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Discussion Starter · #1 ·
I've recently had a strange experiance with charging. When I had an old factory battery, charging voltage on Cobb AP was 12,5-14,14. A week ago I observed occasional drop of charging to 10. I went to Subaru dealer and it turned out to be the dead battery (after 6 years...). I got also an inspection af the alternator and everything was ok. After installing a new battery (BOSCH EFB with equal parameters with the factory one) the charging came back to 12,8-14,14. But the other day, after a long journey it ocassionally dropped to 10,8 or sitted at 12,6 and by WOT came back to 14,1. The same when ligths on. The Subaru Dealer again made an inspection and everything was ok. I was informed that my alternator is "smart" and keeps my battery in good condition not charging it all the time. And now I can see on my AP very different values. Most of the time 13-14V, but sometimes 11,8-12,6 with momental drops to 10,8. So far I didn't know about that. Do You have the same?
 

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Yes most new cars have that. This one's got a few bored people arguing over the same thing:

 

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Little Homie
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Yes most new cars have that. This one's got a few bored people arguing over the same thing:

Oh yeah, the electricity is liberal propaganda guy.

Is your battery still good?
 

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Discussion Starter · #5 ·
Yes most new cars have that. This one's got a few bored people arguing over the same thing:
Thanks for the thread. I have read the same hints at NASIOC... ;) I'm not a technician so I base on wiser than me in this case. The explanations of the Subaru Dealer convince me. I have no CELs, no power drops, sooty pipes :cool: etc. but anyway the drops of voltage to 10 or sitting at 12,0 may seem strange. Although my BOSCH battery is modern and more effective, when I switch off the engine it often shows 12,2-12,3v. After driving with lights on and charging 13-14v it shows 12,5-12,6 after switch off.

My battery is brand new.... ;)
 

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Little Homie
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What do You mean?...
So your car will run without a battery in the car at all once started. It'll report the voltage of the system and not directly the battery. This is a method we used to use all the time to diagnose battery or alternator.

If the system works like I understand it the battery is disconnected from the system unless there is a heavy demand that requires the extra current/voltage/power whatever.
 

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Discussion Starter · #11 ·
So your car will run without a battery in the car at all once started. It'll report the voltage of the system and not directly the battery. This is a method we used to use all the time to diagnose battery or alternator.

If the system works like I understand it the battery is disconnected from the system unless there is a heavy demand that requires the extra current/voltage/power whatever.
As they told me at Subaru Dealer in Warsaw, ECU occasionally lowers charging and then takes power from the battery itself untill some limits and then recharges the battery and so on...
 

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Little Homie
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As they told me at Subaru Dealer in Warsaw, ECU occasionally lowers charging and then takes power from the battery itself untill some limits and then recharges the battery and so on...
Yeah the new systems are not traditional classic setups and I'm sure different OEMs handle it different ways.

For example BMW uses agm batteries. As the battery ages they change the charging rate. If the system isn't reset or you switch to a lead acid battery when changing a bad battery it can destroy the new battery.

Then again, I rarely replace a battery as opposed to 20 years ago. So I'm sure these plans work.
 

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Discussion Starter · #13 ·
As the battery ages they change the charging rate.
By Subaru I think it changes automatically. Before replacing my 6-years old battery the charging rate was constantly 12,6-14,4v. Probably because the old battery wasn't so efficient and needed higher charging.
 

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You.

Just a comment based on us knowing what it's doing but not the idiosyncrasies of it and the seeming low voltage at times.
Ah ok. Yeah haven't noticed anything off recently on electrical side but car's just over 1 year old. Even working on things with interior lights on for 3-4 hrs at a stretch didn't seem to hurt the battery. A few people at a meet said their factory battery didn't last more than 2yrs so maybe the life just goes down.
 

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Little Homie
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Ah ok. Yeah haven't noticed anything off recently on electrical side but car's just over 1 year old. Even working on things with interior lights on for 3-4 hrs at a stretch didn't seem to hurt the battery. A few people at a meet said their factory battery didn't last more than 2yrs so maybe the life just goes down.
Our Forester still has the OEM battery. It's a 2017. It isn't even remotely weak.
 

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2010 Subaru WRX Limited Hatchback 5MT
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Just thought I'd mention to check the alternator belt tension with one of those Krikit gauges. Recently had to replace mine because couldn't get OEM spec tension with the old belt after removing and reinstalling. Seems to have possibly solved an issue or two on the car in the process too that I've been chasing for nearly two years.
 
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