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How is this one going?

Another thing I think I forgot to mention was, your alternator may need an upgrade if you go with electric fans. And with the alternator upgrade, you may also need to upgrade the wiring from the alternator to the battery.

Do you have the stock alternator?
 

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How is this one going?

Another thing I think I forgot to mention was, your alternator may need an upgrade if you go with electric fans. And with the alternator upgrade, you may also need to upgrade the wiring from the alternator to the battery.

Do you have the stock alternator?
Actually, the wiring to the battery should still be adequate. If you are going to run an intermittent high-current device, run it directly off the alternator or close thereby. This may require upgrading the wiring between your alternator and your power distribution block.
 

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How is this one going?

Another thing I think I forgot to mention was, your alternator may need an upgrade if you go with electric fans. And with the alternator upgrade, you may also need to upgrade the wiring from the alternator to the battery.

Do you have the stock alternator?
Actually, the wiring to the battery should still be adequate. If you are going to run an intermittent high-current device, run it directly off the alternator or close thereby. This may require upgrading the wiring between your alternator and your power distribution block.
Looks like we agree more than we disagree. The entire charge circuit should be examined. Anything between the alternator and the battery that can't carry the increased current needs upgrading.
 

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Discussion Starter · #26 ·
Sorry to bring up my thread again. What is your guys recommendation for brand of thermostat fan switch? Thanks again guys. I have the thermostat and fans ordered. I really appreciate your help
 

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Looks like we agree more than we disagree. The entire charge circuit should be examined. Anything between the alternator and the battery that can't carry the increased current needs upgrading.
If the fans are properly wired, none of the extra current will travel through the battery charging wire. Once the engine is running, the alternator is the provider of all current.
 

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Sorry to bring up my thread again. What is your guys recommendation for brand of thermostat fan switch? Thanks again guys. I have the thermostat and fans ordered. I really appreciate your help
From an earlier post:
For a 180°F thermostat, I've used Standard Motor Products TS244 or an equivalent. It turns on about 205°F-210°F, and turns off between 190°F and 195°F. Original application was Mazda 4 cylinder engines from early 1990's (tell the parts guy you have a 1991 Miata and he will get you the right part). The one disadvantage to this one is it's metric threads, so you'll need an adapter bushing or you'll need to drill and tap your own hole for it. For a 195°F thermostat, I use something that cross references to GM 14043275. AC Delco D1855B or Standard Motor Products TS85 are common (although I think the AC Delco may now be discontinued). That one turns on around 215°F and turns off around 205°F, which matches up with a 195°F thermostat well.
The TS244 is metric threads. M16x1.5 is from possibly faulty memory. Check it with your parts source. You'll need an adapter like this one https://www.amazon.com/dp/B07GT8P9H4 to fit it in a 1/2 NPT port on the intake or head.

The other one is 3/8 NPT (again from possibly faulty memory). You may need a bushing for that one as well.

What alternator do you have? You will need to upgrade if you have the stock 10si unit at 63 amps. Your adding about 40 amps of load, often at idle. See my posts above.
 

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Discussion Starter · #29 ·
From an earlier post:

The TS244 is metric threads. M16x1.5 is from possibly faulty memory. Check it with your parts source. You'll need an adapter like this one Amazon.com: Hydraulic Pipe Fitting 1/2" NPT Male to Metric M16 M16X1.5 Female Adapter : Industrial & Scientific to fit it in a 1/2 NPT port on the intake or head.

The other one is 3/8 NPT (again from possibly faulty memory). You may need a bushing for that one as well.

What alternator do you have? You will need to upgrade if you have the stock 10si unit at 63 amps. Your adding about 40 amps of load, often at idle. See my posts above.
It's a 100 amp car quest alternator
 

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Discussion Starter · #30 ·
I'm running 12 or 10 gauge wire from the starter to the alternator at the moment. I have ordered a distribution block though I thought I would run 10 or 8 gauge to the distribution block then 10 gauge to the alternator and 10 gauge again to the fans
 

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It's a 100 amp car quest alternator
That's not so specific. Do you have the part number? Or the application? What car did you tell them you had that originally came with this 100A alternator?

