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How is "cold battery" sensed? I want the charge.

604 Views 21 Replies 6 Participants Last post by  Markgm
I am a new owner of a used 2000 Legacy wagon. I have very interesting living conditions. Living in the car, I use my laptop all day. Sometimes, it's a little cold outside, too. So I break the rules and idle my car from time to time. I have a power bank/inverter that I charge, and the laptop via inverter uses 15-20 watts. Well, this car doesn't give out extra current for much.

When I start the car cold, it gives battery charge current, and I really enjoy using that. It reminds me of the charging of the old days, when the battery was always being charged. Well, I think I want to do this here. I want to charge the battery all of the time. I will use it.

Does my car have a battery temperature sensor, or is it another temp-related sensor somewhere else, that gives me this high cold-start charge? Secondly, I am wondering if there is a way to trick it into always being "cold", so to speak, so that the higher voltage/charge would always be on?

Cheers, Mark
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Some cars have a temperature sensor underneath the battery (Chrysler products)... I am not sure that Subaru does.

I do know that some newer Subaru has a CURRENT sensor on the battery-negative terminal. The computer will boost alternator output based on electrical load.

Some common electrical loads which will increase alternator output:
    • Cold ambient temperatures demands more battery power hence increases this temporary alternator boost
A general search for battery temperature sensor at O'Reilly. I haven't messed around with this before, but I have read about it. General to the industry, there is a known thing, which is to give more charge to the battery when it is cold, and not just when turning the starter, because batteries don't have as much voltage when they are colder.

I have a voltage gauge, and I watch it all the time. When I start my car cold, it goes over 14v for a little while, like 10 minutes. After that, I expect to see less - sometimes 13.2v. Sometimes, when I'm drawing more with my stuff, down to 12.6v or so. But a cold start gives me 10-15 minutes of higher voltage, more current.

Another approach to tricking out my charging system is to install a diode in the voltage sense line from the battery to the alternator. This would drop it by a half a volt, and trick the alternator to make more charge for the battery. It's just that I thought I might be able to fool the thing another way, if I could make it think it was still cold.
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Bruce, I guess what you are saying is that you don't think I should be seeing the behavior that I do. I can say that I am limited in what power I draw, for charging my storage devices for example. When I am charging my power bank (inverter), it's PSU says 15V, 2A, so 30W is its draw, and it drops my voltage to power it. I think that is less than wiper motors would use, but I do not see built-in accessories causing a change in the voltage.
I have taken some measurements. Previously, I had mentioned voltages in a mix of whether I was running or charging something or not. GEAR: I have an inverter in my car wired to the battery. I have another inverter which I will call a power bank. I have a voltage gauge in parallel with the inverter when it is plugged in, and I have a multimeter for tests. [Edit-add: "plugged in" means the inverter is via the lighter socket; my max use would be 50-60W].

On a cold start, ground tests for voltage drop are under 0.1V, except for between the alternator RED and battery RED, which does hit 0.1V.. The system voltage I measure on a cold start is around 14.2V. I can turn on headlights, wipers, or blower on high, and the system compensates and keeps the voltage up. But under these circumstances, when I plug in my power bank to be charged, it will drop a full volt. If I then add the laptop, it will drop a bit more than that. When the car is warmed up, the system goes down to around 13.2V, but at that point, powering my two things will drop it below 12.6V, even down to around 12.2V. So, I guess the problem is that it can give more power to built-in things, but when I want the power, then it won't give it to me. Cheers.
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When the car is warmed up, the idle speed is down, the alternator is will put out what it does. There is a voltage regulator that will prevent it from putting out any more than a set voltage. ... The current (amp service) is what you are lacking, and would need a higher output alternator if you are drawing too much power. ...
This is what I guess I have wanted to point out. It has more power if it needs it at warm idle, to run the blower to MAX. But, this power is not available without turning the blower to MAX (and that just goes to the blower). I think this may be because it is being fuel efficient, and won't induce drag on the motor if it doesn't see the need.
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Yes, there are circuits that when switched on, also boost the idle speed. They run through the ECM and or the VCM.
Again, the idle speed is brought up to increase the amount of current available from the alternator to meet the need. ...
My engine idle speed is not going up when I turn on my blower to MAX, but the voltage stays where it is, and sometimes with a little boost.
I have learned a lot. I will share a few links here to go with it. I came across a guy here who mod'd a gen5 charging system, by cutting the ECM line to the alternator: Alternator Mod for higher output . I then found a guy on youtube who did the same thing in I think a more elegant way, at the fusebox instead: 2015 wrx alternator charge fix .

My old gen2 doesn't have that smart-charge system. I have a voltage sense line from battery, via fuse box, to alternator. I have seen a guy add a diode to another kind of car to raise his alternator voltage successfully. So now it has occurred to me that "default alternator voltage" might be had if I just pull the fuse, and make it so the alternator cannot see battery voltage, and that appears to have worked. I'm holding at 14.2V at idle, when warm.

It would be more graceful to get the diode, I think, because just pulling the fuse leaves my battery light on when the car is running. Perhaps of note, MotorTrend has an article about achieving more power at lower rpm by using a 1/4" smaller pulley on the alternator, which will increase fan speed and cooling. High-Output Alternators Save the Day

The only thing I do not know at this point is how high the voltage goes on a cold start, but I think that is also alternator-controlled. I may come back to post that. Cheers.

[Edited: Had my laptop plugged in at the time... Holding at 14.2V, not 14.]
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I appreciate that what you are saying may apply to some. I have an AC Delco 90 amp alternator. I have not seen more than 14.5V driving around for an hour. I will make sure to watch what I am doing. Fyi, here is a quote from a Delco Remy 3-wire alternator instruction page...
Use of the Sense terminal is optional, however its use is highly recommended for
improved charging performance
- Instructions
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Note: I just did a cold start, and got 14.8V, and so far it looks this is as high as this alternator goes (on its own).
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