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Why not wire alternator directly to battery?

92DripCherokee

NAXJA Forum User
Location
Tampa, Florida
My 92 4.0 has a long run of skinny 10 gauge wire from the alternator + side to the starter relay.

Is there a good reason why there's no shorter, heavier guage wire running directly from the alternator's + side to the battery? (fused of course)

It looks like the whole system was designed to save Chrysler some copper!

Does anyone have pics of their rewired 91 up charging system, upgraded alternators and all?
(Sometimes I wish I had a Renix cherokee! Damn things seem more serviceable.)
 
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That "olypen" writeup does cover the 91' and up, but it describes a different circuit arrangement than what I see on my jeep.


The olypen write up describes the alternator + wire running directly into the PDC, which on my jeep it does not.

On mine, the hot circuit goes from alternator b+, to starter, then battery, and finally to the PDC.

I'm going to find a charge circuit diagram in the mean time, but if anyone has pics of this kind of "bypass", i'd really appreciate it.
 
Why not run it directly to the battery? No particular reason, I suppose - other than accountants making engineering decisions.

Electrically speaking, you can run the alternator output to any terminal that has a direct connection to the battery anode - I've even seen it run to the start motor mains post (which is always hot!)

In later models with the PDC, you can run the alternator into the bottom of the PDC (OEM location,) to the input post on the PDC, and to the battery anode - all are the same connection, electrically speaking. However, once you move the alternator output lead from its original location, you will want to do something to restore fuse protection - because it's either going in through a fusible link, or a pair of MAXI60 fuses in the PDC. That's why I have ANL fuses and blocks (also provides for upgrading protection when installing an uprated alternator.)

10AWG is rather puny for a mains lead tho - that should be 6AWG or larger (larger, if you're going to uprate your alternator. I think 6AWG is right on the ragged edge for OEM output ratings, but that's in keeping with "beancounter engineering." An engineer would put in 4AWG or 2AWG to make himself comfortable - the cost difference is minimal, and Jeeps have a history of being modified anyhow.)
 
Well the +12 stud on the back of my alternator is so stubby, i dont see how i can piggyback a second wire on top of it and still tighten the nut,

is it safe to just completely omit the old run from the alternator to the starter and replace it with a run of 4 gauge straight to the batt (fused at 125 amps)?
 
I suspect they dont go direct to battery simply because that area is more prone to corrosion and more frequently messed with during battery changes.

If you make a run from alternator to battery you really should do away with the run to the starter. You dont want an alternate path through small wires from battery to starter fo obvious reasons.
 
If you run the alt straight to the battery, you will no longer be able to run the engine without a battery.

Stock, you can start the engine, then remove the battery and it will still run, because the alt is supplying all the engines demands.

Also, the volt reg senses the voltage & current needed for the vehicle, NOT the battery. It supplies this and charges the battery at the same time. That way, the battery will charge up AND the vehicle will run.

If you send all this straight to the battery, then as soon as you turn the vehicle off, the battery will be low, since the alt will only be supplying what is needed to recharge the battery. Stock it supplies the vehicle AND recharges the battery. If it goes straight to the battery, it will only charge the battery, while the battery is being sucked down at the same time by the vehicle.

The alt cannot instantly do a full recharge on the battery. So, if you run the alt to the batt, after you start and stop the engine a few times with a short recharge period, you will slowly kill the battery. You lose the full electric power to the vehicle with the excess recharging the battery. Now, you only charge the batt, and you are doing so at the same time it is being drained.

It's kinda like running my laptop off AC while the battery is being charged. If the AC only charged the battery (same as running the alt straight to the batt), I wouldn't be able to use it (like I am now) while the battery is being recharged, until the battery has been recharged enough to run the computer.

Also the battery is kinda a backup power supply for the vehicle. Stock, the alt supplies the vehicle, and when the alt dies, the battery does this until it dies. You don't even need a battery if you jump start it.

If you run the alt to the batt, it will work the same way, but the battery won't be at full charge when it takes over, and it won't nearly as long, since as the alt is dying, the battery charge is dropping. You lose the redundant safety factor of 2 power sources running the alt to the batt straight.

If the gage of wire is what concerns you, upgrade it, but don't run the alt to the batt only.
 
