Voltage drops across the entire system

[irish.david]

You would not need a 12 Volt regulator to protect anything on the vehicle from 14.4 Volts. Everything on our vans is just fine at 14.4 Volts (even car stereos and alarms etc are fine up to 16 Volts)

I was thinking of running a cable from the alternator regulators sense wire all the way to the starter battery so the alternator would regulate to the battery voltage terminals and not just the alternator output.

In almost every vehicle setup the sense wire of the regulator is connected directly to the alternator output which regulates the output voltage directly at the alternator. In most vehicles this isn’t a problem as the battery is right next to the alternator so the cable losses aren’t an issue, but in our vans the battery is miles away so although you might get a nice regulated 14.4V at on the alternator terminals this is significantly reduced when it gets to the battery due to the losses in the cables.

If you connect the sense wire to the battery terminals then the regulator will adjust the output of the alternator so you get 14.4V at the battery terminals regardless of the cable loss. As an extreme example, if the total cable losses from the alternator to the battery were 2V then the voltage at the alternator terminals would be 16.4V and the battery voltage would stay at 14.4V.

With this setup you would get a battery that would always charged pretty quickly (you’d probably actually use a regulator set to 14V to avoid overheating the battery) but you’d have everything on the alternator side of the batter, which is all the engine electrics, connected to a voltage supply that would be going up and down depending on how much the voltage drop on the cable from the alternator to the battery was, which is why you’d need a 12V regulator for the engine electrics.

Another issue would be that you’d have to occasionally check condition of the cabling as any problems with the cabling wouldn’t be apparent at the battery as the alternator would be compensating for it and could in the end overheat the whole cable.

Just a thought anyway


On the subject of DC current measurement, you can pick up a DC clamp meter on Amazon for about £20 that would measure the charging current without mucking about with Shunts or putting anything in series with cabling. Would make things like this a lot easier


On a side note, if you remove the alternators diode block and do some mucking about with the sense wire you can get alternators to produce about 220V AC, but the frequency is obviously all over the place as it’s proportional to the engines speed…

[CovKid]

I will say this, when I came to upgrade my main cables, I was amazed at just how far beneath the insulation, acid erosion had occured (it must surely contribute to voltage drop):



The copper tends to go white/green and gets brittle and as strands break away, the overall conductivity diminishes. It also seems reasonable to assume that the breaks can occur anywhere along this cable too.

I cut the original alternator output cable away completely and replaced it with the same heavy cable I used from battery to starter (at least four times as thick but still multi-strand) with properly soldered ring connectors using a small blowtorch and shrink tubing to seal from the elements. A 90amp alternator has the perfect connection for a thick cable:



I've not finished by a long shot, but wanted a good sound skeleton to work from, pending any other changes next year. I recently created a kind of bus bar at the dash end to take a thicker cable than stock to help reduce voltage loss from battery to dash. I've not completed it yet but from there I ran to fusebox etc.

The gains with starting were very noticeable, and as I say, charging isn't an issue. However, the body itself forms part of most circuits and as its not copper...

Ghost might be able to confirm this but I presume if rather than grounding headlights to body, the earth ran direct to battery via good cable, voltage drop would be minimal, possibly obviating the need for relays?

[ghost123uk]

However, the body itself forms part of most circuits and as its not copper... Ghost might be able to confirm this but I presume if rather than grounding headlights to body, the earth ran direct to battery via good cable, voltage drop would be minimal, possibly obviating the need for relays?

Two things here Ralph, imho, the amount of metal connecting the battery earth place to the headlight earth place is so great that any resistance would be so low as to be unmeasurable with normal meters, even though it is steel not copper (or Silver ). I mean, the starter pulls a couple of hundred Amps via the body so that kinda proves it for me.

Second, re "possibly obviating the need for relays?" doesn’t compute (unless I am misunderstanding) as the relays are there (as I know you know) to circumnavigate voltage drop in the live feed cables rather than the earth path. To anyone else reading this, if doing any upgrades to headlight wiring (or vehicle sound system etc) don't forget to upgrade the earth cables as well as the feed cables as they are an equally important part of the circuit.

[MidLifeCrisis]

Second, re "possibly obviating the need for relays?" doesn’t compute (unless I am misunderstanding) as the relays are there (as I know you know) to circumnavigate voltage drop in the live feed cables rather than the earth path. To anyone else reading this, if doing any upgrades to headlight wiring (or vehicle sound system etc) don't forget to upgrade the earth cables as well as the feed cables as they are an equally important part of the circuit.

As well as the cable loss the relay(s) also performs the important function of bypassing the (very lossy) ignition switch, headlight switch and high-beam stalk switch all of which add their own considerable levels of voltage drop!!

An interesting test to try is to put the headlights onto highbeam then pull the stalk towards you (as you would to flash the high beams) - you'll see the headlights get considerably brighter which is a consequence of the flash action circuit not going through the ignition switch or the headlight switch!!!

[ghost123uk]

An interesting test to try is to put the headlights onto highbeam then pull the stalk towards you (as you would to flash the high beams) - you'll see the headlights get considerably brighter which is a consequence of the flash action circuit not going through the ignition switch or the headlight switch!!!

Aye, it was noticing that many years ago that prompted me to fit a relay upgrade (behind the grill version). I did it to our Scirooco too, years before we even had the T25. That was well before it was covered on here, though I won't claim to be the one that started the subject

[jeffdub]

well I've had a bit of a problem today fitted a new alternator belt and a 14.50 voltage regulator to the alternator and as was well until today when the red dash light came on , checked the back of the alternator and it's only producing 11.90 v according to my multimeter

[ghost123uk]

checked the back of the alternator and it's only producing 11.90 Volts

That is probably just the voltage coming back from the battery (unless you were testing it with the battery disconnected). The alternator is likely producing nothing. Try putting the old regulator pack back on temporarily.

[jeffdub]

bit of a problem there john as I can't find it , think I may have thrown it away by mistake

[CovKid]

John, I was mostly referring to the use of thicker cable from battery to dash to reduce voltage drop. I did note some time ago that the main feed to my fusebox had got warm in the past at the spade terminal which is why I decided to upgrade input cable when I fitted new main cable to starter/alternator, and make better connections all round.

And yes you're right about the use of relays, particularly to save switchgear but I reckon a good sound path from battery to headlights has to be a good thing, especially if you have big spotlamps as I do.

[jeffdub]

new alternator fitted and all's well, fingers crossed