For a standard gasoline powered engine - absolutely none. In operation, the alternator handles the electrical load as the battery is virtually shut out of the system except for starting requirements. The little Odessey battery on the right does everything that big Optima did in my '31 warmed over SB Ford powered Model A and in a lot less space:
I have run 2 of the Odysey motorcycle batteries in my MG (with Gm 3.4L V6) for years. They are ran in parrallel and I am going on 6+ years now with no problems.
Advantage for me is they both fit in a single battery bucket leaving the second battery bucket free for storage.
All batteries have different internal resistance. If you team two in parallel, that means one will be over charged, and one will be under charged. Unless you have a real specific need for two batteries, it is a waste of weight and space.
I have two batteries in my truck with an isolator..I use the second one for my electric trolling motor and always have a charged battery when I need one..
I had one battery in the trunk of my '55 Chevy. I used two lengths of 00 gauge aluminum buried power cables from the battery positive terminal to the starter. I grounded the engine and battery to the frame. Never had any starting problems and I did not need the extra capacity of two batteries.
It was recommended that I use copper welding cables from the battery to the starter but I found the power company 00 gauge aluminum cables in a pile of scrap at a construction site and used those instead. I may not have needed two cables but I used them anyway....couldn't hurt.
In my case I have two batteries ,1 for the alarm and acceaories and 1 to run on with an isolater .you can turn off the starting battery with a marine rotary switch,but the alarm ,radio electric fans will still work without any draw on starting battery ,while running it will charge both batteries with the switch (Bass Pro Shop ) you can run 1.2or both with a twist of switch .on a hot start I use both batterys for more starting Amps then switch to 1 to run on .I also run a 120 amp alternator instead of the 61 amp that came on it .that might be an overkill but that was recommended by marine mechanic ,might not help in normal conditions but in my case it works. little extra for the 496 cu in .
alternator looks a little big but after complete it don't look so big.
The PC680 motorycycle battery is 680 for 5 seconds then down to 525 in 20 seconds.
The red top Optima is 910.
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