Question:
Many people died in fire accident because they found 9V fire alarm didn't work. However, no engineer seems able to design a better battery for fire alarm to work all the time. Do you have any idea?

Answer: -There are Lithium 9v batteries. There are AC powered alarms. In a new design consideration, one could design a new alarm that uses larger cells.

-The reason the battery "didn't work" is becasue it was flat and needed changing. This is THE most common cause of death due to non-functioning smoke alarms

-use double alarms at every relevant places in my home. Have totally 6 alarms. Some of them would always work. All of them have 10 years lithium batteries.

-You can also buy dual power (AC + battery) smoke alarms. Put a lithium 9V in there and you're pretty safe from loss of power.