporary (or incompetent) rewire of the alternator.

 

.............Dave

Yes, we bought the boat a few weeks ago. It has been laid up for about seven years and i believe has remained an unfinished project throughout its existence since 1992! The situation is complicated by a large built in petter generator (defunct) with a shared start/control panel. The only functional item on this panel is a start button but this only operates the starter motor. I don't believe the wires are particularly new, possibly the wire in question may have been connected to the panel in the past and bodged at some stage. 

16 hours ago, Tony Brooks said:

The lack of a warning lamp or far less commonly an "energisation switch" means the battery will be discharged and unfortunately the constant discharge means the batteries have been partially or fully discharged for a while so have probably badly sulphated with little storage capacity.

 

The simplest way to sort it is to take a wire from the "ign. on" terminal on the ignition switch to a 2 watt plus bulb and from the bulb to the push on blade on the alternator.

 

If you don’t have an ignition switch put a push button switch that will not latch on in that black wire so it goes battery - new switch - alternator but I would also like a bulb in that wire. Once the engine is running push the switch to energise the alternator. But a proper warning lamp is better, gives visual indication if the alternator has problems (sometimes) and avoids other rarer problems.

 

 

I should be able to rig up something along these lines. 

1 hour ago, Sea Dog said:

If that's been happening, I suspect little will change there even after a correctly wired, properly functioning alternator is achieved.  Add a new battery to the budget!

 

A new battery was fitted just prior to identifying the fault and is still fine, the previous near new one appears to have survived without any lasting damage after a good charge, fortunately. 

[BWM]

On 03/12/2018 at 09:31, BEngo said:

That is a Lucas A127.Available from ECUs Direct for £50 or so next day delivered. It may be  a diode or the way it is fitted- see below - but for that money why worry?

The wiring looks fine.  The black wire is the field and should have a switch and a 2 W light in it between the alternator and the battery.  If it does not then the battery will discharge through the regulator when the engine is not running.   Charge the battery and then disconnect this wire. If the battery still goes flat the alternator is at fault.  If not sort the field wiring before buying a new alternator.

N

I am not with my son on the boat at the moment but got him to charge and then remove the wire, and then take a multi meter reading across the +/- terminals on the alternator. He had a reading of equivalent battery voltage, i assume that isn't correct?

[Sir Nibble]

That is entirely correct. All you need is a warning light and the system will be corrected.

[jenevers]

Hmm. This is sounding like a problem I mentioned in another topic, where I’m getting a 5Amp drain even though every Electrical device is switched off. I’ve got a clamp meter on its way  via eBay, but I’m going to have another look at my Leece Neville alternator.

[BWM]

1 hour ago, jenevers said:

Hmm. This is sounding like a problem I mentioned in another topic, where I’m getting a 5Amp drain even though every Electrical device is switched off. I’ve got a clamp meter on its way  via eBay, but I’m going to have another look at my Leece Neville alternator.

I identified this problem by disconnecting the alternator after charging the battery, it's worth taking a terminal off the battery beforehand to avoid any flashing over when removing the wires from the alternator.