Help needed to set up a backup battery

Hi

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/sitepages/description1.asp?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.

My questions:-

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.

http://www.cornwall247.f2s.com/backup-battery.jpg

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...

Im no expert (ex = has been spurt = drip under pressure) but i belive you need to restrict the output to 0.5 amps 6 va max - look here http://www.eurobatteries.com/sitepages/description2.asp?id=221&cat=Battery%20Chargers.


I can give you further contacts if you need.

does anybody know if this device could be used to charge a sealed lead acid battery

http://www.maplin.co.uk/Module.aspx?ModuleNo=37762&doy=10m9

Sealed alarm type batteries need a constant voltage charge. Could you not rig it as a backup battery that only gets used when main battery is removed from the system switched via a relay. That way you could charge the backup battery perhaps once a year as there would normaly be no drain on it.

Mark

If you use a relay the problem is this, if it is constantly energised and in one position, when the main battery is removed it will change state, bring in your second battery etc, however as it is constantly energised it will drain the big battery.
Wire your second battery via an on off switch, periodically flick the switch to top it up as you like, normally the switch is in the off position.
The easiest way to bring in your second battery when the main one is removed is simply via a micro switch.
Have the lever forced in by your main battery, and when it is removed the lever springs out fully ,switching in backup battery or latching a small relay with n/o contacts closing and thus bring in battery number two to power the gps transciever

Think you need a split charge relay.
Everything (and more) you wanted to know is here:

http://www.kampenwagen.co.uk/split%20charge%20relay.htm

David