Does the back of the alternator look like this one:

That's the 10Si type alternator that was originally installed in the car. The original was 63A, but there are "upgraded" (modified or "hot rodded") versions available at 100A or more. It has the two pin connection at the 3 O'clock position in that picture, with the terminals labeled "1" and "2". These alternators are know for being very weak at idle, usually producing less than half their rated current when the engine is idling.

Or does the back look like this one:

That's the back of a CS130 alternator. These alternators perform much better at idle speed, usually producing around 75% to 80% of their rated current even at idle. They are much more appropriate for cars with electric fans. That one has the 4 pin connector at the 3 O'clock position in the pic, with the connector marked "PLFS" or sometimes "PLIS". You can get an adapter harness (AC Delco PT2145 or equivalent) to connect this alternator to the harness connector for the Si series alternators. More to follow on how to wire these.

I'm running 12 or 10 gauge wire from the starter to the alternator at the moment. I have ordered a distribution block though I thought I would run 10 or 8 gauge to the distribution block then 10 gauge to the alternator and 10 gauge again to the fans
Is that the wire with the ring that goes on the threaded post on the back of the alternator? Or is that the sense wire, connected to the "2" terminal on the 10si or the "S" terminal on the CS130?

Proper wiring for these alternators:

Main charge power: This is the big threaded post. On the 10si pictured above, it's between 1 O'clock and 2 O'clock and it's marked "BAT". on the CS130 pictured above, it's around the 7 O'clock position, and it's not labeled. AFAIK, the bean counters at GM (who are cheap bastards and known to squeeze every last penny in cost savings) allowed the engineers to use a 8AWG wire from the ring terminal either back to the battery positive terminal, or to a junction block that connects to the battery terminal by a 8AWG or larger wire (or to the starter post). That's adequate for the original 63A alternator, but it's inadequate for a 100A or 105A alternator. If you upgrade to 100A or more, you need a 6AWG wire in this location for a 80A to 120A alternator. The wire should go directly to the battery, or to the starter power post where the battery cable is attached, or to a power junction block that connects to the battery with at least a 6AWG wire. Smaller than that can lead to the wire overheating and the insulation melting, and it can also cause issues with the battery not charging fully.

Lamp connection: This is the "1" terminal on the 10si alternator, and the "L" terminal pin on the CS130 alternator. In the factory wiring harness, there's a (usually) brown wire here that goes back to the incandescent light bulb in the instrument cluster that lights up the "Alt" or "Gen" light (sometimes even "Bat"). The power to that bulb comes from an ignition switched source that's on (12V) when the key is in the "run" position, but off when the key is turned to the "start" position. If you're not running a light bulb in the dash, you can install a 5 watt 52 ohm resistor in the wire from the ignition switched power source. When you first turn the key to the "run" position, before starting the engine, the light should be lit. When you turn the key to the "start" position, it should go off. If the engine stops (technically, if the alternator stops turning), the light should come back on. You can test that by disconnecting the solenoid wire at the starter and turning the key back and forth between start and run.

Sense connection: This is the "2" terminal on the 10si and the "S" terminal pin (the bigger one) on the CS130 alternator. The purpose of this is to sense the battery voltage so that the voltage regulator can adjust the current output of the alternator downward when the battery is fully charged. It should go to the battery with a 12AWG wire. If the run is long (more than about 8 feet), it should be upgraded to 10AWG. A whole lot of folks connect this terminal with a short wire to the ring lug on the "Bat" post on the alternator. That's bad practice. In some cars it will work for a while, especially if the charge wire is oversized for the alternator current. GM always ran a wire back to the battery, or to a power junction block with a 6AWG or larger connection to the battery IIRC. Again, the cheap bastards in the accounting department would not have spent the money on a longer, larger wire if they didn't absolutely have to. When someone brings me an older car with charging issues (battery going dead), and I see that short wire on the back of the alternator, from the sense terminal to the charge post, I'll usually run the correct wire to the battery, and a lot of times, that cures the problem.

