You'll want the secondary battery (and related accessories) on an isolator, so you don't drain your starting battery when the vehicle is OFF. There are two ways to do this - you can use a solenoid isolator (/not/ the Ford starter motor solenoid, but a decent quality 100% duty cycle piece. It's going to cost more, but it's also going to last a good deal longer) or you can use a solid-state isolator (which is essentially a network of high-current diodes.)
The solenoid:
Advantages -
- May be wired with "bypass switches" in various configurations
- Physically smaller (about the size of your fist)
- Available in much higher current ratings
- Less operating heat
- Some are sealed against immersion, most can handle being splashed.
Disadvantages -
- Not always easy to locate
- Requires finding and wiring in a "trigger" lead for automatic operation
The solid-state pack:
Advantages -
- Automatic. No switches
- No user intervention required past installation, in fact.
- No need for a "trigger" lead generally, either.
Disadvantages -
- Greater operating heat - the case is a heat sink. Make sure it can get a useful amount of airflow.
- Limited current capacity.
- Expensive - and not a linear relationship between current capacity and price
- Not always waterproof or weathertight.
Which one would I use personally? It really depends on the application, although I have more experience using solenoid isolators than solid-state packs. A proper bypass switch would allow you to "self-jump" without even getting out of the driver's seat, in fact - and getting a solenoid that has an ampacity of two to four times that of the typical solid-state pack is easy.
However, if I'm designing a system to run on minimal to no user intervention whatever, I'm probably going to go with the diode array. Most people who need the "user friendly" setup aren't going to be the sort of people who are going to remember what to do with which switches, so I'll simplify their system as much as possible.
Feel free to hit me backchannel if you want to start designing a setup. The good news? Most multi-battery isolated setup really don't care what battery is on each side of the isolator - so when you finally kill off your existing Red Top that you're going to use, you can replace it with a Blue Top without making any changes at all to the system (solid-state or solenoid-based!) The only time battery type and age becomes a problem is when you have a "bank" of batteries - if you've got, say, four batteries in parallel and one goes out on you, you'll be replacing all four batteries in the bank. The starting battery, on the other side of the isolator, can be replaced on its own.
Yes, this also applies with "off-grid" storage arrays fed by solar, wind power, or small hydroelectric setups.