I thought I was doing both the environment and me a favor by buying a couple of 9Vbatteries for my smoke alarms, but later realized that they were really 8.4V and set thr low voltage indicator s of right from the start. These were NiMH batteries. So I saw some Lithium Ion 9V batteries that stated they were 9.6V like a regular alkaline 9V. After charging them they were near 11V. Needless to say the smoke detector did not like these either.

So my dellema is this. How do I use the efficiently regulate the 11v battery down to 9V and monitor the actual battery's voltage so that it does not get too low?

 

Have you considered an LM7809?

 

Have you considered an LM7809?

Not that I haven't thought of the LM7809 fixed Voltage Regulator, but I assumed that it would last very long given the dropout voltage and current drain associated with it. Maybe there is a more suitable Low Drop Out regulator I could use. Would a simple voltage comparator be efficient enough to be used to work as a battery disconnect?

 

For your consideration: The smoke detector is a verry low current device so the regulayor should be also. Using a quad comp.[LM339 or lower current version] and a 4066 quad switch to bypass v dropping diodes , & high value resistors , should keep current drain low. If detectors low V det. is insufficient there is still one more comp. to play with.

 

Lost thumbnail again

 

Been sick for a bit, but will try something like this.

 

A smoke detector uses such a low battery current that a 9V alkaline battery lasts a lot longer than the leakage caused reduced life of the charge of a rechargeable battery.
The fire department says to replace the 9V alkaline batteries in your smoke detectors once per year even if the batteries ares still good. After one year the batteries are not new anymore.

 

The reason to replace batteries once a year is to ensure that a one year old battery can still have enough power to produce a loud alarm signal for enough minutes to allow occupants to hear, wake-up, and escape.

Don't jeopradize this requirement.

Retired Volunteer Firefighter

 

The reason to replace batteries once a year is to ensure that a one year old battery can still have enough power to produce a loud alarm signal for enough minutes to allow occupants to hear, wake-up, and escape.

Don't jeopradize this requirement.

Retired Volunteer Firefighter

Understood. I'd set the low battery alarm to trigger sooner than the MFG. trip point for low batt...

 

First, the voltage was 11V due to overcharging.

You can use a 12V lead acid battery and voltage regulator to obtain 9V.

 

First, the voltage was 11V due to overcharging.

Well, let me re-phrase things a bit.

The rechargeable battery that read aprox. 11V(10.38V after sitting for some time) is actually the "9.6V" NiMH. The Li-Polymer rechargeable battery listed as 9V is reading 8.4V. Both batteries were charged with the charger of the same manufacturer. The NiMH that is listed "9.6V"

You can use a 12V lead acid battery and voltage regulator to obtain 9V.

Where would you propose I put this 12V lead acid battery??!!

 

Use the 12V and regulate its voltage down to 9V with a voltage regulator.

 

A smoke detector uses such a low battery current that a 9V alkaline battery lasts a lot longer than the leakage caused reduced life of the charge of a rechargeable battery.
The fire department says to replace the 9V alkaline batteries in your smoke detectors once per year even if the batteries ares still good. After one year the batteries are not new anymore.

+1 or buy mains driven smoke detectors. Mucking around with them may very well void an insurance claim if there was a fire.