home smoke alarm

Discussion in 'General Electronics Chat' started by superway, Jun 14, 2010.

  1. superway

    Thread Starter Active Member

    Oct 19, 2009

    I have just replaced my home smoke alarm batteries about 6 months ago. They are alerted beeping again yesterday. I disconnected batteries and measured them, all are read 9.35 vdc they are good reading. Why did my alarms beeped when my batteries are in good reading? When I replaced brand new batteries, no more beeped. There are 120vac and 9v batterry in my smoke alarm unit.
    can any one help and explan why they beeped while all batteries ar good reading?
    Thanks in advance

  2. SgtWookie


    Jul 17, 2007
    Hi Ken,
    You need to test the batteries while they have a load on them. Simply reading the voltage across the terminals does not give you a good indication of the charge left in them, as the meter is very high impedance; usually 10 million Ohms for a DVM, or 10k Ohms per Volt for a d'Arsonval movement meter.

    A 360 Ohm resistor makes a good battery testing load. Many meters have a battery test function in them. If you measure the current flowing through a 360 Ohm resistor, you should see about 24mA for a fully charged new battery.