Need help understanding my Alternator

[Luhrs28]

Hi guys. I've been reading stuff on the 'Net about alternator wiring, but there's still something I don't understand. There's a little red wire that seems to be called "battery sense" which gets connected to Batt +, and then there's a little purple wire that gets Batt + whenever the ignition key is on. My question is, whenever the ignition is on or the engine is running won't those 2 wires be reading exactly the same thing? (whatever the battery + voltage is) This is for the new Indmar 5.7 I'm about to put in my old Luhrs, and I'd prefer not to fry anything on initial start-up.

 

[bruceb58]

Re: Need help understanding my Alternator

Yes, they will be the same voltage but you need to wire the purple wire through your ignition so that the alternator field is turned off when the ignition is off so as not to drain your battery.

 

[glenn property of pam]

Re: Need help understanding my Alternator

been years since i learned about alternator in school but from what i remember. a alternator does not generate power just because it is turning. a magnetic field has to be made before the alternator generates power by changing the strenth of the magnetic field the output voltage of the alternator can be changed. so battery sense is adjusting the field strenth to control the volt/amp output to the battery the purple run wire is telling the alternator that it is running.(the voltage are normally real close on both wires but may not be same and there is a diode that stops the battery sense voltage going to the ignition wire (or the engine would not stop on the key). i think thats about right.
like bruce said unless you want a flat battery or a engine that runs until the gas runs out theres no skipping the 2 wire set up

 

[Luhrs28]

Re: Need help understanding my Alternator

OK that makes sense now. I couldn't understand why it would need 2 wires that have essentially the same voltage.

Thanks for clarifying that.

Bert

 

[HT32BSX115]

Re: Need help understanding my Alternator

OK that makes sense now. I couldn't understand why it would need 2 wires that have essentially the same voltage.

Thanks for clarifying that.

Bert

It would be more obvious if you had a LONG run of wire with a fairly high load at the other end. You would get a finite voltage drop (depending on actual current) at the other end due to resistive losses in the wire and connections in between the alternator and the load.

The "sense wire" (if connected at or near the load) would "send-back" the lower voltage to the alternator (and regulator) which would then (automatically) adjust the voltage output to maintain desired voltage at the load........

This could cause rather high voltages at the alternator if there were high losses between the power source (ALT) and the load.

This might be ok unless you have a really long supply cable and a large load at the end of the cable...... AND other "stuff" connected to the SAME supply cable near the alternator.

The "sensed" load would get say 13.8 vdc and the "stuff" near the ALT might get 17v.
(This might not be good for some sensitive electronic equipment........... My SGC SG-500 HF marine Linear power amplifier is good up to about 18V, but my ICOM IC-M802 HF SSB marine radio is NOT [13.6V DC ?15%])

Anytime a large (current) load is drawn at the other end of the cable, the regulator could produce too high a voltage at the alternator (for equipment connected close to the alternator) OR a poor connection, switch, ground, etc, that caused the "sense" wire to detect a large voltage drop might also cause the regulator to increase the voltage to high levels.

You must take care in designing your electrical system to place the sense wire in the circuit to minimize this problem. If you have short, large (read: thick) runs of power supply wire that won't have a large enough resistive loss to be a problem, the sense wire can just be connected directly to the alternator output (or directly to the first BUS connector where everything else is connected for DC power......which is usually the case in most automotive and marine power supply systems......it senses itself!)

BUT, anytime the sense wire is connected "away" from the alternator, it becomes EXTREMELY important to have clean, tight (LOW-resistance) supply AND ground connections to prevent false sensing of load/system voltage. Or you might get large swings in voltage that could damage electronic equipment.



Regards,


Rick

 

[bruceb58]

Re: Need help understanding my Alternator

Typically it isn't necessary to have remote sense unless you are charging through a diode isolator. Even if you have a large length of cable to get to your battery, once the battery starts getting close to being fully charged, the current is low enough at that point for the voltage drop in the charge cable to be negligible. The remote sense just reduces the charging time slightly.

 

[HT32BSX115]

Re: Need help understanding my Alternator

Typically it isn't necessary to have remote sense unless you are charging through a diode isolator. Even if you have a large length of cable to get to your battery, once the battery starts getting close to being fully charged, the current is low enough at that point for the voltage drop in the charge cable to be negligible. The remote sense just reduces the charging time slightly.

Yeah. I don't think I would use it. They just put the "sense" connection there to confuse us all!

 

[bruceb58]

Re: Need help understanding my Alternator

Some engineer put it there!

 

[HT32BSX115]

Re: Need help understanding my Alternator

Some engineer put it there!

Damn engineers!