12v question

Exactly. Lead-acid batteries are not supposed to last 8 years in a car.

 

the battery is probably on the edge, and enough finally landed to start it. you're only booting computers and closing relays, but a little below necessary, and the cpu can get confused.

that being said, check the cables for loos connections at the terminals, and neg to ground.

fwiw, the same thing happened to mine a couple months ago, but when aaa got there, it only measured 4.5 volts. a jump got me started, and i bought a new battery at the dealer. $200.

 

If you have a voltmeter, try popping the hood and then let the car rest without disturbing for 12 hours or so.
Then go back and dont open doors or anything, just test the batt at the jump points.

More than likely from sitting the "resting" voltage will be low.

Im sure time to replace.
A light left on or a door ajar could have been the culprit in the past, but still it is time to change it.

 

Get it load tested. At that age, it's probably not taking or holding a charge very well and will soon leave you stranded. When you tried starting, then tested, then it started, it was likely just a voltage bounce from briefly putting a load on it. Old trick with a weak battery in the north with regular car in cold weather was when it cranked too slow to start, let it rest maybe a minute and try again. When you pull down the voltage on a battery, the voltage will tend to bounce back up when you remove the load.

 

Yeah, at that age I would just go straight to replacement.

 

On an old worn out battery, it may still charge to 12.x volts, but the problem is that there is not much charge behind that, so it would very quickly become depleted.

A 7+ year old battery wearing out would not be a surprise. But on these cars, the requirements of the 12V battery are very different from a regular car, where the main thing is to start the engine. Here, there is no such need for supplying a lot of current (which a "load test" would check for). You're more interested in capacity, measured in Ah, or Amp-hours.

In your case, I would double check the battery post-clamp connections - clean and tight. Then I would connect your voltmeter right to the posts / clamps and without turning car power ON (which would engage the traction battery, leading to the 12V battery being charged), do some things that would draw power, such as turning on lights (brake, parking, and interior lights should be live at all times) and see how the measured voltage responds. If it holds steady that's a good sign for the battery. If it drops quickly, that's not. Even better would be to apply an external load to the battery, such as an old headlight.

Another thing you may observe if running a test like that - as I did right before replacing a battery on a different car of mine - is if the measured voltage jumps up or down in ~2V increments. That indicates an intermittently shorted cell is shorting or un-shorting as the current drain is applied.

The test is easier on a normal car because the only way the battery charges is if the engine is running and the alternator is doing the charging (vs. with the Prius where it is charged with a DC-DC converter off the traction battery, even if the engine is stopped). So you can just turn the car on, turn on any accessories, headlights, etc. you want, measuring voltage at the battery, and if you have a clamp meter, current at the battery as well.

In that case, for me recently, I had a battery that usually seemed good, but sometimes bad. That test, seeing the +/- 2V jump coming and going convinced me. Yes, I could have just replaced it without testing, but I felt a lot better doing it having observed the failure.

 

they've been fine, iirc.

 

That does not necessarily mean that those connections are really "good".
Corrosion can hide where you can't see it.

But I agree that it's probably the battery itself.
Not at all unusual for them to get flaky before dying completely.