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

600 Views 21 Replies 6 Participants Last post by Markgm, 2 mo ago

Markgm

Discussion Starter · 2 mo ago

Hello,
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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brucep

· #3 · 2 mo ago (Edited)

Markgm said:
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?

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:

Starting engine.
- Obviously, starting engine drains battery hence there is high electrical load for several minutes after starting.
- Cold ambient temperatures demands more battery power hence increases this temporary alternator boost
Headlights ON
Blower on HIGH
A/C ON
- do not forget that turning on AC also turns on both radiator fans.
Rear window defrost ON
windshield defrost ON
- remember that windshield defrost also turns ON A/C (ref. A/C above)

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brucep

· #5 · 2 mo ago

Markgm said:
I am a new owner of a used 2000 Legacy wagon.

Markgm said:
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?

I am not sure if you found an answer in my last post.... I am trying again...

Your 2000- does NOT have the current-sensor on the battery-negative post. Hence, if you have the 2.5 engine, the alternator should NOT have any funky control from the computer.
In other words, You do not need to "trick it into always being cold".... the alternator should put out full current as-needed when the detected voltage drops below setpoint.

Dont forget, when you think it is being "cold"... it is really just the battery being recharged from cold-startup.
Your inverter should work just fine. If it is not, there is a different issue we can discuss.

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brucep

· #7 · 2 mo ago

Markgm said:
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.

again... It sound to me as if you are mis-interperting the voltage readings you are seeing.

Risking repeating myself a third time:

Starting engine drains battery slightly
hence, for several minutes after starting, the alternator sees higher electrical load and responds accordingly.
- This temporary voltage-increase has NOTHING to do with temperature-sensor. (on your 2000)
After battery-charge is replenished, the electrical-load goes down (and alternator output goes down)

You are correct that higher voltage can mean more current.... but this correlation is based on electrical loading, alternator RPM, voltage-regulator setpoint...etc

It is FARRRR more likely you are dealing with a combination of factors including poor grounds.

Using sensitive voltmeter to take VOLTAGE DROP measurements would be the approach I would begin with to isolate where the voltage is getting lost.

BOTTOM LINE ==> Do not assume that higher voltage always means more current. You have jumped to conclusions which have no basis in facts.

Infact, it is more likely that the lower voltage you are seeing (after few minutes) may represent HIGHER current which is pulling down the voltage. (if the regulator were not involved)

Before you move forward with adding diodes to 'trick' the regulator to produce more voltage (0.7V per diode BTW)... it would be better if you perform the VOLTAGE DROP measurements at appropriate nodes in the circuit.

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brucep

· #13 · 2 mo ago

Markgm said:
Bruce, I guess what you are saying is that you don't think I should be seeing the behavior that I do.

Incorrect! I am not saying that at all.

I have tried to help you several times by explaining from various perspectives.

I truly hope you are not relying solely on the display of your fancy 'inverter' to troubleshoot the issue you are seeing. If so, that may explain the lower voltage readings. I hope it is obvious that the voltage drops when a load is applied... this is why a voltmeter DOES NOT draw any current (high impedance input)

Have you taken the VOLTAGE DROP measurements with voltmeter yet? Do not forget that there MUST be a load on the circuit to get valid readings.

Feel free to use [search] to review my other postings which detail VOLTAGE DROP measurements.

If you wish more help from me, PLEASE describe how your inverter is connected to the vehicle and detail the results of your VOLTAGE DROP measurements. I troubleshoot these kinds of things for breakfast and I am sure we can get you going if you participate.

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