I am buying a gps traker for my boat in case of theft.
I want to set it up with its own backup battery so that if the main battery is removed it will continue to work for at least 24 hours.
Something like this http://www.eurobatteries.com/sitepag...id=595&cat=SLA
Im planning to mout the GPS unit with its backup battery in a waterproof box and hide it in the boat somewhere and i would like the battery to be kept topped up in normal use (ie because its connected to the main battery) and only ever come into play if the main power goes down.
However im not sure exactly how to wire something like this up I have basic GCSE level electronics but i was hoping someone here will be able to help.
Can I just wire it in paralel with the unit and it will keep itself charged as 99.9% of the time it will have the boats main battery in parallel with it?
Will simply being connected in parallel with the main battery not fry the backup battery (ie do i need to limit the current to a trickle chanrge - if so how do i do this)
Do i need to put a diode in the system to stop the backupo battery powering up other eledctrics (and therefore not lasting as long) if so what rating of diode will i need.
This is what im proposing but Im sure iuts not quite right.
OR am I completely mad - if so please suggest the correct solution. for instance there may be a simple little circuit i can add that will trickle charge the backup battery until the main voltage drops and then cut in the backup one. I dont know...