If you have a multimeter measure the battery voltage when turning over if goes low battery is knackered. Might show 12 or 13 normally till put load on. Check connections tight too a bad connection can make it picky too.
Or could be alternator not charging/drain as mentioned
Poor connections can also seem like a bad battery, I had a seemingly dud battery, morning starts would see the revs die to a clicking starter then nothing. I thought it must be the battery but after seeing smoke from one of the terminals I realised it was bad connections, cleaned the battery terminals, reseated the earths, cleaned and reseated connections on starter and it turns over like a new one, wipers run faster and lights run brighter.