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

brucep said:

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

Click to expand...

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.

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.

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.]