5.4 Starting and House batteries - do they need to match?

[daveb]

Depends on how your system is configured. From what I read here was that your spare battery isn’t like a 2nd starting battery and W/O knowing how your system works it’s hard to say. I would rather have a standard system with two starting batteries paralleled together that charges as one bank if in fact you need that. It seems like you’re concerned about ending up with a dead starting battery.
If your van uses the chassis battery to operate something while the vehicle isn’t running and you’re worried that you’ll end up with a dead battery in the morning, then paralleling another starting battery might be a good solution. If not, you might be better off just adding a good separator that monitors the chassis and house system which has the ability to manually combine the two systems for an emergency start. I do hope the guy that designed your system did incorporate some kind of separator or isolator between the starting and house systems.
Every time you add a battery there is the possibility of one going bad and taking the other down with it. It can be expensive if you have 4 batteries on board, then one goes south and you loose em all. The better made separators are designed to monitor and protect the other battery or other battery systems from each other. A separator also keeps the starting system isolated from a house system which generally has different designed batteries, usually deep cycle types. Most all experts suggest that battery banks should contain batteries that are the same make, size, type, and age. Is that absolutely necessary? No, and I know some who have used oddball batteries mixed together on boats, trailer, etc for years… its just not the best solution.
[edit] BTW AGM batteries handle freezing weather well, but it does screw with their state of charge a little more than wet cell types IIRC.