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97 Legacy OutBack, 09 Forester XT, 12 Silverado 2500HD
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Discussion Starter #1
What is considered normal?

I finally got around to hooking up my VOM for the ride to work this morning...highest voltage I saw was right after starting, 14.0VDC; and it soon dropped to 13.75VDC. On the cruise in, it ran 13.75VDC, sitting at a stoplight, 12.85VDC. Turning on an amplifier would cause a 0.2VDC drop at cruise and around 1.5VDC drop idling...amplifier should only pull about 20 amps.

I would think you would see higher than 13.75VDC, since a battery is 13.8VDC and you need some voltage to drive a charge into it? Even after putting the amplifier on, it only came back to 13.75VDC (no jump past 14VDC, then settling down; it just rose back to 13.75VDC).

Thoughts?

Are these alternators internally or externally regulated? I've seen this once before on a completely different make, and it turned out to be a corroded connection on the field wire (the field wire controls the charging rate).
 

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What you're describing sounds normal to me.

The 95 OBW I had behaved just like what you're describing and I never had any battery/charging issues.
 

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97 Legacy OutBack, 09 Forester XT, 12 Silverado 2500HD
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Discussion Starter #3
I guess I'm used to the newer stuff that charges based on temperature...my truck charges at over 15VDC at times. I've never had a failure to start, battery has been in the car for about eight years now...the voltage just sounded a little low to me.
 

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What is considered normal?


I would think you would see higher than 13.75VDC, since a battery is 13.8VDC and you need some voltage to drive a charge into it? Even after putting the amplifier on, it only came back to 13.75VDC (no jump past 14VDC, then settling down; it just rose back to 13.75VDC).
A fully charged battery is 12.7 volts, not 13.8. What you're describing is normal.

15 volts is overcharging.
 

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97 Legacy OutBack, 09 Forester XT, 12 Silverado 2500HD
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Discussion Starter #6
A fully charged battery is 12.7 volts, not 13.8. What you're describing is normal.

15 volts is overcharging.

13.8VDC is nominal for a "12VDC" system...

On the newer vehicles, 15VDC is NOT overcharging...you need a driving voltage to put a charge into the battery. The more drained the battery or the colder the outside temperature, the higher the voltage will be going into the battery. I've owned four newer model pickups, all bought new off the lot, two different brands at this point (Three Dodge, one Chevrolet); and the actual voltage at the battery will read 15VDC at times. Three of those trucks went well over 200k miles each without a hitch. As I stated, there is more to a modern charging system than just a battery, alternator, and regulator.

And for the record, I'm not talking about what the dash gauge reads; I'm talking about reading the voltage with a VOM. The gauges in the newer vehicles are not direct reading, and often either heavily dampened by the BCM or completely faked based on other operating parameters. For instance, the voltmeter in a Dodge Cummins is dampened on initial starting to prevent the needle from fluctuating during the cycling of the grid heaters, the oil pressure shown by the gauge is completely fake as there is no variable pressure sensor but only a 7psi switch.
 

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97 Legacy OutBack, 09 Forester XT, 12 Silverado 2500HD
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Discussion Starter #7
Steved, check your grounding.

Grounding...I was headed that direction.

OK. Next question, should I consider pulling the alternator and cleaning its mounting surface since it likely grounds through the case/block? I'm thinking galvanic corrosion...
 

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Looks as if you have a digital VOM; probably has a low DC Volts range, perhaps 0-200 or 0-400 mV. This can be used to check the grounds quite effectively.

Turn on some loads (lights, the amplifier etc), run the engine at more than 1000 rpm. Measure the voltage between the battery negative post (not the clamp) and the engine block, the alternator case, and a few reachable points on the car body. In all cases, the voltage should be less than 0.2 Volts (200 mV) and ideally less than 0.1 Volts. If the voltage is high, check between various points, such as between the battery negative post and the negative cable clamp, then between the negative cable clamp and the other end of the negative cable at the starter, etc. Each of these measurements should be low; anything high identifies a bad connection or bad wire. This might help narrow down where to focus the ground cleaning or fixing.

The same technique can be used to check the charging (positive) side, by measuring between the alternator output post and the battery positive post. There shouldn't be any significant voltage drop (same as above).
 

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97 Legacy OutBack, 09 Forester XT, 12 Silverado 2500HD
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Discussion Starter #9
Yes, digital VOM...