Ground connection: These alternators ground through the case to the mounting bracketry, and through that to the engine. You'll sometimes see a ground wire from the atlernator case to the negative battery terminal, or to a ground point on the frame or body. DO NOT DO THIS! If you're having a grounding issue with the alternator, it's acceptable to run a ground wire from the alternator to the engine block or head. That should complete the circuit for the alternator. If you use an anodized aluminum bracket or the brackets are powdercoated, this alternator ground wire may even be necessary. The wire should be sized the same as the big wire on the ring terminal for the "Bat" wire/charge wire. If you connect a wire from the engine or the alternator case to the frame or to the negative battery terminal, it becomes a potential ground path for the starter. If the other ground strap(s) and large ground cable(s) from the engine/transmission fail, you'll be running hundreds of amps from the starter through whatever "alternator ground wire" you installed. The wire will vaporize when you turn the key to the start position, and it can start a fire under the hood. Don't ask me how I know this.

10 gauge is entirely adequate for connecting the battery to the alternator. Install a 14-gauge fusible link in series.
No, it's not.
If the fans are properly wired, none of the extra current will travel through the battery charging wire. Once the engine is running, the alternator is the provider of all current.
OK. RIddle me this. I've got a 105A CS130 with a harness adapter on the smaller connector, and the alternator wired with a 10 gauge wire on the big threaded post (which is inadequate for even a 63A alternator). How do I make sure "the fans are properly wired"? Where does the alternator output current go if it doesn't go down that 10 gauge wire (and overload it)?

Let's say one nice sunny spring Saturday, I jump in the car (it's been parked for almost a week), roll the windows down (it's to nice not to, and too nice to turn on the A/C, too warm to turn on the heat, so the HVAC stays off), and I start the car. It's on the fast idle, and the alternator is making at least 80A, which is 20A more than the stock wiring was designed to handle. The thermostat hasn't opened. I don't have the headlights or the wipers on. The only "load" currently active is the ignition coil. Where does the extra 20A go if it doesn't go through that stock wiring (or the undersized wiring you seem to be advocating)? How is that charge wiring not going to see 80A coming from the alternator?

Now, I need to be somewhere, so after the engine is started, maybe 30 seconds later, I back out of the driveway and start driving down the road. The engine is now running over 2,000 RPMs, and the alternator is now producing it's rated 105A (or very close to it). The engine is not up to operating temp, so the fans won't activate on their temperature switches. So still, the only load on the electrical system is the ignition. Again, where is all that extra amperage going if it's not going through that wire attached to the threaded post on the back of the alternator? How will an undersized wire on that post not be overloaded?
 

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No, it's not.

OK. RIddle me this. I've got a 105A CS130 with a harness adapter on the smaller connector, and the alternator wired with a 10 gauge wire on the big threaded post (which is inadequate for even a 63A alternator). How do I make sure "the fans are properly wired." Where does the current go if it doesn't go down that 10 gauge wire (and overload it)?

Let's say one nice sunny spring Saturday, I jump in the car (it's been parked for almost a week), roll the windows down (it's to nice not to, and too nice to turn on the A/C, too warm to turn on the heat, so the HVAC stays off), and I start the car. It's on the fast idle, and the alternator is making at least 80A, which is 20A more than the stock wiring was designed to handle. The thermostat hasn't opened. I don't have the headlights or the wipers on. The only "load" currently active is the ignition coil. Where does the extra 20A go if it doesn't go through that stock wiring (or the undersized wiring you seem to be advocating)? How is that charge wiring not going to see 80A.

