[peejglass]

So I finally got around to adding a temporary voltage meter for my audio system and I am slightly concerned with the voltage I am seeing. If I turn my AC and lights off after a few minutes of the engine running my alternator will completely stop charging and my batteries (yes there is a second one) will drop down to the mid 12V range. I turn my lights on and almost instantly jump back up to 14.2 and stay there. Is this some new age crap? My other cars have been 2005 and older and the alternator would put out 14+ regardless of the draw on it. Will someone with a multimeter go out and check their car after it has been on for a few minutes? Just turn all the accessories off that you can and see where your voltage sits and then turn your lights/ac on and see where it sits then. I would greatly appreciate it!

 

[SkierDave]

Well you piqued my curiosity so I took the VOM with me on an errand. All measurments made at the cigarette lighter socket.

With no major electrical loads, I saw 13.3 - 13.5 volts. This is independent of engine speed ... didn't matter if idling or cruising down the road. EXCEPT after sitting at a red light, accelerating away when the light turns green, voltage rises to 14.5 for a few seconds then back down.

Turning on any major load - headlights, rear window def, even directional signal, causes the voltage to hold HIGHER... 14.3 - 14.5. Certainly not what you'd expect. And it drops back down to 13.x when the load is shut off.

But I never saw the voltage drop into the 12's like a discharging battery would. Is there a relay or some kind of module between your aux battery and the car's system, that is cutting out?

 

[ILuvMySubaru2]

14+ volts is the system charging the battery. I wonder if the car has a clutch on the alternator, sort of like on an AC compressor, to take the load off the engine if the battery is fully charged?

 

[flash75]

I checked these voltages at the battery posts.

Battery voltage, engine and lights off 12.81 volts
Engine running all accessories off 14.23 volts
Engine on, headlights on 14.35 volts
Engine on parking lights on 14.22 volts


Clifton

 

[peejglass]

Thank you all for the info! I am going to put my car on the charger tonight and report back because when I first bought the car I checked and was seeing 12.6 with the car off and I did leave it with the dealership for 3-4 days where they didn't have the engine on for probably the entire time because they were waiting for a part to come in. I think my battery is just discharged. Lead acid loses its charge pretty quickly and maybe my car was sitting around on the lot for a while before I picked it up on top of sitting around waiting for service.

 

[peejglass]

So after more testing I found that I was still dropping when I turned everything off. My idle would drop down to 750 and my voltage would steady around 12.4. The second I turn my AC on even one click I am right up to 13.3-13.5 and I would idle right around 800. Then if I turn my headlights on one click (parking lights I think?) the idle creeps up a tiny bit and my voltage holds at 14.2 for as long as I have the lights on.

If however I turn the car off and then back on with no electrical loads it stays at 14.2 for a solid minute or two and then drops right back down into the 12s.

 

[peejglass]

Also if anyone could check their voltage after letting the car sit all night that would be great I am resting at 12.67 this morning.

 

[flash75]

After sitting all night my battery voltage is 12.71 volts.

Clifton

 

[peejglass]

Well that says to me that my battery is still good. My alternator is clearly still good because the second there is a load on the system I'm at a stable 14.2 and even with the car audio system going I don't drop below 13.9. But the idle voltage with no accessories on is a bit concerning.

 

[peejglass]

Well you piqued my curiosity so I took the VOM with me on an errand. All measurments made at the cigarette lighter socket.

With no major electrical loads, I saw 13.3 - 13.5 volts. This is independent of engine speed ... didn't matter if idling or cruising down the road. EXCEPT after sitting at a red light, accelerating away when the light turns green, voltage rises to 14.5 for a few seconds then back down.

Turning on any major load - headlights, rear window def, even directional signal, causes the voltage to hold HIGHER... 14.3 - 14.5. Certainly not what you'd expect. And it drops back down to 13.x when the load is shut off.

But I never saw the voltage drop into the 12's like a discharging battery would. Is there a relay or some kind of module between your aux battery and the car's system, that is cutting out?

There is not. The batteries are wired in parallel so it just acts as if it is one battery. You didn't have any air blowing? Because what you described is EXACTLY what I experience when I turn the AC on 1 click (regardless of if I press the AC or recirculate buttons)

 

[SkierDave]

There is not. The batteries are wired in parallel so it just acts as if it is one battery. You didn't have any air blowing? Because what you described is EXACTLY what I experience when I turn the AC on 1 click (regardless of if I press the AC or recirculate buttons)

I'm sure the vent fan would have been on low for all of my test run. So I would conclude the fan on low was not enough electrical load to make a difference. I did note try half the trip with the AC on and saw the 14+ readings. So apparently the compressor clutch was enough load to act like lights, etc.

(( I have never heard of wiring two batteries in parallel. Is this a common practice? Without any type of control, I would think that any tiny voltage difference would result in a large current between them, causing heat in the batteries and cables. And whatever energy is wasted in those losses would slowly discharge them both. ))

 

[peejglass]

I'm sure the vent fan would have been on low for all of my test run. So I would conclude the fan on low was not enough electrical load to make a difference. I did note try half the trip with the AC on and saw the 14+ readings. So apparently the compressor clutch was enough load to act like lights, etc.

(( I have never heard of wiring two batteries in parallel. Is this a common practice? Without any type of control, I would think that any tiny voltage difference would result in a large current between them, causing heat in the batteries and cables. And whatever energy is wasted in those losses would slowly discharge them both. ))

In car audio yes this is common practice however after disconnecting my aux battery I am now seeing 14.2 solid at idle with no electrical load at all not even the vent fan. SO it is definitely possible that I have a bad ground which is pulling the voltage down. The joys of electrical problems lol.

 

[peejglass]

Well my ground is better than the stock ground. No big surprise there seeing that I am using 2/0 welding wire and the stock is maybe 10ga. Well I am clueless but it doesn't seem like it is an issue as long as I keep the parking lights on I am sitting at a healthy 14.2