Battery light coming on and off towards end of journey

[Transit-man-van]

What is the likely cause?

I was on the longer driver today, about 2 hours after all, and noticed towards the latter 3rd of the journey the battery light was on, then went off after a couple of minutes, then came on again maybe 15 minutes later and stayed on until I got the to destination.

I really didn't want it to be the alternator having died again as I only got in not a year ago. I took a look under the hood as soon as I parked and put the voltmeter on with engine stopped and it read 12.9/13.0v. So fully charged right? I know you should let it rest a while to get a true reading but in time when it has been working normally and checked the voltage I didn't seen it deviated much above the suggested voltages for full charge In fact I have never read it as high as this, usually 12.6 or so but then again I don't usually drive that far and check. If the alternator hadn't worked at all I would be looking at much lower voltages right? It is a normal sealed batter btw, not lifepo4 or the like.

I ran the engine and saw the belt spinning so relieved to see it is still operational. I just read an article which advises to put the voltmeter on when the engine is running, which I haven't done yet, to look for a jump in voltage on the battery to indicate the battery is charging.

Likely causes?

I will also note that it is a new battery, bought only last year.

MOT test is tomorrow so fingers crossed it won't affect that! Have to try and solve it if is going to be an issue, only a few hours!

[metalworker0]

With you messing around so much ., you are more than likely the culprit

your last few [posts ..without reading all of them
have titles below and just reading for few lines dc to dc charger etc .

last posts titles

Post subject: Destroyed my starter battery, replacement and thoughts?

Post subject: placing or DIYing alternator electronics lead cap?

Anyway sorry, just says to me, i should not get involved , i barely have the ability to diagnose a standard set up never mind a modified set up.
if i was doing such a thing it would be another battery in "parallel" (negative to negative , positive to positive ) then all your gubbins connected to that ...if you make the mistake of in "sequence" then you double the voltage ..including that returning to the alternator. and double in all electronics of the van ..risking frying everything!
Even if in parallel - Put too much load and capacity in the circuit ...you overload the alternator.

al;l the best.mark

[metalworker0]

meant series not sequence .

all the best.mark