I have a 1990 740GL that runs great despite a persisting problem.

For the past year or two, my volvo's low battery indicator would come on very dimly, but would never cause any problems. Then one day in November last year, I went to start it up and it didn't start at all. I got a jump and eventually bought a new battery because it kept happening and it was older than my alternator leading me to think that it needed replacing.

Everything was fine until a month later when the symptoms started again where the indicator would come on, I would park my car, and when i started it again, it would either start up very reluctantly or not at all.

I've installed a voltmeter on the dash to keep an eye on the battery and have found that my voltage would strangely slowly drop to 11 volts over the course of about 1hr of constant driving and stay that low for a very long time (after about 150 miles) until it would then suddenly jump to 13.5 volts and be perfectly fine.

I have noticed that idling for about 10 mins would cause my voltage to lower to about 12v but other than that, I see no reoccurring trends. I would be grateful for any advise!