Every package of RAYOVAC L13ZA-8 batteries that I open shows maximum voltage of 1.38 volts instead of 1.4 volts as listed on the package. Why is this? Why don't the batteries read 1.4 volts? My hearing aid fades away at about 1.31 volts. The additional .02 volts would (should) give me better than 20 percent longer battery life.
The resting open circuit voltage of a non-sealed zinc air battery is approximately 1.4 volts when they are manufactured. After the battery air vents are sealed, Rayovac relies on a minute amount of air that goes through the tab to maintain a voltage. The amount of air that migrates though the tab is insufficient to maintain a voltage level of 1.4. As a result, the tabbed voltage is significantly lower than 1.4 volts during storage. This characteristic allows for a final check of seal quality.
Prior to packaging the product, the voltage of every battery is checked and any product above 1.38 volts is rejected. This is one of the few times where a high voltage can be a sign of a problem. High voltage indicates that additional air is getting into the battery signifying a poor seal. With time this will cause the battery to dry out and cause short life or dead in pack batteries. It is to the end-users benefit that our batteries can be checked in this way because it insures that the battery is properly sealed and will give full service life when put into use.
Some, but not all of our competitors add a small amount of manganese dioxide to their batteries. This causes their batteries to have a open circuit voltage above 1.4 volts even in a well-sealed condition. They, however, can't use the open circuit voltage as a test to insure seal quality.
Rayovac tabbed batteries will generally have an open circuit voltage between 1.1 and 1.38 volts. Within 15 seconds of removal of the seal tab all of the batteries in this population will have an open circuit voltage above 1.3 volts. While it will be above 1.35 volts within a minute or two it make taken an hour to get the level to 1.4 volts.
A open circuit of 1.3 volts or higher is sufficient to operate a hearing aid. When the door of the aid is shut a load is imposed on the battery, providing power to the aid. Normally the battery operates at a 1.25 closed circuit voltage. Some high power aids will require more power and the battery will operate at a lower voltage level.
There may be an occasional battery that is extremely well seal and the open circuit voltage may be lower that 0.9 volts with the tab on. Accidentally shorting the battery for even a second or two can also drop the open circuit voltage to these low levels. If this occurs, it does not mean the battery will give short service in a hearing aid, but these batteries may take several minutes after removing the seal tab before they reach a level where they can power an aid. Giving the battery a few extra minutes can save throwing away a battery that might give perfectly normal service life.
For more information on Rayovac, visit www.rayovac.com
Denis Carpenter, Rayovac Corporation, Zinc Aid Technical Manager. BS-Chemistry UW-Lacrosse. He has 25 years experience in zinc air research and development.