electrical/electronic gurus

Think I have this figured out. The "keep alive" circuit will be powered by this aux battery (don't need this large but is the smallest SLA i can find) https://batterysharks.com/12-volt-5-5-a ... ttery.html The positive of this battery will be connected to the vehicle battery through a 1N5404 power diode (400 volt@3 amps). The negative to ground. That will allow the vehicle to keep the aux battery charged, but the diode will prevent the aux battery from discharging through the vehicle electrical system. easy/peasy simple and inexpensive.
Bought the diode today, 50 cents, and ordered the battery.
PS: I was told by several sources, that I really should use the lead acid type battery, as other types don't play well with automotive charging systems.
Thanks for the suggestions