Alternator or battery?

[ConorE]

Hi, bought a 2009 Grande Punto yesterday and an hour after I bought it, it refused to start. After a jump start from a mate I took it for a 15-20 minute drive with no issues then it started again about an hour later. However, this morning the same thing is happening as before where the car simply won't start.

I believe it is the battery as the car didn't stop at all when driving it so the alternator is (I hope) keeping it charged, but any other ideas would be appreciated, thanks.

[stevieturbo]

Test voltage when stopped and when running. It should be pretty obvious if the alternator is not charging.

And usually if it isnt the alt light would be lit. Usually...but not always.

[annodomini2]

Could be something draining the battery as an alternative option.

e.g. Badly wired Aftermarket stereo.

[S6PNJ]

After a very nice man (AA) helped me jump start my car one day (many moons ago) he told me that an alternator doesn't like charging a flat battery (as it senses voltage which rises rapidly on a flat battery) whereas a dynamo will happily charge a flat battery (as it works on current sensing not voltage). If you can, put your battery on a charger overnight and try it again for a few days. You could always go to KwikFit (or any other suitable motoring chain) and ask them to do a battery test for you. Might cost a tenner or something but it will tell you properly if the battery is life-ex or not.

[paintman]

If the jump start was required because your battery was flat a trip round the block isn't going to put much charge into your battery. It certainly won't fully charge it.
Contrary to popular opinion car alternators aren't designed to charge flat batteries:
https://www.optimabatteries.com/en-us/experience/2...

Fully charge the battery with a battery charger & see if it then holds charge.

If you bought it from a garage take it back as a warranty issue.

Edited by paintman on Friday 2nd February 14:21

[jefword]

stevieturbo said:

Test voltage when stopped and when running. It should be pretty obvious if the alternator is not charging.

And usually if it isnt the alt light would be lit. Usually...but not always.

Could you or anyone else by any chance explain why the light does not always illuminate because I had this happen on my Volvo S60.

A few years ago I put a brand new battery on, unnecessarily, as I thought it would cure a fault. Then I accidentally ran the battery fully down whilst driving until the ignition etc. totally packed up.

Turned out to be the alternators voltage regulator needed changing but at no point before or after my problems started did the alt/charging light come on.

Still baffles me to this day.

[stevieturbo]

jefword said:

Could you or anyone else by any chance explain why the light does not always illuminate because I had this happen on my Volvo S60.

A few years ago I put a brand new battery on, unnecessarily, as I thought it would cure a fault. Then I accidentally ran the battery fully down whilst driving until the ignition etc. totally packed up.

Turned out to be the alternators voltage regulator needed changing but at no point before or after my problems started did the alt/charging light come on.

Still baffles me to this day.

Because modern cars are s lol. Alternators etc used to be more simple...they too now have more electronics so if they're wonky...the light may not behave correctly, and on some cars the whole lot is ecu controlled.

I had the same happen myself, probably drove for 150 miles or so before realising alternator wasnt charging, and that was only noticed after the car started having a high idle and I started investigating only to find it was doing so to try and combat low battery voltage...except it couldnt fix it because the alternator was buggered. No alt light, no warning of failure etc.

[buggalugs]

jefword said:

Could you or anyone else by any chance explain why the light does not always illuminate because I had this happen on my Volvo S60.

A few years ago I put a brand new battery on, unnecessarily, as I thought it would cure a fault. Then I accidentally ran the battery fully down whilst driving until the ignition etc. totally packed up.

Turned out to be the alternators voltage regulator needed changing but at no point before or after my problems started did the alt/charging light come on.

Still baffles me to this day.

I had this on an old beemer. It’s a certain kind of failure with the diodes or something, probably.