battery backup circuit???

[Mosaic]

Well Mike, I guess you never met a self limiting wall wart charger.....transformer, rectifier, maybe a cap and little else, no fuse, no bimetal strip or PTC. The charger is 'sized' to charge to 13.8V or so as current load drops to a trickle. Higher currents cause increased voltage drop loss due to DCR winding loss plus rectifier and wiring losses and is thereby self limiting. Such chargers are not suitable for every size battery, they have a window of compatibility and are as cheap as they come. No dedicated current limiting electronics at all. That approach sidesteps hysteresis as well and would probably serve elecv better than Colin's circuit (which I found on other sites as well, so it may not be Colin's at all)

Edit:
BTW I converted an AC3U Nokia clone cellular charger ($3.00 ea) to deliver 13.5V (changed a zener) and used a 5 ohm, 5 watt resistor as a series current limiter , effectively making my own Wall Wart 'extra efficient' charger and there have been no 'fires'.

 

[elecv]

Other than improving the entire design, you might replace the 22uf with a 1000uf or so to slow the chattering. The transistor's base bias is hovering near its linear region and causing the heating.
This might be want you want:
http://www.next.gr/circuits/lead-acid-battery-charger-circuit-l35827.html

Thanks mosaic I will try the same.

 

[MikeMl]

Well Mike, I guess you never met a self limiting wall wart charger.....transformer, rectifier, maybe a cap and little else, no fuse, no bimetal strip or PTC. ...

I have a box-full of about 30 DC output wall-warts (not switchers, they are in another box, so are the AC output types ). About a third of them have unregulated output voltages of ~15Vdc at 500mA to 2A, so might be candidates for a low-current 12V battery charger. Our dilemma is if we grab a charger at random, we have no way of knowing if it will it will be suitable to power a non-current limited charger. Without more info, any given charger might catch fire, blow an internal fuse, or repeatedly overheat if used for LA battery charging. For this reason, I always include purposeful current limiting in the regulator, such as I posted here.

 

[Mosaic]

Yes, well, those charger warts are custom built to work. An average adapter needs customizing to become a charger. Right tool for the right job. You can't drive a screw with a spanner.