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)
