Hearing aids use different sizes of batteries, so it's essential to buy the correct size as the wrong size won't work. The audiologist can suggest where to buy the appropriate battery size and tell you the correct size. Some manufacturers offer rechargeable batteries, but they can't power the hearing aids for a full day. Mercury-free batteries are now available, and many audiologists will collect and recycle them if you are their patient. Battery technology may improve, so keep an eye out for future developments.