Now, I need to be somewhere, so after the engine is started, maybe 30 seconds later, I back out of the driveway and start driving down the road. The engine is now running over 2,000 RPMs, and the alternator is now producing it's rated 105A (or very close to it). The engine is not up to operating temp, so the fans won't activate on their temperature switches. So still, the only load on the electrical system is the ignition. Again, where is all that extra amperage going if it's not going through that wire attached to the threaded post on the back of the alternator? How will an undersized wire on that post not be overloaded?

First off, you should understand how alternators work. An alternator is a device that rotates a copper conductor through a variable magnetic field to produce an electric current.
Just because it is rated at 80A at a specific rpm doesn't mean that it always produces 80 amps at that rpm. The voltage regulator (VR) is designed specifically to handle this problem. Let's say your system is drawing 20 amps while your car is idling in the driveway. At this time the VR is regulating the field current to produce the proper magnetic field density (gauss) that will keep the voltage at the setpoint (14V) while supplying the necessary current. At this time your alternator is not producing the rated full load current because it has nowhere to go. It's Ohm's Law.
Let's say you turn on the headlights and run the heating fan while still idling in the driveway, running the load up to 40A. In an unregulated system the voltage would drop dramatically as the gauss is insufficient to produce this amount of current. Fortunately, the VR is paying attention to this voltage drop and increases the field current and gauss, which causes the alternator to produce more current and maintains the voltage at the setpoint. The VR does this via the 'sense' wire, which should be attached to the main power distribution point.
You're still not producing rated power. If you happen to exceed the power/rpm rating the voltage will droop and you have to increase the rpm, which will move more conductors through the magnetic field and get the voltage back up.

Addressing the wiring issue -

The wiring between the main power distribution point (MPDP - the horn relay on many old GM cars) and the battery serves two purposes - to power the ignition when you are cranking the car and to provide current to recharge the battery while the engine is running. Since the ignition key circuit is designed to remove power from almost all devices except the ignition when you are cranking, it's not a large current and the stock wiring will be quite adequate regardless of the rating of the alternator. The instant that the alternator is spinning fast enough to create a voltage higher than the battery voltage, current flow in this wire is reversed and the battery is being recharged.
Let's say that you crank your engine for 3 seconds before it starts, and the starter current is 150A. In terms of amp-hours, this is 0.00083 a/h. When the alternator takes over and provides 10 amps of charging current, how long will it take to replace the charge lost from the battery? Roughly 45 seconds. Once the battery is charged this current goes to some minimum determined by the characteristics of the battery, basically a trickle charge.
Consequently, there is no reason for a larger wire between the battery and the MPDP. But there are solid engineering reasons to install wiring between the alternator and MPDP reflective of the expected circuit current, to use the MPDP as the primary source of power as well as connecting the sense wire to the MPDP.
 

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You've left the battery charge current out of your calculations. What happens when the driver leaves the lights on, and the battery runs down and the car has to be jump started?

Charge current can be calculated fairly easily. It's alternator (or charger) output voltage minus actual battery open circuit voltage (when discharged, this is low) divided by the internal resistance of the battery (in Ohms). A fully charged car battery typically has an internal resistance around 20 milliOhms (0.020 Ohms). It's possible to measure that using a volt meter, a known load and some calculations with Ohm's law. When the battery is discharged, the internal resistance increases, and when the battery is "dead," the resistance is generally about forty percent higher than when it's fully charged. To make the math easy, let's say it's 30 milliOhms for out car that had the lights left on.

