Diode to prevent backflow from a second 12v battery on accesory power or would a relay be better?

I'm wanting to have a second battery to have power available when the car is off without risking draining the main 12v battery. I only want it to charge when the the car is on. The 2nd 12v battery is it to keep the system board of a head unit I'm building running for temporary power offs (keep navigation from resetting for short stops and such). Also I'd a phone run a phone charger off it.

Two approaches I'm considering:

1) Run the battery backed system board on accessory power. Use a diode to prevent back flow when the car is off.

2) Run the battery backed system on the always on circuit but put in a relay that disconnects when the accessory circuit is off.

I'm thinking 2 would be safest typing all this out. 1 puts a lot of trust in the diode not to fail and cause bad problems with back flow, but 2 would be least likely to cause damage if it failed I believe.


The battery is going to be a small 5 AH deep cycle VLRA AGM battery. It seems like it'd be most compatible with the Prius 12v system since that's what the main 12v battery is a bigger version of.

 

A low voltage disconnect is something to consider. Thank you for the suggestion. The diode causing a voltage drop isn't something I though about and would rule it out.