Final answer:
The best way to increase server availability when replacing a UPS battery is to install a second UPS, which allows for maintenance without interruption to power. Adding a redundant power supply is also a viable option.
Step-by-step explanation:
To increase the server's availability during UPS maintenance, the best recommendation is to install a second UPS in the rack. This setup is known as a redundant UPS configuration and allows for one UPS to be taken offline for maintenance or battery replacement while the other continues to provide uninterrupted power to the server. Alternatively, adding a redundant power supply to the server can also help. This means the server can be connected to two separate power sources, so if one fails or needs maintenance, the other can keep the server running without interruption. However, a generator would not typically be considered the best option for immediate UPS replacement as it is a long-term power solution for extended outages and does not address quick switchover during maintenance. A surge protector, on the other hand, would not provide power if the main source is lost, hence it is not a suitable replacement to increase availability in scenarios of power source failures.