Recommended battery voltage

Hello guys .Today I changed my battery (Nissan Micra k13 2015) because a few days ago the car couldn't start without external help.The mechanic changed the battery and checked the voltage with the engine on .It said 13.1 and he told me that the recommended is 14 .Do you know the recommended price for this model ? Do you think that the alternator is the problem?
 
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