The issue of testing batteries and the battery voltage are related, so we will cover these two issues under separate headings.
Batteries will not show their correct voltage unless they are under a load. If you simply take a multimeter (or battery tester) and connect the leads to each battery terminal, you will not get an accurate reading.
Expensive, high-end and specialised battery testers are designed to place the battery under a load in order to get an accurate reading, while multimeters provide a guide as to the voltage potential.
When testing batteries with a multimeter, you must have the meter set to measure DC voltage and place the battery under load by using a resistor in parallel with the test leads of the meter. A battery can be used until it is down to about 30% capacity. After that, it typically will not supply enough voltage to power the product it is being used in. The actual cut-off voltage will vary with the battery type.
If you are just wanting a device to let you plug in a battery and know whether or not it's still good, a battery tester is a better choice (we have these on our website). Battery testers are really designed to give users an indication as to whether the battery is good or dead. This is especially the case for people who get their old and new batteries mixed up when changing batteries.
Voltage in Zinc-Air Batteries
Removing the sticky tab activates the battery as air enters through the holes. After the tab is removed, the battery literally requires a moment to “catch its breath.” As air starts to enter the battery, it becomes activated. Zinc-air battery manufacturers always recommend letting the battery sit for a full minute before you insert it into the device and shut the door. This allows the voltage in the battery to rise and ensure that you will not have start-up problems with the hearing device.
Although the package of most batteries will state 1.4 or 1.45 volts, batteries that are still tabbed will measure anywhere from 1.1 to 1.3 volts if you were to test them with a multimeter. After you take the tab off, the voltage will rise enough to power a hearing aid within one minute, but the entire cell could take as long as 24 hours to rise to the maximum 1.4 or 1.45 volts. Un-tabbed voltage after a maximum of one day is 1.45 volts.
The important thing to know is that the hearing aid does not require 1.45 volts to operate. Most hearing aids are designed to operate until the voltage drops to 1.1 volts.