When is a "dead" Battery not Dead?

My hearing aid notified me that it was time to replace it. I waited until the hearing aid notified me the battery was "dead" completely (which then set me as mostly deaf). After changing out hearing aids, I tested the 1.4V battery for voltage which registered at 1.35V. Why would the hearing aid stop working if the battery still has voltage?
- Anonymous
Edwardsvile, IL

When certain types of batteries run down they develop a high internal resistance.  This means when they are driving a load, like the electronics in your hearing aid, the output voltage is dropped due to the current flowing through the internal resistance.  Now when you take the battery out and check it with a voltmeter it reads almost the full voltage because the voltmeter draws almost no current, so no internal voltage drop.

LeeH

(published on 12/29/2009)