Assuming your fanbelts are not shot and slipping, Check the voltage coming from the alternator when running should be 14.4V anything below 14 would suggest the alternator isn't charging the battery enough. If it is low it would be worth checking all the earths but could be time for a new alternator.
For example when my alternator went at 150k , it was charging at just under 11v.
Usually if its starts and then the battery light comes on its charging system related rather than the battery.
However, you maybe right might just be the battery, so fully charge it and see if the voltage drops off, over a few days, could be a drain or battery dead, mine seem to last 3-4 years. Usually you get a host of other errors or bongs if the battery is getting old and tired.
Hope that helps
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