electrical/electronic gurus

Working on my truck. I'm planning on installing an old school "driving" computer. Without going into a lot of detail about the unit, I need some help. In wiring up the unit, I need constant batt power, and ign switched battery power. The constant power is for "keep alive" memory. I would like to add a backup keep alive power source, in case of a dead battery or needing to disconnect the battery while doing other work on the truck. I'm thinking of a rechargeable battery connected to the truck battery positive with a diode in the wire. That way if I lose truck power the backup battery will power the computer, but the diode will prevent discharging the backup battery, and will allow charging the backup battery.
My question is: What kind/size battery can be used for backup (min 10 volts- max 12 volts nominal)? What size type diode would be best?