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Old 05-15-2024, 11:39 AM   #1
C0lsanders_
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Join Date: Nov 2022
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2016 Impreza
Red

Default Low voltage after new high-end battery?

So, will try to not be long-winded.
Got back from a 6-month deployment a few weeks ago. Dad said the car ('16 Impreza, "Lucy") was beginning to be slow to start, suspected a bad battery. I got back, and yes, slow to start. AutoZone tested, and yes. Bought a new battery, Optima Yellowtop. Now quick to start, all better (or so I thought...).

Next, I have a DC distribution bus under the passenger seat (power to the HAM radio, dashcam, a few others) and it includes a under- and over-volt alarm, to warn if the voltage is getting high or low. After I got the new battery in (ONLY after, not before) something like every other time I go driving, on acceleration, the bus will alarm for under-volt. Stuck a voltmeter on it to watch while driving (plugged into the ports left side shown in https://forums.nasioc.com/forums/sho....php?t=2948875, which is port -> DC bus -> battery, via multiple fuses). When it alarms, it is dropping as far down as 11.35v or so. Again, only on acceleration, and not every day. It will reach a normal 14.4v just fine, eventually.

So, I have been back to both Autozone and Subaru, and neither has any good idea of the issue. Could be a simple bad battery, or I think perhaps the Impreza system can't appropriately handle the higher-end deep-cycle battery (only a theory).

Any ideas? Advice?
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Old 05-15-2024, 12:06 PM   #2
NickVu
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I'm not sure on later impreza's, but for my 04 sti ecu, it sends an alternator cut signal when going near full throttle and rpms over 4-5k. It delays the the signal a bit if the headlights are on. It coincides with AC cut on hard acceleration. It could be the new battery doesn't hold the voltage up to power your accessories when alternator cut kicks in.
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Old 05-15-2024, 12:12 PM   #3
C0lsanders_
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NickVu, would suspect that's a fault on the part of the battery, or some other problem? On the whole, everything runs fine, I'm just slightly nervous of the fact that it goes so low, and wonder if that's an indication of future problems.
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Old 05-15-2024, 12:15 PM   #4
Mulder
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02 RAV4 MT
15 WRX/11 CRZ

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Yes it may have to do with the battery management. Also sometimes when you buy a new battery it may not come fully charged.
It's possible the new battery may have to be "registered" to the car for proper charging, this would be a dealer thing unless you have access to a Subaru Monitor tool.
One thing you can try is leaving the parking lights on when driving so the battery management sees a drain, in my '15 this works to keep the battery charging and the voltage up.
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Old 05-15-2024, 05:17 PM   #5
NickVu
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It does sound like the battery, but before jumping to conclusions, put it on a trickle charger and fill up fully overnight. See if the problem goes away.

I'm not sure if newer models have alternator cut on high rpms so just assuming.
On my car, when doing full throttle pulls, ecu cuts the alternator and it would drop to 12.3v, very occasionally dip under 12. Voltage comes right back soon as you let off gas.

Also how long has the car been driven on the failing battery? It could be that the bad battery kept the alternator running full tilt on every drive and wore down the regulator.

Diagnosing electrical problems is the worst, no easy answers. I'd probably start inspecting and cleaning terminals of all the big wires leading out from the battery. Make sure the battery terminals are scrubbed. The charging cable from top of the alternator to battery likes to corrode at the battery side.

And wow, I'm glad not having to deal with needing the car programmed to the battery. First I've heard of it on a subaru.
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Old 05-15-2024, 07:01 PM   #6
super_subie
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could be the DCM issue which Subaru just sent a notice for, for some of their cars...
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Old 05-15-2024, 07:53 PM   #7
nwood1976
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Have you had the alternator properly tested? If you remove the alternator many parts stores can test it under a load. It's a good place to start troubleshooting and verify a bad battery.
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