Alternator wires

[Maverick]

Does a very basic Alternator set-up just need a negative and a positive wire connecting to it to return output charge to the battery or are more wires required such as a live feed in and a live feed out?

[Guest]

Does a very basic Alternator set-up just need a negative and a positive wire connecting to it to return output charge to the battery or are more wires required such as a live feed in and a live feed out?

The ones I have require a small wire via a bulb to apply a field, a larger wire from the output to the positive terminal on the battery, and an earth to the battery from the engine block. There is also a wire to the tachometer. Snibble and others will be able to give you far more detail.

[mayalld]

Does a very basic Alternator set-up just need a negative and a positive wire connecting to it to return output charge to the battery or are more wires required such as a live feed in and a live feed out?

 

An alternator needs 3 connections.

 

The negative (B-) is almost invariably supplied via the casing, rather than by a cable.

 

The output (B+) is the one that will connect to the thick cable connecting to the battery.

 

Input (D+) is used to energise the alternator, and must be connected to the battery positive. In a very basic set-up, it would be acceptable to simply connect B+ and D+ together.

 

Terminal W is used to drive a rev counter.

[Maverick]

An alternator needs 3 connections.

 

The negative (B-) is almost invariably supplied via the casing, rather than by a cable.

 

The output (B+) is the one that will connect to the thick cable connecting to the battery.

 

Input (D+) is used to energise the alternator, and must be connected to the battery positive. In a very basic set-up, it would be acceptable to simply connect B+ and D+ together.

 

Terminal W is used to drive a rev counter.

 

Thanks for that. Just one query - you say Input (D+) the energiser must be connected to the battery possitive. would this be the norm or would this feed wire come from the ignition switch so it was only live when the engine had started?

[mayalld]

Thanks for that. Just one query - you say Input (D+) the energiser must be connected to the battery possitive. would this be the norm or would this feed wire come from the ignition switch so it was only live when the engine had started?

 

It is normally fed from the ignition switch, via the alternator warning light.

 

It must be live *before* the alternator starts turning to energise the field before the alternator becomes self energising.

 

Once the alternator is producing sufficient output to energise itself, current will cease to flow to D+, and the light will extinguish.

 

If the output falls away, then D+ will keep the field energised (and the light will come on).

 

Running alternators with a de-energised field is a "bad thing", and likely to blow the regulator, so D+ serves two purposes;

1) get the alternator up and running

2) provide a safety net in case the revs fall too low for charging, so as to prevent the alternator killing itself.

[Maverick]

It is normally fed from the ignition switch, via the alternator warning light.

 

It must be live *before* the alternator starts turning to energise the field before the alternator becomes self energising.

 

Once the alternator is producing sufficient output to energise itself, current will cease to flow to D+, and the light will extinguish.

 

If the output falls away, then D+ will keep the field energised (and the light will come on).

 

Running alternators with a de-energised field is a "bad thing", and likely to blow the regulator, so D+ serves two purposes;

1) get the alternator up and running

2) provide a safety net in case the revs fall too low for charging, so as to prevent the alternator killing itself.

 

Brilliant Thanks

[Sir Nibble]

STOP!

DO NOT connect D+ to the battery even through the ignition switch, you will blow your alternator's field diodes.

D+ should be connected to the charge warning light and thence to ignition +ve.

What type of "basic" alternator did you have in mind?

[mayalld]

STOP!

DO NOT connect D+ to the battery even through the ignition switch, you will blow your alternator's field diodes.

D+ should be connected to the charge warning light and thence to ignition +ve.

What type of "basic" alternator did you have in mind?

 

Ah, that was the bit I forgot.

 

D+ needs the resistance of the warning light in series to ensure that it doesn't get the full 12v.

 

I shall go to the back of the class.

[Maverick]

STOP!

DO NOT connect D+ to the battery even through the ignition switch, you will blow your alternator's field diodes.

D+ should be connected to the charge warning light and thence to ignition +ve.

What type of "basic" alternator did you have in mind?

 

 

No probs got that.

 

Just trying to make some sense of the awful wiring on a boat I've recently purchased.

It had a starter and a leisure battery linked together with just one isolater switch. Turn the switch and the wire to the starter becomes live as it should. Then on the same connection is the main leisure feed to all electrics plus a main feed to the alternator.

 

The starter motor even has a negative feed atached to its casing as well as the main negative feed from the battery.

 

Bit by bit I'm making sense of it all and changing things with the advice of the forum members and Tony Brooks excellent website.

[Gibbo]

Running alternators with a de-energised field is a "bad thing", and likely to blow the regulator, so D+ serves two purposes;

 

?

 

Gibbo

[Gibbo]

STOP!

DO NOT connect D+ to the battery even through the ignition switch, you will blow your alternator's field diodes.

 

Surely only if B+ is disconnected will this be a problem?

 

Gibbo

[Sir Nibble]

No. If you connect D+ to the battery then the alternator will try to use the field diode output to charge the battery alongside the main output, and since these are only 5Aish diodes.... need I say more?

[Gibbo]

No. If you connect D+ to the battery then the alternator will try to use the field diode output to charge the battery alongside the main output, and since these are only 5Aish diodes.... need I say more?

 

It would only be a problem if B+ was disconnected.

 

Gibbo

Edited July 10, 2007 by Gibbo

[Sir Nibble]

I do not understand why you believe that to be the case. Some years ago when identical looking machine and battery sensed alternators were about, It was common for machine sensed alternators to blow their field diodes by charging through the sense lead.

A few months back, Elessina posted a picture of some explosively demolished field diodes, he had a wrong connection on his alternator and the same problem occurred.

The electrical circuit from stator to D+ is identical to the circuit from stator to B+, only the diode rating differs.

