Only one way battery charge (zener diode?)

A diode is the cheapest. Or get a "splitter" at an RV place.

 

Not sure what it is called, but on my cargo van I have an inverter installed with a separate bank of batteries. When the ignition is on, the thing-a-ma-bob they installed combines the van battery and inverter batteries for charging. As soon as the ignition is turned off, a relay automatically separates the inverter batteries and starting battery so you don't drain the starting battery while using the inverter.

Had it installed at Camping World. Think they are common in the RV industry.

 

Here is what ya need...

http://www.mastervolt.com/marine/products/bi-battery-isolators/bi-1203-s-120-a-12-24-v/

Of course this a marine unit so it probably costs four times what it should, but you get the idea...

 

there is a switch you can buy. been around since the 60's.

grandpa had three batteries in his truck. the first was the main standard battery to truck only. the other two were secondaries to the camper. the alternator charged all three but the truck only used one like normally. while the camper only used the two secondaries.

all you had to do was install the switch. there was no flipping.

 

They are called battery isolators. They are/were very big in the car audio scene, when there was still a scene. Should be less than 150 bucks depending on the alternator amperage you're looking for.