Device defective due to battery

Unfortunately, the quality of the built-in batteries seem to be very poor (I am not the only one). Even with a 2 devices, the battery is already empty after just 9 months, acc. To print this but keep until 2021. The trader refers to the warranty conditions which after 6 months passes the burden of proof on the consumer and does not accept an exchange from (Great control of the EU). Since the consumer but the evidence can not lead, it is so off in the bin with the part because the battery is firmly installed. Despite the recommendation Warentest Fingerweg Foundation.
Update 29/04/2015: In the meantime also the second device has failed with a defective battery. I have but I replaced two new batteries and those concerned, goes smoothly. We'll see how long they keep this time.