Oh and there is nothing to really"look" for in this situation, you probably won't even notice a difference. It's just not a good idea. Just like in any electronics, they tell you not to mix batteries or use dead batteries with good batteries. It will just hinder the performance. The weaker battery actually becomes a load for the better battery to try and support.

 

Ya no one has come outright and said it, but the system might work a little better if you just bought two of the deep cycle batteries. I use two Optima yellow tops without an isolator and it charges up and starts just fine. Both batteries were bought at the same and spent the same amount of time in the truck.