The connection issue makes more and more sense. That would explain why stepping on the brakes causes the lights to dim, turn signals cause the interior lights to to pulse, etc. I'm reading the voltage through the cigarette lighter, not directly form the battery...so that could also explain why the voltage seems lower than it should be. I went through the grounds on a while back...I'm wondering if I missed some.

Yet another project for this old car...
 

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With the battery being 8 years old, I wouldn't discount the possibility that it isn't performing properly. As cardoc has quite clearly demonstrated, battery voltage is not an indicator of battery strength. Indeed, the factory battery I replaced last December was showing "good" voltages (static, engine off, and with the engine running) up till the time it wouldn't start the car. It might be time to have the battery tested.

See: http://www.subaruoutback.org/forums/66-problems-maintenance/90658-battery-amp-test.html
 

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With the battery being 8 years old, I wouldn't discount the possibility that it isn't performing properly. As cardoc has quite clearly demonstrated, battery voltage is not an indicator of battery strength. Indeed, the factory battery I replaced last December was showing "good" voltages (static, engine off, and with the engine running) up till the time it wouldn't start the car. It might be time to have the battery tested.

See: http://www.subaruoutback.org/forums/66-problems-maintenance/90658-battery-amp-test.html
^ battery under no load can show healthy numbers an old battery can easily go into the dumper the second a load is placed on it. All electrical systems on the car place loads on the Battery even with the Alternator cranking the Battery is the middle man that evens out the power think of it like a lake. The river running into the lake can trickle at times or rush at times but the out flow at the other end of the lake can be held constant till the lake runs dry.

A crummy old battery might show basic numbers that look OK but the second a load is placed on it the battery might lack the ability to meet ie send the power thats needed. Bingo lights that dim when added loads are applied etc.

The newer charging systems will run in the higher voltage ranges when there is a big enough power demand not to mention many of the newer tech batteries actually require a faster higher voltage charge process when the initial charging step is done or the battery simply doesn't store energy very well.
 

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What you're describing sounds normal to me.

The 95 OBW I had behaved just like what you're describing and I never had any battery/charging issues.
What is considered normal?

I finally got around to hooking up my VOM for the ride to work this morning...highest voltage I saw was right after starting, 14.0VDC; and it soon dropped to 13.75VDC. On the cruise in, it ran 13.75VDC, sitting at a stoplight, 12.85VDC. Turning on an amplifier would cause a 0.2VDC drop at cruise and around 1.5VDC drop idling...amplifier should only pull about 20 amps.

I would think you would see higher than 13.75VDC, since a battery is 13.8VDC and you need some voltage to drive a charge into it? Even after putting the amplifier on, it only came back to 13.75VDC (no jump past 14VDC, then settling down; it just rose back to 13.75VDC).

Thoughts?

Are these alternators internally or externally regulated? I've seen this once before on a completely different make, and it turned out to be a corroded connection on the field wire (the field wire controls the charging rate).
NORMAL RUNNING,NOTE THE HEAD LIGHTS STAY ON IS 13.25 ----14.35,,SET METER ON 20 V GET SOMEONE TO START YOUR SUBIE AT TURNOVER IF ITS 10.5 YOUR BATTERY IS LOW SURGE MODE ,ITS BAD THIS STARTS ABOUT 41/2 TO 5 YEARS NORMAL FOR THE JUNK BATTERYS THEY MAKE,EXILE IS THE STRONGEST HOLD ,THEY MAKE WAL-MART 330 CC ABOUT $45.00 AND TAX ON SALE!!
 

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13.8VDC is nominal for a "12VDC" system...

On the newer vehicles, 15VDC is NOT overcharging...you need a driving voltage to put a charge into the battery.
I too am not sure where you're getting those numbers. Go read any battery on a car that is not running. It will not be at 13.8 volts. That would be a fine voltage to charge the battery though.

I miss the day when vehicles came with altmeters--not sure the term. It would be a gauge that would measure whether the battery was charging or discharging (exclusive of the starter motor draw). I can't even seem to find an example of one on Google, so maybe I'm not remembering the right term.

FWIW, my Duramax Colorado comes with a AGM battery and it never gets to 15 volts, but its system is designed where the voltage does at times drop to about 12.8. If you need constant higher voltage to charge a trailer battery you either need to turn on tow/haul mode or the headlights.
 

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Two wire alternators only know a source voltage, usually at the bus, and the indicator light.(LS) Three wire like after 2000ish is a CLS. The third terminal is an ecu switch to ground for things like WOT. The voltage drops to 12 point nineish and then just goes back to a regular voltage regulator when that condition ends. It's marketed as soft-start.
 
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