Today's hearing aid batteries are very small.  They are not like watch batteries and they require a lot more energy.  Batteries now are made with a chemical that requires air and more chemicals are used in the batteries.

Hearing aid batteries range in size from the largest, labeled 675, down to 13, 312 and 10 being the smallest.  A size 5 battery is currently in production, but will not last very long.  Size 5 batteries will be used in very small deep canal hearing aids.