Polo 1.6 9n not starting

[Ayaan61]

Hi guys any advice on what to do next, I've replaced the starter and the car started perfectly fine after that, checked voltage on the battery was idling at 14v which I'm told is correct but after I drive for a while the car just won't swing again and looks like there isn't enough power, does the battery need to be replaced in this situation or the alternator.. Any advice