The Express Power Pak is a self-contained rechargeable battery back-up for the Vodavi DVX Plus Mach I. It contains gel-cell batteries, an aluminum case and a cable to attach to the battery connection on the power supply unit. The Power Pak is 24 volts and 15 AMP hours. Customers like the units because they have uninterrupted phone service, even during a power outage, at a competitive price.