Flat (ish) battery

[milu]

Just getting my head round this. Had car 3 months so not sure how old battery is but thought seller mentioned replacing it. No receipt though.
Thought it seemed low sometimes but got through ok. Been using more over the last 2 weeks hols finally doing 200 miles on Thursday.
Then last night it’s too flat to start car (cl500)
11.5v ish. I’ve charged overnight so now at 14.4v

So... iffy battery or drain elsewhere? Would a battery just finally give up like that even after a long drive a few days before.
Obviously the next few days will give me a clue but I’m inpatient!

[PaulKemp]

Fully charged automotive batteries should measure at 12.6 volts or above. When the engine is running, this measurement should be 13.7 to 14.7 volts.
If your battery is holding 14.4 V without it running or being on the charger I’d be very surprised.

I’ve just had an issue with a much older battery, would take a charge to 12.5 V but die off on cranking.
Adding batt aid tablets increased the time it would crank for but it was to far gone.
Looking at the plates by looking down the cell fill hole showed a lot of buckling and sulphation.
I bought a new battery

[littleredrooster]

milu said:

...So... iffy battery or drain elsewhere?

You've missed the other candidate: Alternator.

Is the alternator bringing the battery back to full charge? Put a voltmeter across the battery terminals with the engine running; it should read over 14v. If it is less, the alternator becomes the prime suspect!

[Gary C]

milu said:

So... iffy battery or drain elsewhere? Would a battery just finally give up like that even after a long drive a few days before.
Obviously the next few days will give me a clue but I’m inpatient!

In my experience, thats exactly how a failing battery behaves. Starts fine after a drive, flat in the morning.

[milu]

It was showing 14v on the charger readout
Once that was disconnected it was at 13.1v
I’ll check tonight but of course it won’t prove car drain or dud battery unless I disconnect it for a day or so
I’ll check with engine running but there’s been no dashboard warnings

[GreenV8S]

Take it somewhere that sells batteries and get it load tested. A faulty battery is the most likely diagnosis and the easiest to fix, so test for that first.

[littleredrooster]

milu said:

It was showing 14v on the charger readout
Once that was disconnected it was at 13.1v
I’ll check tonight but of course it won’t prove car drain or dud battery unless I disconnect it for a day or so
I’ll check with engine running but there’s been no dashboard warnings

Half-wave rectification won't necessarily trigger a dashboard warning, but will gradually flatten your battery. Battery-charger voltages and idle voltages tell you pretty much nowt worth knowing.

What you need to know is, mainly: 1. system voltage when charging and, 2. battery voltage when cranking the engine over. This can then more closely pinpoint the problem.