After we leave the lights on for a few hours, the open circuit voltage of the battery might be 6V or lower, but let's say there was still a little bit of charge left in the battery, just not enough to start the car. To make the math easier, let's say the open circuit voltage was 7.8V. And let's say the alternator is trying to maintain the system at 13.8V. That's 6V difference at 30 milliOhms internal resistance. Doing the math, 6/0.030=200A. Obviously we don't have a 200A alternator, so the voltage regulator is going to be running the field coils in the alternator full strength, sending the full 105A to the output terminal, and the charge terminal voltage at the alternator will be lower than 13.8V. Whatever isn't consumed by other loads (ignition, lights if they're on, HVAC fan if it's on, etc. etc. including electric radiator fans, if they're running) will go to the battery. If the battery needs 40Ah to reach full charge, and we have a 105A alternator operating at full rated output and the other loads (ignition, etc.) average 25A during the charge time, it'll take at least 30 minutes to charge the battery fully (105A alternator output - 25A loads = 80A left for battery charging). It will actually take longer, because as the battery approaches full charge, the voltage regulator will throttle back the current some, and the rate of charge will be slower at the end of the charging cycle. This also assumes the engine is running above idle speed, where the alternator is capable of delivering full output. Idle time will lengthen the time to fully charge the battery even further.

Now, unless you're planning to change that wire any time you leave your lights on, or just put a jumper cable from the alternator post to the battery while the battery is recharging from an "accidental discharge," the wire from the alternator needs to be able to safely carry the full output of the alternator. That's why vehicle manufacturers always use a wire that's at least marginally adequate for the full output of the stock (or optional) alternator that was installed in the car. That's also why when you upgrade the alternator to a higher output unit than was offered on the car when the car was built, it's a really good idea™ to upgrade the alternator charge cable (the cable on the threaded post) to match the new alternator.
 

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Discussion Starter · #35 · (Edited)
Sorry for the late
That's not so specific. Do you have the part number? Or the application? What car did you tell them you had that originally came with this 100A alternator?

Does the back of the alternator look like this one:

That's the 10Si type alternator that was originally installed in the car. The original was 63A, but there are "upgraded" (modified or "hot rodded") versions available at 100A or more. It has the two pin connection at the 3 O'clock position in that picture, with the terminals labeled "1" and "2". These alternators are know for being very weak at idle, usually producing less than half their rated current when the engine is idling.

Or does the back look like this one:

That's the back of a CS130 alternator. These alternators perform much better at idle speed, usually producing around 75% to 80% of their rated current even at idle. They are much more appropriate for cars with electric fans. That one has the 4 pin connector at the 3 O'clock position in the pic, with the connector marked "PLFS" or sometimes "PLIS". You can get an adapter harness (AC Delco PT2145 or equivalent) to connect this alternator to the harness connector for the Si series alternators. More to follow on how to wire these.


Is that the wire with the ring that goes on the threaded post on the back of the alternator? Or is that the sense wire, connected to the "2" terminal on the 10si or the "S" terminal on the CS130?

Proper wiring for these alternators:

Main charge power: This is the big threaded post. On the 10si pictured above, it's between 1 O'clock and 2 O'clock and it's marked "BAT". on the CS130 pictured above, it's around the 7 O'clock position, and it's not labeled. AFAIK, the bean counters at GM (who are cheap bastards and known to squeeze every last penny in cost savings) allowed the engineers to use a 8AWG wire from the ring terminal either back to the battery positive terminal, or to a junction block that connects to the battery terminal by a 8AWG or larger wire (or to the starter post). That's adequate for the original 63A alternator, but it's inadequate for a 100A or 105A alternator. If you upgrade to 100A or more, you need a 6AWG wire in this location for a 80A to 120A alternator. The wire should go directly to the battery, or to the starter power post where the battery cable is attached, or to a power junction block that connects to the battery with at least a 6AWG wire. Smaller than that can lead to the wire overheating and the insulation melting, and it can also cause issues with the battery not charging fully.

