One major disadvantage of using MOS RAMs to construct main memory is that the stored information may be lost as a result of even very short power failures.The solution is to provide a backup power supply which will support the system if the main supply fails.

The type and numbers of batteries required in the backup supply are determined by the following factors:

1. The supply current required by the memory modules.
2. The battery discharge characteristics.
3. The size,weight and cost of the batteries.
4. The maximum length of time the memory must be supplied by backup batteries.

                                           Nm x Pm + Ps          
                     Discharge current = ----------------    
                                                 V