In short, it does happen even if B+ is soundly connected, seen it with my own eyes, dozens of times.

[Gibbo]

I do not understand why you believe that to be the case.

 

Because the cable from D+, up to the ignition switch (which is what is being discussed here) and back down to the battery has far too much resistance to create enough current through the field diodes unless B+ was disconnected.

 

Gibbo

[Sir Nibble]

How far from the alternator is the ignition switch? How far from the ignition switch is the battery? What is the total resistance of the cables and battery? Sounds like too many crossed fingers to me. Like I said, seen it happen dozens of times.

[Gibbo]

What is the total resistance of the cables and battery?

 

Well think about it. If the cable from D+, up to the ignition switch and back down to the battery is only 1 Ohm in total, you'd need a 5 volt difference between the battery post and the D+ terminal before you exceeded the rating of the field diodes. And as the B+ terminal is nominally at the same voltage as D+ and B+ is connected to the battery post you'd have to have something seriously wrong to achieve that.

 

Gibbo.

[Sir Nibble]

Fine, there's the maths applied to a hypothetical situation, but I repeat, seen it happen, dozens of times. Bear in mind that a good proportion of the load on a vehicle is electrically closer to the ignition switch than the battery. A boat is different of course, and I cannot fault your logic, nonetheless it does seem to be a crossed fingers tightened sphincter form of "safe" that I am not personally happy with.

[Serendipity]

I know I posted this on another thread, but perhaps more in place here, where I reckoned from memory my charging lead was about 3-4mm. Well I measured it this weekend and it's 5mm including sheath, and my alternator is an 80A Mitsubishi. After Gibbo saying that was seriously under-rated I squeezed my digits round it, and yes it's quite warm for the entire length, as is the split relay casing. So I know I ought to upgrade the cabling AND relay (I didn't check but am assuming it's a 40A unit) - thanks for the heads up.

 

On that subject, can any of you guys explain why my relay (connected to the same terminal as the charge indicator lamp), energises with the switch in the auxiliary position before starting the engine? I disconnected it and measured the voltage and got 1.8V?

 

My concern with this is that the domestics at a state of reasonable charge will be a drag on the starter battery at best, and surely could be dangerous if they are completely flat when suddenly linked to the starter through such a feeble cable? I've got in the habit of just switching on the main starter battery, then the domestics once the engine is running.

 

Julian

[chris w]

I know I posted this on another thread, but perhaps more in place here, where I reckoned from memory my charging lead was about 3-4mm. Well I measured it this weekend and it's 5mm including sheath, and my alternator is an 80A Mitsubishi.

 

That'll make it a 10mm² cross-section cable - certainly one would expect to find maybe 40mm² as being more normal for the charging lead from the alternator.

 

On that subject, can any of you guys explain why my relay (connected to the same terminal as the charge indicator lamp), energises with the switch in the auxiliary position before starting the engine? I disconnected it and measured the voltage and got 1.8V?

 

What's the other side of the relay connected to? If it's a 12v source, then the relay will energise on auxiliary because current can flow through the relay to earth via the alternator rotor winding (this is why the charge lamp lights as well initially).

Chris

[Sir Nibble]

?????? The relay pulls in at 1.8V?????

Bloody hell!

A while ago, someone had a problem with their alternator refusing to cut in until revved. We all advised ever less and less resistance across the warning light until we saw a picture of the alternator and realised we were on course to blowing the regulator 'cos it was not the type we thought we were dealing with. So, can you find a part number on the alternator?

Worst case is a flat starter battery causing the domestics trying to meet the demands of the starter through cables and relay not robust enough to take it. There is no way this should happen and it needs sorting.

[chris w]

?????? The relay pulls in at 1.8V?????

Bloody hell!

 

I suspect, Snibble, that he was probably measuring the voltage at the D+ terminal to ground and found 1.8v. If the other side of the relay is at battery potential then that would mean there was around 11v across the relay when it pulled in. Just a guess.

 

Chris

[John Orentas]

Hi Julian.

 

Your split charge relay has been wired wrongly, it should never energise until your engine is running and the alternator charging. One of the incidental functions of the relay is to prevent overloading of your wiring, for example if your starter battery happens to be near flat when you attempt to start your engine the domestic circuit will have a path through your then energised relay. A potential 150+ amps load could try to pass through the light wiring and the relay contacts which would probably overheat or even melt.

[Gibbo]

I suspect, Snibble, that he was probably measuring the voltage at the D+ terminal to ground and found 1.8v. If the other side of the relay is at battery potential then that would mean there was around 11v across the relay when it pulled in. Just a guess.

 

Chris

 

That means the relay would energise to enable him to melt all the insufficiently sized cables between the relay and the batteries when he starts the engine. Then when the engine starts the relay will de-energise and ensure he has no split charging.

 

Split charge relays and split charge cables should be capable of handling starter current and any maximum domestic loads including inverters and chargers. I use 95mm and 120mm on my boat and *proper* 200 amp continuous relays.

 

Those little plastic cube relays aren't fit to light a headlamp let alone split charge huge battery banks.

 

Gibbo

[Sir Nibble]

I suspect, Snibble, that he was probably measuring the voltage at the D+ terminal to ground

You may be right. When I saw this figure, the first thing that went through my mind was not "Oh look a 1.8V relay" but "Human error". I am reluctant to say so even when it's fairly obvious 'cos an offhand "you're doing it wrong" can be seen as derogatory, and after all, the magic word "disconnected" was used.

This looks to me like a relay connected into a system on the assumption that this is an ordinary 9 diode alternator. Bet it aint!

Maybe this is a job for my alternative split charge relay circuit, (ign +ve to start solenoid 50).