Alternator has trickle charge mode?

[Mark R]

Just wondering whether alternators are now advanced enough to do this.

I was playing with a volt meter and data logger on my lexus, and found that for about 10-15 minutes after starting the battery voltage is 14.0-14.4, but afterwards it drops to 12.4, with short pulses up to 14.4.

I'm guessing that this is deliberate trickle charging, but I've never come across this before in a car. All the others I've seen, hold the voltage at a steady 14.x

 

[Bartman39]

Would bet it has an advanced charge detection circuit that besides being easier on the battery (wont over charge) it is also part of a fuel saving idea...?

 

[Ferzerp]

You sure you don't have a clutched alternator that only turns part of the time? It would be on or off. No "trickle" (like an ac compressor). When off, it would be battery voltage only. It is basically extra complexity in the name of a little less fuel consumption.

 

[Meghan54]

What year is your Lexus?

Just curious because the factory repair manual(s) for my Lexus, an '06 GX 470, states that voltage while running from idle to 2000 rpm is to be between 13.2 to 14.8V, if lower, it's the voltage regulator or the alternator.

 

[DaTT]

You sure you don't have a clutched alternator that only turns part of the time? It would be on or off. No "trickle" (like an ac compressor). When off, it would be battery voltage only. It is basically extra complexity in the name of a little less fuel consumption.

That sounds viable. I have never heard of this by it also doesn't seem like a feature that would be advertised as most of the general public wouldn't understand or care.

 

[Ferzerp]

Considering you can find a lot of "lexus alternator pulley with clutch" parts online, I assume that's what it is.

 

[LTC8K6]

A clutched alternator is usually not primarily to turn off the alternator, but to cope with speed changes and inertia.

http://en.wikipedia.org/wiki/Overrunning_alternator_pulley

http://www.ina.de/content.ina.de/en...e/mot3400/mot3410/mot3414/mot3414.jsp?_r=1&hp

 

[Meghan54]

A clutched alternator is usually not primarily to turn off the alternator, but to cope with speed changes and inertia.

http://en.wikipedia.org/wiki/Overrunning_alternator_pulley

http://www.ina.de/content.ina.de/en...e/mot3400/mot3410/mot3414/mot3414.jsp?_r=1&hp

This.

Like I said, you may have a problem with your volt. regulator or alternator itself. Even if not running lights, radio, A/C, etc., your vehicle is still drawing power---engine, ECU, etc. And the ECU looks for, according to the service manual I have, around 13+V.

And by service manual, I don't mean a Chilton's or Haynes, rather the factory service manual, typically a 3 volume set (4 if you get a set with the wiring diagrams.) If you're planning on keeping your Lexus and you wrench on it, a worthwhile investment. The manuals were written by the guys who designed and built the vehicle and are vastly superior to any aftermarket repair manual---which are more directed towards the backyard/weekend mechanic who is uncomfortable with doing a brake job without a book beside them.

They're rather spendy, the set for our '06 GX 470 ran around $400 for four volumes, but worth every penny.

 

[alpineranger]

My 1998 Dodge has an externally regulated alternator. This means that the ECU will take into account factors such as the ambient temperature, electrical load, engine temperature, battery state of charge, etc, to intelligently control the alternator output voltage. For example, if the ambient conditions and battery state of charge are such that the battery is pretty full, and charging the battery might result in damage, the voltage of the alternator is limited to to the voltage of the battery, typically around 12-13V.

As far as I know, this sort of thing is even more common today than it was in the late 90s, although not universal. Basically, instead of a dumb voltage regulator built into the alternator that gives you about 14V all the time, the alternator field current is directly supplied by the ECU. I have never heard of a clutched alternator, but it strikes me that that arrangement offers few if any benefits over this sort of external voltage regulation.

 

[Ferzerp]

Thanks for the corrections guys. I was just taking a stab at what it possibly was.

 

[tortillasoup]

Just wondering whether alternators are now advanced enough to do this.

I was playing with a volt meter and data logger on my lexus, and found that for about 10-15 minutes after starting the battery voltage is 14.0-14.4, but afterwards it drops to 12.4, with short pulses up to 14.4.

I'm guessing that this is deliberate trickle charging, but I've never come across this before in a car. All the others I've seen, hold the voltage at a steady 14.x

What kind of car is this? I've worked on many cars I just recently came across the same unusual behavior on a particular car I've been working on. At first I thought it was a defect but I believe it is actually part of its design in order to help it save fuel.

When using your data logger, try to slowly increase the electrical draw and see if this behavior of it dipping to battery voltage goes away. So try head lights, then blower fan (no a/c), then Blower fan w/A/C, and finally rear defroster... The car I'm working on is an Acura 3.2TL and it does this exact same behavior except when: the headlights are on, radiator fan is on, A/C is on (that turns on radiator fan though), and obviously if the rear defroster is on... Now if the car is doing these voltage drops under heavy load like when the rear defroster is on, then you definitely have a real problem on your hands as the alternator is suppose to keep up with those loads but it can't likely due to dirty electrical grounds or something to that effect.

I should also mention that I don't think the car dips the voltages occasionally to battery voltage if it's in park but will in Drive which would indicate it's definitely for fuel economy purposes since the CAFE fuel economy tests aren't done with any electrical loads (though the tests for for the 2008+ Monroney sticker ARE done with the A/C on).

 

[Mark R]

What kind of car is this? I've worked on many cars I just recently came across the same unusual behavior on a particular car I've been working on. At first I thought it was a defect but I believe it is actually part of its design in order to help it save fuel.

It's just a regular car, 2006 IS250 - gas, no hybrid, start-stop or anything weird like that.

When using your data logger, try to slowly increase the electrical draw and see if this behavior of it dipping to battery voltage goes away....

I'll give that a go and see what happens. I think it must be deliberate; I got a copy of the alternator wiring diagram for this car, and it is ferociously complicated compared to the cars I'm more used to.

Alternator wiring

Essentially, the alternator is modulated by the ECU, which can adjust the alternator voltage.

The ECU is bristling with sensors including battery temperature, battery voltage and battery current sensors, as well as CAN bus signals for electrical load (e.g. heated window controllers, heated seat controllers, etc.), and presumably the ECU adjusts the alternator output according to all these parameters.

Service manual states normal battery voltage under electrical load/engine running is between 12.0 and 15.1 volts (variable according to conditions) which suggests that the modulation is intended and normal.

I guess this is normal these days to preserve battery life. I did notice a whole bunch of electrical widgets on the battery cover, when I had the battery changed a while ago. I guessed they were current and temp sensors, but the backstreet mechanic didn't believe me, never having heard of a battery temp sensor before.