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2015 Outback Premium
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Discussion Starter · #1 ·
I have a 2015 2.5L Premium Outback. I have put in a tablet with Torque Pro and the ActiveOBD plugin to monitor certain engine parameters. One that I check is battery/alternator voltage. I expect and most of the time it shows 12.X volts when the engine is off and 14.X volts when it is running.

Problem is that sometimes while I am actually driving down the road the reading will drop to the 12.X range and stay there until I come to a stop. While I am coasting to a stop it will jump up to 14.X volts. Sometimes when I stop it will stay at 14.X and sometimes it will drop into the 12.X range again.

I have wired in my volt meter directly to the battery terminals to double check the tablet and OBDII set up and get the same readings on it.

The dealership has seen this occurring and supposedly tested the alternator, etc. and tell me that this is normal operation. I have been working on/modifying car for 50 years and have never seen this before where it wasn't a belt slipping (mine is fine) or some other problem.

I worry about it because right after the first time I noticed this behavior my battery went dead and had to be replaced.

Anybody have any information on this type of situation?
 

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'15 Outback 2.5i Premium
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From what I've read here, this may indeed be normal operation. Alternator voltage regulation is apparently handled by the BIU, and had been programmed to maximize fuel economy by reducing (or stopping?) alternator output under some operating conditions.

[ETA] This post describes voltages similar to what you're seeing.

It appears that the charging system performance as initially implemented may have been too stingy to provide adequate battery charge for some people, and one or more TSBs have been released concerning reprogramming the BIU to improve charging system performance. I'll edit this post with the TSB numbers unless someone pipes up with those.

I will probably have my next oil change performed at the dealer and have the TSB procedure performed then.

[Edit to add]

TSB 11-174-17R (latest revision 10-31-2017) Reprogramming File Availability

TSB 11-176-17 (11-16-2017) Reprogramming to Optimize ECM for Improved Battery Life
 

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It's called "opportunistic charging."

You can eke out a little better MPG if you do more of your battery charging when the car is coasting or braking. The computer is supposed to make sure that the net total charge is adequate for each trip- this may or may not be the case, but that's what they're trying for anyway.

In other words your seemingly odd voltage readings are the new normal.
 

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Discussion Starter · #4 ·
Thanks for the info. First time dealing with a charging system set up this way, so I was a bit skeptical. Some comments the tech at the dealership made about my setup made me feel like they were blowing off my concern,

Note: after watching the voltage some more, when I am making short trips around town the voltage stays in the 14.x range. But any trip over ~15 miles will have it going into "fuel conservation mode" on acceleration. This "intermittent " behavior is what had me confused.
 
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