Receive Defective battery

The battery is inserted and charged exactly as prescribed (3x complete loading / unloading to instructions). Nevertheless, had the battery at full load and 100% charge indicator only for a period of about 60 minutes in sleep mode between 90 and 120 minutes.
In the first 2 months, has remained so, but then performance decreased steadily. Tip to the Net-Electronic Customer Service: Repeat 3 times charge / discharge process from the beginning.
I did that, the performance has not improved. He now holds under full load still about 45 minutes (at 100% load performance) and in sleep mode just under 90 minutes.
Net Electronic thinks that the termination would not be in a defective battery; I do not think so, but since I've communicated that silence reigns. Thanks a lot for this ...

Apparently I have an old battery, or simply caught with a particularly weak cells. Can happen, but the customer service from the vendor somehow behaves quite wrong.
So if you already want to buy a battery so, then perhaps better with a provider, the defective goods also takes back or exchanged.