2005 s80 2.5T AWD, 176k.

I've got a minor mystery on my hands. This car is driven by my teenage son. On Tuesday the car was dead, no electrical power at all. I'm pretty sure he left his bluetooth adapter, which plugs into the 12v slot, plugged in. He's "just not sure," LOL.

Before jumping it I measured 12.5v at the battery. Seemed a little low to me... high 12s would be better/normal, yes? It jump started very easily. With the engine running and no loads, I got 13.5 at the battery. That seemed a little low too.

I took the car, drove it that afternoon and all day Wednesday. Many miles, many starts. Then car is starting and running fine. Today (Thursday), I again tested the voltage at the battery before starting it. Only 12.4! But then the car started easily.

So...12.5v, totally, completely, utterly dead. 12.4v, two days later, it starts right up.

FWIW I now measure 13.9 at the battery with the engine running, no loads. Turned on the blower and lights, voltage stayed at 13.9.


Thoughts on this?

Hi Dan,

The battery should measure 12.6 V with engine off.
with the engine running you see the alternator and voltage regulator.
around 14 volts.
14.0 is good higher and you start blowing lights faster.

Check your battery out, how old is it. Many parts stores will check it for you.

It's possible that something decided to draw current when the car is off.
Maybe the radio at the Submarine Races (Lovers Lane) for hours? :-)
Things such as a trunk light or glovebox light can discharge the battery over time.
You can check by measuring current flow at your battery with everything off,
and the car locked so nobody can run the windows and blow out your ammeter fuse.
(Ask me how I know this)
You should find some fluctuating current to run stuff like your clock and
some ECU functions. Expect to see around 20-25 DC milliamps for a normal car.
If it's way higher, start pulling fuses to locate the culprit.
Pull the courtesy lights fuse/s first.

Cheers, Bill

Hi,

I would like to mention what B.B. should have said about where to look for a battery drain.

You want to check between a removed negative battery cable with the ammeter.
Place it in between the battery negative post and cable. A drain of approximately 20 to 50 milliamperes.
A few LEDs worth is about normal with the cars alarms being off.
Any higher do what he suggests, look for something on.

The charging voltage seems to very normal.
A battery at 12.43 is about half empty. From there down, things can get very questionable fast.

Battery age effects cells individually.
As B.B. is saying, you may have a weak cell. Sulphate flaking shorts the bottom of the cells or a the lack of good electrolytes level in a cell cause low voltages.
Not all cells age or die equally.

The newest battery maintainers can double a battery life, if used consistently, to avoid internal discharging by their own nature.
I don’t like going over a week without float charging or driving.
But I have about ten batteries to stretch their lives out on.
A little float charge of 13.2V maximum with a minimum amperage can be endured to avoid fluid losses.

Same technique goes for a flashlight or hand torch.
One bad battery in a series stack kills a flashlight.
In this case you can check them all to find if you have only one being a culprit.

I have studied car batteries in the past, that had cap openings, by putting a probe right into the electrolyte and to a post or the next cell.
Back when they were tar topped a screw put down in them made them a six or nine volt source. A Handy cheat for my hand held six transistor radio.
Jeez I’m old 🥴

A bad cell will show more drop or an unevenness across a car battery.

A current loading or a 15 second cranking with a voltage bounce back up with in 30 seconds is the best for a better confirmation of its status of reliability.

Phil