Getting ready for a long road trip next week and had a question. Going to be running a large power inverter the whole time and I wanted to put a extra battery or 2 in. My question is I have a 1 year old blue top from my boat and plan on buying a new blue top to go also, would it be a bad idea to run them both together.

As long as they are both the same models, you should be fine.

the only issue with mixing batteries is if they sit for a looong time the bigger battery will drain the smaller one... but other than that you should be ok

if after charging them up, they rested differently, then they would effect each other all the time even during driving.

 

All a battery is is cells.  It's just a box of 6cells.  Now you have 2 boxes of 6 cells.

 

When 1 box of cells rest higher than the other, the alternator will STOP charging(maintaining) them once the level of one of the two batteries reaches full charge.

 

Because they are paralleled together, they will share a common voltage.  That means, in this scenario, one of the two may always be at 93% charged and the other at 100%.  Eventually, that will deteriorate the better battery by discharging more to equalize the voltage and then kill both equally.

 

This won't happen in 1-2 weeks, it would take time.

 

this is also why it's not wise to charge "banks" of batteries without an equalization ability on the charger.

Thanks for the great info guys

what about using an isolator

if anything, a relay, not an isolator.  A relay, also called a solenoid.

 

They are fine, but in my experience if the output side is going to a crucial battery, they still have ~0.1-0.2v drop due to limitation of being able to overkill on the wiring.