Lamp connection: This is the "1" terminal on the 10si alternator, and the "L" terminal pin on the CS130 alternator. In the factory wiring harness, there's a (usually) brown wire here that goes back to the incandescent light bulb in the instrument cluster that lights up the "Alt" or "Gen" light (sometimes even "Bat"). The power to that bulb comes from an ignition switched source that's on (12V) when the key is in the "run" position, but off when the key is turned to the "start" position. If you're not running a light bulb in the dash, you can install a 5 watt 52 ohm resistor in the wire from the ignition switched power source. When you first turn the key to the "run" position, before starting the engine, the light should be lit. When you turn the key to the "start" position, it should go off. If the engine stops (technically, if the alternator stops turning), the light should come back on. You can test that by disconnecting the solenoid wire at the starter and turning the key back and forth between start and run.

Sense connection: This is the "2" terminal on the 10si and the "S" terminal pin (the bigger one) on the CS130 alternator. The purpose of this is to sense the battery voltage so that the voltage regulator can adjust the current output of the alternator downward when the battery is fully charged. It should go to the battery with a 12AWG wire. If the run is long (more than about 8 feet), it should be upgraded to 10AWG. A whole lot of folks connect this terminal with a short wire to the ring lug on the "Bat" post on the alternator. That's bad practice. In some cars it will work for a while, especially if the charge wire is oversized for the alternator current. GM always ran a wire back to the battery, or to a power junction block with a 6AWG or larger connection to the battery IIRC. Again, the cheap bastards in the accounting department would not have spent the money on a longer, larger wire if they didn't absolutely have to. When someone brings me an older car with charging issues (battery going dead), and I see that short wire on the back of the alternator, from the sense terminal to the charge post, I'll usually run the correct wire to the battery, and a lot of times, that cures the problem.

Ground connection: These alternators ground through the case to the mounting bracketry, and through that to the engine. You'll sometimes see a ground wire from the atlernator case to the negative battery terminal, or to a ground point on the frame or body. DO NOT DO THIS! If you're having a grounding issue with the alternator, it's acceptable to run a ground wire from the alternator to the engine block or head. That should complete the circuit for the alternator. If you use an anodized aluminum bracket or the brackets are powdercoated, this alternator ground wire may even be necessary. The wire should be sized the same as the big wire on the ring terminal for the "Bat" wire/charge wire. If you connect a wire from the engine or the alternator case to the frame or to the negative battery terminal, it becomes a potential ground path for the starter. If the other ground strap(s) and large ground cable(s) from the engine/transmission fail, you'll be running hundreds of amps from the starter through whatever "alternator ground wire" you installed. The wire will vaporize when you turn the key to the start position, and it can start a fire under the hood. Don't ask me how I know this.


No, it's not.

OK. RIddle me this. I've got a 105A CS130 with a harness adapter on the smaller connector, and the alternator wired with a 10 gauge wire on the big threaded post (which is inadequate for even a 63A alternator). How do I make sure "the fans are properly wired"? Where does the alternator output current go if it doesn't go down that 10 gauge wire (and overload it)?

Let's say one nice sunny spring Saturday, I jump in the car (it's been parked for almost a week), roll the windows down (it's to nice not to, and too nice to turn on the A/C, too warm to turn on the heat, so the HVAC stays off), and I start the car. It's on the fast idle, and the alternator is making at least 80A, which is 20A more than the stock wiring was designed to handle. The thermostat hasn't opened. I don't have the headlights or the wipers on. The only "load" currently active is the ignition coil. Where does the extra 20A go if it doesn't go through that stock wiring (or the undersized wiring you seem to be advocating)? How is that charge wiring not going to see 80A coming from the alternator?

Now, I need to be somewhere, so after the engine is started, maybe 30 seconds later, I back out of the driveway and start driving down the road. The engine is now running over 2,000 RPMs, and the alternator is now producing it's rated 105A (or very close to it). The engine is not up to operating temp, so the fans won't activate on their temperature switches. So still, the only load on the electrical system is the ignition. Again, where is all that extra amperage going if it's not going through that wire attached to the threaded post on the back of the alternator? How will an undersized wire on that post not be overloaded?
Sorry for the late reply. The part number is 7861-7A

Also, isn't there a fusible link that is on the alternator wire as well?
 

