The car is a 2007 Subaru Forester with an automatic transmission and 155,000 miles. I started noticing that when I accelerated harder than normal I saw on the dashboard that the battery light and brake light would flicker at the same time for half a second. It was hard to get a picture while driving but I will post a link to the picture below. Over the course of the last couple weeks it started happening more and more, as in normal and even mild acceleration I would see the lights flicker on for half a second. As well as going over bumps would sometimes activate the lights. Then one day I went to start my car and I was unable to start the car at all. I checked the batterys voltage and it was about 12.1V so I hooked up my battery charger for a bit and then everything started just fine. Later that day I tried starting the car again and it wouldn't turn on with the first try but I let it sit for a minute and tried starting it again which did work. Since then, sometimes I can and sometimes I can't start it and when I can not the batteries voltage is always lower than 12.4V.
The battery is about 1 year old and the alternator is about 6 months old. I tried Scottys Parasitic Battery Drain Test and I was not able to find anything wrong. When doing the test the voltage I measured across the resistor was 14mV which seems pretty reasonable given all the computer that are in cars these days. No codes pop up when I plug my OBD2 sensor in although when the lights are on I am in the middle of driving and I cant really stare at the screen while I'm driving.
I am not particularly knowledgeable when it comes to cars but I have been trying to learn lately since I started watching Scottys videos. If anyone has any suggestions on things I can try or possible causes for this I would very much appreciate it!
As well as going over bumps would sometimes activate the lights.
This sounds like a bad connection.
Yup, check the battery terminals and clamps for corrosion and tight fit. Check the wire harness, as well.
The alternator could be nearly new, but new or nearly new is not the same thing as known good.
Battery lights come on when the alternator isn't sufficiently charging the battery. So, that's one issue. Have the alternator retested. Aftermarket alternators, particularly Chinesiums, are nowhere near the quality items that OEM alternators are.
So, one bet is on the alternator.
Another possibility is the belt is the wrong size, i.e., slipping. That would also undercharge the battery.
The brake light coming on is just another manifestation of the power/charging issue.
Have that alternator checked. The problem is likely coming from there.
Thanks for the suggestions!
So here are the updates since I made this post. As geriscan suggested, I checked the belt and it seemed to be quite taut but I went ahead and loosened and retightened the belt tensioner anyway to make sure. Then checked the voltage of the battery when off, voltage when on, voltage with peripherals systems (a/c, lights, radio) on, and while revving the engine. The voltage while off was ~12.4V and voltage with the alternator running was ~14.4V. When I first turned the A/C on it seemed like it was running particularly hard and I saw the voltage dip to 12.4 but I tried turning it off and on again and but the voltage did not dip a second time, stayed at ~14.4V. I then went to O'reilly and checked with their tool and they said the everything was fine. Although the guy kind of seemed like he was trying to rush me out the door. On my way home I tried to pay close attention to when specifically the lights would flash on. It seemed like anytime there was a large jump in RPM that the lights would come on, small RPM changes and didn't seem to activate it as often. Actual speed of the car did not seem to matter as I was able to get the lights to turn on even when parked but revving the engine. Bumps only sometimes activated the lights, not every time though. I also noticed that when I would rev the engine that the headlights would dim a bit.
I cleaned the battery terminals and clamps and reran some of these tests. Then I hooked my multimeter up using the parasitic battery drain test method and started wiggling wires and connectors to see if I saw a change in the voltage. I tried to wiggle as many of the wires as I could find/reach but I did not see any changes.
Are there other tests I can do on the alternator to figure out if that is the issue? Also, what are my options for trying to figure out where the short is since, if it is a short, it only shows up for a second when reving the engine?
I forgot about this posting for a while but I wanted to update it on how I ended up solving this issue in case someone else come across a similar problem. Basically since the lights were only turning on when I was driving, during acceleration and going over bumps, I was unable to do any tests or measurements while the issue was happening. What I ended up doing was to have someone sit in the driver seat of the car with the car on while I opened the hood. I then took a stick and started pocking all the wires I could see and eventually my helper said they saw the lights turn on. It turns out that my specific issue was in the alternator wires. Not the large bolted on wire with the protective covering over it but the smaller wires that are plugged into socket on the alternator. I then when along the wire lightly bending it until I found the spot that activated the error lights. I bought a new plug that had a length or wires already attached to it and then cut my wires off just behind the point where I found the issue. I used some waterproof heat shrink wire solder connectors to connect the new plugs wires to the old one. I wrapped the length of wire in some tape for extra protection and it has been working perfectly ever since.
Usually you have to buy big sections of wiring harness from Subaru but this is the company that I was able to buy the individual plugs from.
https://www.iwireservices.com/
Anyway, hope this helps someone else someday.