How do i hardwire an originally battery powered smoke detector? And still maintain a battery backup?

Question

We have battery powered smoke detectors. I was hoping to hard wire an ac/dc adapter that gives it same , 9V (not sure amps), directly to the battery leads.

I would previously just remove the battery but hoping I can leave some type of battery back up in case power goes out.

Can I just put in a rechargable 9V battery into where I now already have 9V dc coming into the leads?

Thanks for any insight and help.

Answer 1

That doesn't sound like a very good idea to me, line powered (with battery back up) detectors are readily available and not too expensive.

Most rechargeables require the charging voltage be somewhat higher than the battery voltage, just connecting a rechargeable battery to the DC input gives the problem of either not reaching full charge, or over charging.

You could try using diodes to isolate the adapter and the battery (non-rechargeable), but due to the voltage drop of the diode, you may get a low battery warning if the power goes off, even with a good battery. Also, a cheap adapter may not provide very "clean" DC, and a battery powered smoke detector would have been designed assuming no ripple and a fairly stable voltage.