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Thread: How does the alternator regulator sense wire work?

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    How does the alternator regulator sense wire work?

    Hey Guys,
    I've been wondering... how does the alternator regulator "assess" the battery SOC(state of charge) using the SENSE WIRE??!!,
    Regulator can adjust the alternator current output(B+ wire) to the battery accordingly by controlling the amount of current it pass to the field coil.

    Both SENSE wire and the B+ wire joins the battery, either as separate wires or joined each other somewhere between the alternator and the battery positive terminal.

    Voltage of the alternator charge wire(B+) is always at minimum of 13.8V, and that's usually the voltage we measure at the battery as well(may be a fraction of a volt difference behind the alternator and at the battery terminal due to slight voltage through wire harness)

    I guess, what I'm asking is whether the voltage that the regulator sense wire sense from the battery is somehow partially "compromised" by the alternator B+output to the battery itself!!! In other words, is it possible that the sense wire can't accurately assess the battery SOC, 'cos it can sense the voltage output of the alternator B+ as well?! Or does chicken or the egg question make any sense?
    For example, if a battery is at 90% SOC, does the alternator regulator sense wire sense that accordingly and adjust the alternator output current? Or Is this the reason that the alternator can fail to charge a battery closer enough to battery's highest capacity due to "error" of regulator getting feedback from the alternator output voltage??!!

    Or since the voltage and current are electrical measurements that are "directly proportional" to each other(assuming the battery resistance does not change with voltage), regulator can't increase the current flow to the battery without increasing the voltage(but keeping it regulated at maximum of 14.4V)??!!
    Or if the battery SOC is pretty low, would me measure lot less than 13.8V(the minimum alternator output voltage or what is required to charge the battery at the battery terminals)??!!

    Thanx in advance for any explanation(s)
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    The "Sense" wire is designed to be connected directly to the battery or as close as possible. The reasoning for it is to ensure the correct voltage is at the battery overcoming any volt-drop (Vd) and resistance of the main output cable to the battery and any other devices in the path like dual-battery controllers or diodes (0.4-0.7 Vd across each diode in series). When working, maximum safe voltage is kept on the battery to keep in best state and power all devices.

    Never tap anything into this wire or mess with it. I have seen people shorten it to the back of the alternator which results in poor charging and way shortened battery life. If you do a battery relocation, a new sense wire should be run to the new battery location.
    There is no legal cure for STUPID!

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    Quote Originally Posted by LostBenji View Post
    The "Sense" wire is designed to be connected directly to the battery or as close as possible. The reasoning for it is to ensure the correct voltage is at the battery overcoming any volt-drop (Vd) and resistance of the main output cable to the battery and any other devices in the path like dual-battery controllers or diodes (0.4-0.7 Vd across each diode in series). When working, maximum safe voltage is kept on the battery to keep in best state and power all devices.
    Maximum voltage should be around 14.4V, isn't it(unless the sense wire has gone open circuit)?!
    Isn't that safe enough?

    Never tap anything into this wire or mess with it. I have seen people shorten it to the back of the alternator which results in poor charging and way shortened battery life. If you do a battery relocation, a new sense wire should be run to the new battery location.
    Factory setup of the GQ(not sure if all of them) was to join the sense wire with the charge wire, only few centimeters from the alternator rear. I've done the modification of cutting that wire from the join and re-wire that directly to the battery with a new piece of wire & a fuse. The idea was to improve the alternator sense wire sensing capacity & charge the battery better with an extra half voltage or so, as the idea mentioned in your above post.
    I'm still confused about how the sense wire circuit is able to accurately sense the battery voltage without being affected by the alternator's charge voltage output.(It says alternator's charge output always put out a minimum of 13.8V as measured at the battery terminals).

    I'm wondering whether the concept of voltage sensing or "measuring" is involved at all with this process of sense wire's sensing battery SOC.

    Does the sense wire sense the changes of current flow across it, towards the battery, based on the battery's SOC(low charge battery drawing more amps out of it)?!

    Or does my question make little sense from the point of view of the fundamental concept of electricity measurements like voltage and current(they are interdependent electricity measurements, not separate)??!!

    In other words am I asking a non-sensible or nonsensical question??!!
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    Alternators are a rectified AC source providing a current limited regulated DC output.
    They have no 'idea' what the SOC of the battery is nor the servicieability of the battery.
    The Battery +ve is really just the common point for the vehicle load and is after any voltage drop in the high current wiring.

    The alternator will just increase the current to try and maintain the sense voltage at that common point as more load appears.
    It will do that until it cannot provide any more current at the RPM that it is turning.
    At that time when you are essentially current limited and the voltage will start to sag if further load appears or RPM drops.

