We have developed car security device. This device takes data from existing security controller from Autocop and then alerts the users. You also get the location information from tower data.

This device need to work even when car battery is removed. So we need to modify the power supply by including 6 volts SLA battery.

I was thinking of converting 12 volts to 6.8 volts using LM317, and then float the battery at the output. This output is again taken through an LDO to generate 3.3 volts. 6.8 volts will also be used to power the GSM modem.

The second option I that came to my mind was to convert 12 volts to 6.8 volts and 8 volts separately with the battery floating at the end of 6.8 volts, and then OR the outputs using diodes. This method also doesn't have over-discharge protection for the battery. Since diodes are used to OR the voltages, in the backup mode, when the battery is discharged to 5.4 volts, output after diode will go below 5 volts.

Won't the first method work satisfactorily? Is there a better way of doing this? Kindly guide.