need advice for gps tracker battery backup idea

[Gruelius]

hi all

im after some sort of small battery that can be wired in series to the car so that the car battery charges it and that if the main battery is removed that it only gives power to the gps unit, i was thinking that a resistor and diode could do this as the charging current would not need to be large current to charge.

need something i can hide in the parcel shelf or behind the boot trim somewhere.

[StereoInstaller1]

You have a GPS that doesn't have a battery?

[Gruelius]

cheap gps transponder unit that works via a sim card, doesnt come with backup battery. I only need the backup battery for situations where theifs remove the main battery before i can get the location.

[StereoInstaller1]

Yeah, then a simple resistor and a relay will make a perfect backup.

Close the relay to charge when the key is on is what I would do.

You have like a little gel cell?

[Gruelius]

nah i have no battery whatsoever so i need to find out what batteries are small and good for that.

[StereoInstaller1]

I used to use these little gel cells, like 50mmX100mmX150mm, were 12V and could run an alarm for a couple of weeks...I just have no clue what they were for, we always got them from car stereo suppliers like AAMP of america.

Sorry I have no better help, but those batteries are perfect for what you are doing.