Wierd battery failure

[carrera]

So, emergency call out to rescue the black Aero on Sunday evening, which wouldn't start after filling up (engine barely trying to turn over). Jump start & off she goes. 14.2v at the battery when running,

Put the battery on charge, saw it go up from 12.8v to 13.3v, full bars on the little charger so disconnected & left to stabilise. 2 hours later shows 12.8v, so seems OK

Next morning she started, but only just, never mind, 230 miles to charge her up. 230 miles later, turned off, left 5 minutes & it wouldn't turn over. Bump start & visit to motor factor for a new battery. All is good

Tested the battery tonight. Still showing 12.8v. I expected 12.0, or even less

It seems the battery can retain a voltage but is unable to deliver current

Any ideas what's going on, I haven't seen this failure mode before

By the way, OpenSid tells me the car start time is now 0.9 seconds, when it was 1.4-1.5 seconds before. I did suspect the battery was getting weak.

[sgould]

This sounds very similar to the fault I reported earlier this year.  All the tests said that the battery was “good”, but the car turned over for a long time before starting.  A new battery has sorted things out, at least for now.

I can only assume that there’s a high resistance somewhere in the battery.  That would reduce the available current. The voltage checks would still show OK as there’s is very little current flowing during a check with a voltmeter.

[carrera]

It is a strange failure

Checked the knackered battery this evening:12.9v

New battery on the car after a short run: 12.59v

Fully charged  lead acid batteries cannot hold more than 12.6v due to chemistry.... still confused

[sgould]

I understand (well, that's overstating my knowledge!!) that modern cars use the battery in various ways to control the electronics, etc.  I'm wondering if there's some electronics in the battery itself.