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Sorry for the late reply. The part number is 7861-7A
OK, so you already have a 105A CS130 alternator. Did you do a serpentine belt setup on this engine? Or did you swap pulleys on the alternator when you installed it?

It looks like maybe you have the early 1990's 4.3L V6 serpentine belt accessory package from a S10 Blazer or S10 pickup truck, or maybe from a full size truck/SUV V8 from the late 1980s through 1993 or so. It looks like all the truck TBI small block engines used that alternator. That's what that alternator fits. If it's working, don't mess with it. That one is definitely adequate for an electric fan setup with your car. That's the same alternator as I was recommending, but I was recommending it with a v-belt pulley and a different clocking on the rear cover (plug for the wires facing different direction), to make it easier to install with the original stock v-belts.

If it's working, I take it someone installed a wiring harness adapter, or rewired the harness in the car for the 4 pin plug (2 wires populated, the L and the S terminals).

Also, isn't there a fusible link that is on the alternator wire as well?
There probably was on the stock wiring. With modifications, it's up to you. Personally, I prefer a "megafuse" or "midifuse" at the firewall or near the battery to protect the charge wire (depending on which way the alternator charge wire is routed). I would use a 150A fuse with a 6 AWG wire to that alternator. Modern cars built to the latest safety standards use a fuse on the battery terminal assembly for the alternator, and that's the best protection for the battery and the wiring, and the best place to run the big alternator cable to. But that's a little more complicated than most folks want to do.

If you want a wire with a fusible link, there is a pre-assembled 6 AWG wire assembly available from AC Delco with a fusible link included in the wire at the alternator end. AC Delco part number 6ST-44. It's 44 inches long, if that works for you, that's an easy bolt on solution for a charge wire upgrade.
 

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After we leave the lights on for a few hours, the open circuit voltage of the battery might be 6V or lower, but let's say there was still a little bit of charge left in the battery, just not enough to start the car. To make the math easier, let's say the open circuit voltage was 7.8V.
Here's a chart of open-circuit voltage versus % charge.
Font Material property Rectangle Parallel Number


It will actually take longer, because as the battery approaches full charge, the voltage regulator will throttle back the current some, and the rate of charge will be slower at the end of the charging cycle.
How does the voltage regulator sense the battery voltage? On most cars it only knows the voltage at the output of the alternator.

A fully charged car battery typically has an internal resistance around 20 milliOhms (0.020 Ohms). It's possible to measure that using a volt meter, a known load and some calculations with Ohm's law. When the battery is discharged, the internal resistance increases, and when the battery is "dead," the resistance is generally about forty percent higher than when it's fully charged. To make the math easy, let's say it's 30 milliOhms for out car that had the lights left on.
If the internal resistance of the battery is lower when it is fully charged, why doesn't the charging current increase as the battery approaches full charge? If we have a fully charged 20 mOhm battery at 12.89V with 13.8V impressed on it, what is the current? It's not 45 amps. In my car, my alternator voltage is 14.5v. Do I have 80 amps going into my fully-charged battery all the time?
Or is the internal resistance not a real factor in calculating charging current?
 

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Here's a chart of open-circuit voltage versus % charge.
View attachment 443913



How does the voltage regulator sense the battery voltage? On most cars it only knows the voltage at the output of the alternator.



If the internal resistance of the battery is lower when it is fully charged, why doesn't the charging current increase as the battery approaches full charge? If we have a fully charged 20 mOhm battery at 12.89V with 13.8V impressed on it, what is the current? It's not 45 amps. In my car, my alternator voltage is 14.5v. Do I have 80 amps going into my fully-charged battery all the time?
Or is the internal resistance not a real factor in calculating charging current?
[/QUOTE
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