Hearing aid batteries operate at full power until they are almost fully drained, and then they stop abruptly. They do not slowly get weaker like most other batteries. Waiting until the hearing aid stops working is the most common way to know when to replace the battery, so be prepared by carrying extra batteries. 

Although your hearing aids may have on/off switches, opening the battery doors will insure that the hearing aids are completely turned off.  It is recommended that you do this every night or when the hearing aid will not be used for an extended period of time.