Charging system voltage drops low occasionally

[ChrisW]

I have a dashcam setup to run until battery voltage drops to 12.5 volts when the car is off, and then it shuts off. I've noticed that it somewhat frequently turns off as soon as I shut off the car, so I started paying more attention to the voltage gauge when driving and when I get home.

While driving I notice that the gauge fluctuates from 14.1 down to about 12 volts. I would think it should stay at 13.8-14.1 the entire time, so this is odd. Also, the car was at 12.48 volts (measured at the terminals with a meter) as soon as I parked after driving for awhile.

Has anyone experienced something similar? Does anyone have an explanation? The car is completely stock aside from the Optima yellow top I put in to get more camera runtime when the car is off.

[guybo]

If the battery is at 12.48 V after driving, it is taking a good surface charge. To test the deep charge, leave it over night and in the AM, check the voltage, it should be close to what you left it at. Turn the lights on and see how quickly the voltage drops. If you get more than a few 1/10's difference over 30 secs or so, you may have a battery that's not taking a deep charge.

If your alternator is putting out 12V sometimes, that may indicate an issue with the battery or alternator. Get the alternator tested.

[ChrisW]

I should add that I have a carbon pile load tester and when I tested it at half the CCA for 15 seconds it held good voltage, so I'm assuming the battery is taking a good charge. I remember reading about using a carbon pile tester for alternator testing before, I'll give that a shot.

[guybo]

It's a 17? That'd be pretty unusual for such a new alternator to go out. You don't have a big sub hooked up or anything do you?

My wife's Camaro will reduce voltage to reduce parasitic load on the engine when the battery is fully charged. It made me nervous when her gauge went down to 12V, but it's normal for a Camaro. Maybe the 17's do the same thing? I have a 16 and have never seen the system voltage drop below 13V

[ChrisW]

Yea that's what I think it's doing. I know the alternator works but it's really annoying coming home to a battery that isn't fully charged.

I even tried locating some kind of onboard battery maintainer that would accept alternator input but there's nothing like that for starting batteries, just auxiliary batteries.

Edit: No sub or anything either. A dashcam and a radar detector, plus swapping out the factory battery for the Optima are the only changes made to the car...

[ChrisW]

I did some research, and apparently, Subaru has done something like this at least once before. The computer controls the alternator and does whatever it wants with the voltage, I won't try and deduce it's programming for that. What I do know is that if I turn on the headlights, the voltage rises to 14.1 and stays there. This explains why it seemed to only drop occasionally because at night it wouldn't do it at all.

I'm looking in to see if there's a way to get the alternator to always put out 14.1 volts so I don't have to drive around with my lights on just to ensure the battery stays charged.

[PhyrraM]

Your battery is fine, and will be fine. The ECU is playing with the voltage to maximize both fuel economy and battery life. It is monitoring the battery temperature also, through a sensor in the negative terminal. It calculates what the battery needs at that moment and provides it.

If you really want to try and mess with it, you might be able to intercept the ECU provided PWM signal and provide your own.

[ChrisW]

I don't think that's a temperature sensor, I think it's a current shunt to measure load. Aside from that, the ECU keeping my battery at under 12.5 volts is not a good thing. At that voltage, my AGM battery is only 80% charged and my dashcam will not record when parked.

Has anyone with a 2016 or older model noticed voltage fluctuations? If so I can definitely see it contributing to the battery death these cars are known for.

[CJRhoades]

This is normal and my '17 shows the voltage drop to around 12 on the in dash display sometimes. It removes load from the alternator to save fuel when it can.

[nico_rsx]

Like a couples have said, it's a new feature in the 2017. I think it's in the manual somewhere. If the battery is almost fully charged, the alternator will reduce voltage, but if you lift the throttle and are braking or coasting, the voltage will increase to 14V (to make use of the decelaration to charge the battary, similar to an hybrid), as soon as you press the gas again, the voltage will drop. Of course if will stay at 14V if the battery need to be charged, of if you have enough load (lights/defrost, etc).

Edit: the 2016 and older didn't have this function.

[ChrisW]

Well, this is crap. Multiple times it has resulted in me getting home with a less than fully charged battery.

[cjd]

This could explain why I have had issues on short trips with the Shorai battery... If the same thing happens on a 2014. Guaranteed if I left the dash cams on while parked for a half hour, but a couple times without even that. I may have to switch to a supplemental battery for the cameras.