Switching power sources, battery charging

[Demon]

... overheating is only a potential issue if you are charging a totally pancaked battery. In normal use you really don't need to worry.

What if the battery dies, sulfates, whatever?

The 317 is a bit light, but conceptually you could use it as a reference with a couple power transistors that turn on as required.

The circuit in the link seems fine.

I'm open to using higher rated components, I'm not merchandising. I try to follow his math on how to select resistors but I get lost.

Robert

[Demon]

I checked this, but I'd prefer to build and learn instead of using a charging module.

http://www.picbasic.co.uk/forum/show...5713#post95713


I could use a PIC as they did, but I haven't seen much on google or here that does exactly what I need.

The PIC could do it all, switch between sources and auto charge. But I would need more hand holding going that route.

Robert

[pedja089]

If I were doing same thing as you, I'll check if siren works form 10-15V(It should).
If wont work then put step up, to boost output voltage to 12V. So if battery voltage drop below 12V, you still get 12V for siren.

And supply everything from battery, and put battery charger with little hysteresis(Turn on your charging if battery voltage is below 13.2V, and turn it off when battery voltage reach 14.2V). So you wont worry about battery, just replace it every 2-4 years.
For battery charger you can use transformer with 1A rated output, and 12Vac. Then just put rectifier capacitor and voltage regulator to limit maximum voltage to 14.2V.

For lower power consumption of board with PIC, LCD, LED, keypad etc, use step down convertors. So you your input current will be less then output(Pout=Pin*0.9). If you have 12V input, output 5V out 1A, it will only take about 0,5A from your battery.