The Activair Battery Tester is a very popular hearing aid battery tester with 4 testing slots for each size battery; 10, 13, 312, and 675. Just slide your battery into the correct size slot and push the lever! The color-coded easy-to-read meter display shows how much strength is left in the battery, ensuring your hearing aid battery is working at its maximum strength. Built-in, convenient storage compartment for 6 spare hearing aid batteries. Measures 3.5 in L x 1.75 in W x 1 in D. Includes instructions.