    Alternators or generators are there to run the vehicle load, the ability to 'sort of' charge a battery is just a by product.
    In the most basic way to look at it the Alternator has nothing to do with the battery, that just happens to be the common point for the sources (Alternator and Battery) to connect to the load.

    The sense wire is there to determine load and has no idea about battery in isolation.
    Which is why alternators and generators are shit at charging batteries.
    That isn't why they are there nor is it their primary purpose
    The only time the sense wire can "see" the battery SOC is when the Alternator isn't running
    Last edited by the evil twin; 5th June 2016 at 01:09 PM.
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    May be better of asking about amps ......

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    The alternator has no idea what SOC the battery is. The alternator doesn't even know a battery is connected. It just reacts to the electrical load demand.

    Try thinking of the battery as an electrical load. The lower the SOC the higher the demand on the electrical system ( for e.g. park lights draw less current then high beam lights).

    The alternator sees the higher demand (low SOC of the battery) and increase the AMPS it produces to try and bring the voltage level up to the alternator's regulator preset value.

    The sense wire or the point where the alternator senses the voltage has changed over the years. When the first internal regulated alternators were introduced into production the sense point was within the alternator itself, there was no external point or sense wire connected directly to the battery.

    As the electrical demands have increased in motor vehicles over time they have moved the sense point directly to the battery. This helps overcome any potential voltage drop between the battery and alternator.

    The B+ terminal on the modern alternator is not directly connected to the alternator regulator sensing terminal. This is why the alternator voltage will go sky high if the sense wire is disconnected or goes open circuit.

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    Thank you ET and Yendor.
    You guys nailed it.
    I don't think I should be asking anymore questions about this.

    But, please spare with me to post something from 'cyber university'.

    I just came across this web page yesterday, which explains the workings of the charging system, but also advises about something I'm bit worried about.

    http://www.chargerr.com/Alternators/ALT.HTM

    SENSE
    The SENSE line is a positive input to the alternator, and is meant to be feedback giving reference information on the load of electronics on your car. So its supposed to be hooked up the most electrically distant part of your car to let the alternator know if it should be upping the current to compensate. If its hooked up close to the BATT wire, which will be virtually the same voltage, then the alternator doesnt think it has to work that hard, and just does enough to keep the voltage steady at 13.8V. Not a bad thing if other systems in your car, like amplifiers, or fuel pumps dont mind getting a little shy of 13.8V.
    The above is confusing because of the below thread and the ideas we discussed there, and of course in this thread as well.

    http://www.patrol4x4.com/forum/auto-...-cable-319529/
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    Quote Originally Posted by dom14 View Post
    The SENSE line is a positive input to the alternator, and is meant to be feedback giving reference information on the load of electronics on your car. So its supposed to be hooked up the most electrically distant part of your car to let the alternator know if it should be upping the current to compensate. If its hooked up close to the BATT wire (B+ terminal alternator?), which will be virtually the same voltage, then the alternator doesnt think it has to work that hard, and just does enough to keep the voltage steady at 13.8V. Not a bad thing if other systems in your car, like amplifiers, or fuel pumps dont mind getting a little shy of 13.8V.
    It's a bit vague but I think there trying to say if the sense wire it's hooked up close to the battery wire B+ terminal at the alternator

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    Originally Posted by dom14 View Post
    The SENSE line is a positive input to the alternator, and is meant to be feedback giving reference information on the load of electronics on your car. So its supposed to be hooked up the most electrically distant part of your car to let the alternator know if it should be upping the current to compensate. If its hooked up close to the BATT wire (B+ terminal alternator?), which will be virtually the same voltage, then the alternator doesnt think it has to work that hard, and just does enough to keep the voltage steady at 13.8V. Not a bad thing if other systems in your car, like amplifiers, or fuel pumps dont mind getting a little shy of 13.8V.
    Quote Originally Posted by Yendor View Post
    It's a bit vague but I think there trying to say if the sense wire it's hooked up close to the battery wire B+ terminal at the alternator
    Yeah, that's what I thought.
    I couldn't justify hooking up the sense wire closer to the headlights.
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    The alternator doesn't settle @ 13.8V, the is the resting voltage of a healthy battery, not its charge.
    I will say it again, sense wire should be a the Batt+ terminal, not on a amplifier or other load. Placing the sense wire on the output of the alternator is not help with any losses on the cabling between the alternator and the battery (yet to see an OEM lead that is of suitable size cable).

    If we were supposed to place the sense wire on the output terminal, you think that all alternators would just omit the sense wire and go back to just two terminals, one output, the other for the alt-fail lamp?
    There is no legal cure for STUPID!

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