SLA charger from Car socket

Hi Everyone,

I'm trying to put together a circuit to charge a 7Ah SLA battery from a car lighter socket.

I am part of a neighborhood watch group in my country (South Africa). As part of this group we all use flashing lights on our scheduled "patrols". This is a simple strobe type LED light that plugs in the car lighter socket. What I would like to do is use an SLA battery with the light so that when I turn off the engine, the flashing light stays on even with the lighter socket off. I do not want to run off the car battery when the ignition is off.

Points I would like to achieve:

• Have the light running when ignition is off
• Not use the car battery when ignition off
  
• Use an SLA battery (7Ah)
• Charge the SLA battery whilst ignition is on
• Have the light on whilst charging the SLA battery.
• Protect the SLA battery to extend its life.

I have done some searching around and so far I have decided on the simple circuit supplied in the datasheet for the PB137(link) regulator (ST). I am concerned that this simple circuit will not provide necessary protection for the SLA battery to prevent premature "battery death".

I would appreciate your advice!

My background:
Trade Electrician
Diploma Mechatronic Engineering
8 years Work experience in Maintenance Department of Factories ranging from Heavy metals to Food processing
Not much experience on circuits < 220v but I have dabbled.

Thank you!