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Battery Backup 12v

Hello, I wanted to add a battery backup to a Picaxe circuit so when the AC main fails the power will switch from the AC-DC transformer to a 12v battery which is then lowered to 4.5v to power the PICAXE board.

after some research I found this circuit which seems pretty simple and I wanted to know if you people think its good or will I have fried PICAXE for dinner.

Thanks in advance!  :D

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Also, I think you should put a better charge limiting on there other than just a 10 ohm, 10 watt resistor. Also where you are going through any diode, you will be losing 0.6 to 0.7 volts

A couple things here.

1) if your mains supply is just 12 volt and never exceeds it, then it will not charge your 12 volt battery to full. Since the drawing shows a 30 amp fuse, I presume this is a lead-acid "car" battery. Those are 6 cells that run 2.2(+) volts per cell full charge.

This means a "12"-volt lead-acid battery is actually 13.2 volts. If you try and charge it up with 12 volts only (and after going through the diode, will be only 11.3 to 11.4 volts), the battery will never be fully charged and this will eventually result in a dead cell.  The mains circuit needs to be higher than the battery float voltage in order to charge it. (typically in the 14 to 15+ volt range.)

2) Let's say you do have an AC supply that is high enough voltage to charge your 12 volt battery... That means when that supply is on, you have even more voltage drop to get the voltage down to where the picaxe (or any other microcontroller) can use it.

But it also presents another problem. Once your batteries are fully charged, the charge part of the circuit needs to automatically switch off, or your battery will over-charge. If the battery has vents, the boiling acid will boil out and evaporate; your battery will go dry and will go dead. If the battery is a sealed type, (maintenance-free), then it can actually build up enough pressure to explode, spraying sulfuric acid on everything nearby. --and the battery will be dead after that, of course.

What I am getting at is that the circuit, the way it is drawn will work, short-term... -but before very long will cost you money for a new battery at the very least, and could be much worse.

I suggest that you make your mains transfo somewhere in the 14-15+ V range as mentioned above and use a DC-DC regulator to get to 12V.

About the over-charging: this will only happen if he's above a tenth of his Ah's. If he's below that (trickle charge) he can charge his battery forever without damaging it, it would just be a giant waste of electricity...

Unless you have other circuitry that uses 12 volts, why not just go with a 6 volt supply? This is way overkill just to keep a PICAXE running, IMHO.