If a new battery becomes unserviceable within a few days after its installation it may be due by one or more of following reasons:
An incorrect charge
A short circuit in the electrical system or battery terminals are dirty or not properly connected
Electrical capacity of the battery is insufficient for the size of the vehicle (see also capacity of the battery is not at its correct level!)
The battery has been inadequately activated, dissipating its electrochemical strength
The battery presents a sulfating effect due to time between activation (acid filled in)  and first charging need to completely activate electrochemical properties.