Japan will not suffer a power failure. However, Thailand frequently blackouts. I started life in Thailand and immediately purchased an uninterruptible power supply (UPS).
The battery has deteriorated.
I bought a replacement battery at a parts shop on the 3rd floor of Fortune Town. True It is near the service center. One replacement battery was 750 Baht. I replaced the battery of UPS which is running normally. This UPS has one battery.
There is space for adding a battery.
There is also spare wiring on the + side.
I soldered the minus side with a cord.
I bought a soldering iron for 220 volts in Japan. The battery did not fit in the case.
I bought a power cord. It is 60 Baht.
The power cord was soldered to the internal wiring. Then I will protect with electric tape.
I drilled a hole with an electric drill on the back side of the main body in order to pass the cord outside.
There is no dedicated connector to connect to the battery. I soldered the pulled out cord to the battery.
I also protect with electric tape.
I installed an additional battery between the two UPS.