Charging circuit - reduced load on ammeter circuit

[67Dart273]

67Dart273 PM'ed me about this thread as he recalled mine linked below. The dual diodes allow your dash ammeter to still work at lower currents, but shunts progressively more current straight to the battery at higher outputs. The ammeter is intended to read all current flow in and out of the battery, except for underhood loads (starter, horn) or special cabin loads like convertible motor or spotlight.
Modernized Engine Wiring | For A Bodies Only Mopar Forum

1963 and 1965 have the dedicated buss-bars which 67Dart273 shows. 1965 is better since it has more terminals. In 1964, they moved the large IGN and BAT wires to regular 57 terminals, which might have caused melting issues (familiar?), so apparently walked that back in 1965. In 1966, they moved the wiper motor into the engine bay, so added a 3rd terminal row and put IGN and BAT back to spade terminals, I recall bigger ones but not big enough (hence these hacks). I changed my 1964 Valiant to a 1965 bulkhead from a Fury. I had to cut the square opening ~1/8" taller to fit.

Thanks Bill. For those getting bogged in the convoluted "discussion" in that thread, the gist is that you use those diodes back to back across the ammeter, WHY?

Because if you BYPASS the ammeter with wire it is useless. If you want to do that and use a voltmeter, fine, that is OK too. But if you use those diodes, they have a point at which they do nothing. Because they are doing nothing, the ammeter reads NORMALLY. When the ammeter read more and more current, at some point the voltage drop across the meter becomes great enough to turn on one of the diodes, depending on which way current is flowing. (charge or discharge) This then forms a parallel path across the ammeter which limits the current through the thing.

 

[Mattax]

Right? Even though the ammeter won't read accurately anymore, you'll still know when there's a "problem".

Not always.
For examples you won't know of any high charging and overcharging.
Add a voltmeter and you may be wise to a high voltage situation, which would cause overcharging.

 

[RustyRatRod]

Yep, and that us what I am doing. The extra wire is essentially a "hard to see" safety update for a "stock" resto on my car. If something goes wrong with my car, I sure don't want it to be caused by the ammeter circuit!!

How does that make the ammeter "act"?

 

[BillGrissom]

... at some point the voltage drop across the meter becomes great enough to turn on one of the diodes, depending on which way current is flowing. ...

Correct, except the diode doesn't suddenly turn on, but starts conducting current progressively more as the voltage drop across it increases. Think of it as a "current relief valve" (opens proportionally like a back-pressure regulator instead of popping open). Before picking a diode, I measured the voltage drop from ALT+ to BATT+ (which the diodes would see) with the alternator maxed out (by applying full-field). The drop was ~0.5 V (or whatever I wrote). I recall I did this in my 1965 Chrysler, which might have had a round-back alternator then. Regardless, its ammeter was almost pegged. No worries if your alternator outputs more, as the diodes quickly conduct more current as the voltage drop increases, up to their rating of ~200A. The rating depends on cooling, so might check how hot they get at max load. Heat is conducted away by the thick copper cables (6 awg, I recall). I used a different PN on my 1965 Dart (lower rated germanium diodes, I recall) before I found this PN on ebay which installs easier (cut in two, flip and bolt together), which I put on my Valiant and Chrysler.

Re over-charging, as in olden days, if you see your ammeter needle staying on the "C" side for what seems too long, you have a problem. As I recall my 1969 Dart long ago, when driving it daily, the ammeter would run ~1/8th scale after starting for ~5 min, then return to center. But, depends on how long you must crank. If you also want a voltmeter, you can buy cig-lighter ones cheap from ebay or bald-guy. Some are integral w/ a USB adapter.

 

[RustyRatRod]

Not always.
For examples you won't know of any high charging and overcharging.
Add a voltmeter and you may be wise to a high voltage situation, which would cause overcharging.

I'm not arguing....just asking/trying to learn here.
Let's say I run a hefty wire from the alternator to the battery. So I get used to how the ammeter reads after that. "WHY" would I not notice a change "either" way whether it's charge or discharge? I mean I'm used to how it works now. It works correctly. If I get used to how it reads with the new wire run, what will keep me from noticing a big change? I get that running the extra wire will reduce the ammeter's reading, but won't it still register "change"?

 

[Mattax]

Sure.
Given:

Let's say I run a hefty wire from the alternator to the battery. So I get used to how the ammeter reads after that.

Then

WHY" would I not notice a change "either" way whether it's charge or discharge?

Because this

I get that running the extra wire will reduce the ammeter's reading, but won't it still register "change"?

The ammeter won't see any flow along the new, hefty, wire.
If the wire direct from the alternator the battery is hefty, electricity for recharging the battery will take that path.
So what does that mean?

The ammeter only works when electricity flows through it. The electomagnetic field created by the electricity flowing through the meter deflects the needle.
Why would there be any current through the meter if there is a shorter path to the battery?

It still should show discharge.
Why?

Where does the electricity wants to go?
And what's the shortest path there?
Is it all the way across the engine bay to the alternator, and then from the alternator all the way to the main splice?

Probably not.
Bottom line will come down to which route has the least resistance.

@Mopar Tim did a wire around on his '66 and this what he observed.

 

[Demonx2]

Hopefully about 4 months from now I'll be able to shed some real-world experience too as this is what I am doing (adding a 10ga wire direct to batt in parallel with the normal route through the big splice).

