Start problem with 12v battery

Hello , I have like others , now already for the 3rd time start problems because of too low voltage of the 12V starter battery at the Toyota Yasiss Cross . Yes , I drive in Munich and surroundings mostly short distances and the max. 2-3 times per week . 2years I had no problems . The “workshop” – longer distances drive and battery every 4 weeks properly recharge (have no electric connection in the underground garage ) is cynical and little helpful ! I have to my problem in the forum following notes found: Replace battery with AGM battery ( warranty case ??) or download new update – who knows more about it -how can I get this to the workshop? As well as replacing the 12V charging module, so it is said that there is a callback at Toyota. Who can help? I am grateful for any help – otherwise the Yaris Cross is a great city car !!