I had a flat battery, and charged overnight. After charging, voltage read 12.6ish on the multimeter, and started fine.

After starting, the voltage measured at the battery did not increased to 13 or so (what I would expect if the alternator was charging). 

I've driven it a couple of times, and it looks like the battery is slowly losing charge, but I have no warning lights now (The engine light was previously on for low battery charge, but I cleared it with a friends OBD tool after charging the battery).

I've also checked fuses under the bonnet, and they all look fine. Is there anything else to check before sending in for an alternator replacement?

 

ps - I'm still puzzled why I wouldn't get a warning light for what seem like a basic problem (my 1971 Datsun would have let me know!).

 

The link fuse (is that the right name, as per picture below?), looks ok, but if it blew, might not be obvious? A guess a continuity check from top to bottom with the multimeter might rule this out.

 

Also, would putting the multimeter on the positive terminal of the alternator and the other end on the car body (while running) would also be a check for the alternator charging, though need to be careful obviously?

 

Your charging system probably doesn't work the way you're expecting it to.

The car has start-stop, I imagine?

 

There may be nothing wrong but you can indeed do the two checks you suggest. (Carefully) measuring voltage at the output stud of the alternator relative to chassis ground will tell you what voltage it's putting out, and you can measure continuity through the strip fuse if that voltage is significantly different to that measured across the battery posts.

 

If you do have start-stop, you'll have an EFB or AGM type battery, and the charge management system actively avoids charging these to 100% charge; so having charged it with external energy, it may be that the alternator isn't being asked to charge it yet.

Edited 24 May, 20215 yr by Wino

Thanks for the tip, yes it is start-stop, I haven't owned one of those before and hadn't thought about that.

Do you have an idea at what battery voltage should it kick in to charging mode? I'll start on some googling, but I hope I haven't stuffed up by charging the battery with an older style of charger (I removed terminals to charge).

I expect it'll be fine, but check the owner's handbook for instructions on connecting chargers before doing it again. Did the car have very little use prior to the flat battery problem?

 

As for why it was flat, I may have left the ignition on on the weekend. At home I don’t tend to lock the car, and I had been checking the windows seals, but not sure.
I will keep an eye on it. It was charging on my last car trip from the alternator (14.something volts) so fingers crossed. Thanks for the tip on the start stop! I was ready to book it in and thinking about which kidney I was going to sell.

Also, there are some amazing YouTube tutorials on how to remove and check the alternator out of Skoda’s, but the amount of stuff to remove makes me miss my Old Datsun - three bolts to take it off!

8 hours ago, mbn said:

At home I don’t tend to lock the car

An unlocked car uses more electricity than a locked car.

A bit like the difference between a mobile phone being on stand by and being completely turned off. 

 

Thanks, AG Falco

Is it the original battery on a 2015 stop/start car? If so it's probably near end of life - especially if it has been lightly used in  lockdown etc. The original on my facelift 2014 Superb had to be replaced a few months ago - funnily enough after I left the sidelights on for a whilst diagnosing a troublesome rear number plate light. When the battery is near the end it has much less storage capacity.

 

Replacement will have to be a EFB or AGM battery.

 

NB I find running my car with the headlights on provokes it to charge. I can tell as I have a split charge relay to sometimes run a coolbox from the boot 12v socket.- and I can tell when it's not charging as it turns the coolbox off.

 

 

Edited 24 May, 20215 yr by bigjohn

Quote

Is it the original battery on a 2015 stop/start car? If so it's probably near end of life

True this; I know other people who've had to replace batteries on stop/start cars by about 3 years old.

I haven’t had to replace battery, everything seems fine now.