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Spoken like a true bookworm.

Disconnecting the vehicle's battery while the alternator is charging is a great way to destroy voltage sensitive electronics. Oh and the battery is the main supply for the vehicle, the alternator charges the supply (battery).

The main output stud on the alternator is always 'HOT'. FYI.
 
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TRCM said:
If you run the alt straight to the battery, you will no longer be able to run the engine without a battery.

Stock, you can start the engine, then remove the battery and it will still run, because the alt is supplying all the engines demands.

Also, the volt reg senses the voltage & current needed for the vehicle, NOT the battery. It supplies this and charges the battery at the same time. That way, the battery will charge up AND the vehicle will run.

If you send all this straight to the battery, then as soon as you turn the vehicle off, the battery will be low, since the alt will only be supplying what is needed to recharge the battery. Stock it supplies the vehicle AND recharges the battery. If it goes straight to the battery, it will only charge the battery, while the battery is being sucked down at the same time by the vehicle.

The alt cannot instantly do a full recharge on the battery. So, if you run the alt to the batt, after you start and stop the engine a few times with a short recharge period, you will slowly kill the battery. You lose the full electric power to the vehicle with the excess recharging the battery. Now, you only charge the batt, and you are doing so at the same time it is being drained.

It's kinda like running my laptop off AC while the battery is being charged. If the AC only charged the battery (same as running the alt straight to the batt), I wouldn't be able to use it (like I am now) while the battery is being recharged, until the battery has been recharged enough to run the computer.

Also the battery is kinda a backup power supply for the vehicle. Stock, the alt supplies the vehicle, and when the alt dies, the battery does this until it dies. You don't even need a battery if you jump start it.

If you run the alt to the batt, it will work the same way, but the battery won't be at full charge when it takes over, and it won't nearly as long, since as the alt is dying, the battery charge is dropping. You lose the redundant safety factor of 2 power sources running the alt to the batt straight.

If the gage of wire is what concerns you, upgrade it, but don't run the alt to the batt only.

I'm certainly no EE, but you are making no sense at all. Whether the alternator is hooked up to the battery or the PDC, it's providing power in the same way to the same places.

Does which side of a wire you tap for an accessory matter? If both sides of the wire are hot then it does not, all other factors being equal. I would assume the same thing holds for the charging side also.
 
Speed_racer said:
Dude, are you telling me you cant AT LEAST UNDERSTAND that a battery that is constantly being drained and charged simulatenously will live a shorter life?

And that you don't understand that like this you couldn't pull your battery while running w/out stalling the motor?

No I can't. If the alternator lead was hooked to the battery terminal then the battery terminal would simply become a connnection for the alternator lead to the wire from the battery to the PDC. In which case everything would still function because its getting the same amount of power. The big difference would be the alternator going nuts and the battery not being around to smooth out any voltage spikes from the alternator.
 
"Dude, are you telling me you cant AT LEAST UNDERSTAND that a battery that is constantly being drained and charged simulatenously will live a shorter life?"

Boats use this exact system and I have seen battery lives in those of almost five years. Battery maintenance is the key there though!
 
"Dude, are you telling me you cant AT LEAST UNDERSTAND that a battery that is constantly being drained and charged simulatenously will live a shorter life?":confused1

Someone should know when to keep quiet, you can't both charge and discharge at the same time, if system voltage is above 12V then the battery is charging... current IN. Switch on a winch and even with the alt voltage will drop and the battery will drain to assist the load...
I am an Electronics tech... go read ohms law and some basic DC voltage stuff... elementary..
Run it to the Bat and leave the factory one there... the purpose is to not overload the 10 gage wire... and if you add lights, winch etc at the battery, stereo well the alt has to send all the current through the skinny wire and since these accesories are more load than the factory calls for it gets hot and melts..
Leave whats there for the stock wiring harness and tandem over from the alt to the batt and you will be fine...
Don't worry about stray current... more hogwash.
The only stray current that will bother you is a ground loop. IE using different points of ground on the vehicle like the radio and an amp way in the back. If conncections aren't grounded well on both ends the "loop" is formed by the ground shield on the rca cable that connects them. If you don't know about them, just google.
 
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