I recently bought a van that has a starting and an auxillary battery, who ever installed it didnt isolate it from the starting battery. Im planning on installing a simple switch to isolate it, but I need to know how many amps it needs to be rated for. How can I find this out? I have a multi- meter, but worry that the amprage may be high enough to blow it even unfused (it says 10A max) if I connect it in series between the aux battery and the incoming + lead. Thank you in advance.

I recently bought a van that has a starting and an auxillary battery, who ever installed it didn't isolate it from the starting battery. I'm planning on installing a simple switch to isolate it, but I need to know how many amps it needs to be rated for. 

How can I find this out? I have a multi- meter, but worry that the amperage may be high enough to blow it even unfused (it says 10A max) if I connect it in series between the aux battery and the incoming + lead.