 

[Mattax]

Why would there be any current through the meter if there is a shorter path to the battery?

Could it happen? Sure.
When the new hefty wire got so overloaded that the longer path through the 12 ga wire and 16 ga fusible link had less resistance.


If it helps, picture a river delta.
When the river is relatively low its all going to flow through the deepest, widest channel.
The water in the side channels hardly move.

 

[RustyRatRod]

Sure.
Given:

Then

Because this

The ammeter won't see any flow along the new, hefty, wire.
If the wire direct from the alternator the battery is hefty, electricity for recharging the battery will take that path.
So what does that mean?

The ammeter only works when electricity flows through it. The electomagnetic field created by the electricity flowing through the meter deflects the needle.
Why would there be any current through the meter if there is a shorter path to the battery?

It still should show discharge.
Why?

Where does the electricity wants to go?
And what's the shortest path there?
Is it all the way across the engine bay to the alternator, and then from the alternator all the way to the main splice?

Probably not.
Bottom line will come down to which route has the least resistance.

@Mopar Tim did a wire around on his '66 and this what he observed.

Yeah. You seem to not understand you're tellin me everything HERE that I already know. The big wire....while it reduces current to the ammeter, does not stop it altogether. So it will still "do something". It will still register "something" although at a reduced capacity. It will now have a "new" operating range.

My question is how do you know for 100% sure that I cannot get used to this new operating range and gauge what the system is doing accordingly? From what I am gathering, you're simply telling me the same thing over and over that I already know and are not answering my question. Have you not actually tried it?

 

[Mattax]

Yeah. You seem to not understand you're tellin me everything HERE that I already know. The big wire....while it reduces current to the ammeter, does not stop it altogether. So it will still "do something". It will still register "something" although at a reduced capacity. It will now have a "new" operating range.

LOL. I'm trying to get you to see what you're missing.
But now I'll tell ya.
It basically redirects all the flow through the ammeter from alternator to the battery.
You will see zero charging on the ammeter.

What's it take to recharge a battery?
If its 5 amps or 30 amps, its all going to take the easy path through your new hefty wire.

Get up to 50, 60 or 100 amps and sure maybe some takes the route through the ammeter because the new path got too crowded.
By then its too late anyway.

 

[RustyRatRod]

LOL. I'm trying to get you to see what you're missing.
But now I'll tell ya.
It basically redirects all the flow through the ammeter from alternator to the battery.
You will see zero charging on the ammeter.

What's it take to recharge a battery?
If its 5 amps or 30 amps, its all going to take the easy path through your new hefty wire.

Get up to 50, 60 or 100 amps and sure maybe some takes the route through the ammeter because the new path got too crowded.
By then its too late anyway.

I get what you're saying. You've got me to see it, believe me. Is this something you've actually tried?

 

[Mattax]

Have you not actually tried it?

No. Tim tried it. I did the parallel feed. That keeps the ammeter seeing everything the battery is doing. I even moved the MSD's power supply over.
If Tim doesn't answer here, you know where to find him. He can tell you exactly how much charging, if any, he saw on it.
What he did see was discharge. That will continue to work pretty much as before.

 

[Mattax]

@Mopar Tim I think is who said what he saw, and then got me thinking about why that would happen.
Like you, I had thought it would simply split the load, and so the ammeter would still show something.

 

[RustyRatRod]

No. Tim tried it. I did the parallel feed. That keeps the ammeter seeing everything the battery is doing. I even moved the MSD's power supply over.
If Tim doesn't answer here, you know where to find him. He can tell you exactly how much charging, if any, he saw on it.
What he did see was discharge. That will continue to work pretty much as before.

Yeah, I understand you'll see discharge. That's been made abundantly clear. lol Remember, the needle doesn't have to be on the charge side to go "up". I guess I won't know until or unless I try it myself to see if it's something I can live with.

 

[mopowers]

Wouldn't it depend on the sensitivity of the gauge? If the difference in current that the gauge sees is minimal, it may not be enough to actually move the needle, would it? Kinda like adjusting the resolution on a multimeter?

 

[67Dart273]

If you see anything I doubt it will be much. The ammeters seem to be about 40A scale. Or so I've heard. A direct bypass wire with only two end terminals, and much shorter wire, even if only no12, should have much lower resistance than the olc ammeter circuit

 

[Mattax]

By co-oincidence I'm headed down to the garage now to amongst other things, fix the alternator wire on my heep.

It seemed to start choking up flow around 23 amps. was testing the output with a clamp meter.
I tried to clean the wire up last week but was short on time.

Bought some new 10 ga marine wire. Not sure I'll get it today or not.

FWIW this is an '85 grand wagoneer so it has electric choke, two electric solenoids, and the parking lights include side markers.
Only load it doesn't have, in theory, is field current

Current draw measured for each major accessory is on the far right.

14.5 amps Headlights is not just headlights. Its headlights plus parking and instrument panel. Subtract 5.3 amps for 55 Watt headlights alone.
Bat V is what I measured at the battery, Alt V is what I measured at the alternator output stud.
The alternator is putting out 4 amps to just run the Duraspark and those solenoids.

So there's some real world measurements of power needed to run stuff.

 

[Mopar Tim]

You do not see anything on amp meter. Did it. Installed volt meter. Problem solved.