Dual power source

[Charlie]

Charlie,
The resistor might permit charging, but will it shut off when the battery will be full, or increase charge rate after battery has been drained after a power failure (I'm guessing no). Also, won't constantly charging the battery be bad for battery life over the long run? That would mean adding a sort of current monitoring circuit.

About decoupling caps on regulators, yup, leaving them out was intentional. It's the hexfets I'm interested in knowing if I have them wired properly. As far as I saw, I only need curent limit and pull-down resistors.

It's a Solex SB1270 12V 7Ah sealed lead acid battery.

Robert

That battery can be left on trickle charge forever, which is how a UPS works (or the telephone network for that matter). There is really no need for a fast charge cycle after a power outage unless you get those outages really often. Say you do your calculations and you have 10 hours capacity in the battery. After a 4 hour outage, you are down to 6 hours capacity until it recharges. How likely is it that you would have an outage greater than 6 hours within the next few days?

So use a 14V regulator and any old diode you have around (4000 series will be perfect for this). You will then have 13.3 to 13.5 (depending on load) available for your siren when on house power, and between 12 and 12.2 available on battery (that will ramp down as the battery discharged. The siren will still work even when the battery is almost flat - no need for schottky diodes or other low forward drop devices.

The hexfets will work in that configuration, but you only need one of the 10K resistors to VSS, and I'd be temped to put a resistor in series with the gates in this application. I'd also change the logic so that you can turn both devices on to charge the battery somehow, maybe in some sort of duty cycle to pulse charge... actually, I'd just use the diodes, with the resistor around the second diode, value set to maybe 1K (need to think about it a bit more).

[Demon]

That battery can be left on trickle charge forever

That is what I needed to know.

There is really no need for a fast charge cycle after a power outage unless you get those outages really often.

No, not really. I have a generator for extended outages..

I can easily keep my little charger for new batteries, charge them, then replace the old one.

I still have a lot of work to do on the control panels. Then I'll be able to determine exactly how much maximum current the circuit will need. I'm aiming for 1A max (700-800mA with siren).

I still have a few unknowns, like the fire and CO detectors, window break detectors, etc.

Thanks guys. I'm really liking the diode technique now that I understand more what goes on.

Robert