You do not need yellow top battery, (personal feelings speaking, the red-tops are better spec-wise for car stereo anyway, but I don't really want to go into that right now...) or any other specialty type of battery. Just remember, if you add another battery, just make certain it is the
SAME as the front one. As far as the cable size between the two, it only has to be big enough to support the current capacity of your alternator. If you have a 150 amp alternator, use a number 4; 200 amps need a number 2... and on and on. Make certain you fuse
BOTH ends of this cable for safety, also!
As far as an "enclosure"? Not really necessary, except for appearance. As long as it is strapped down, so it can't fly around in the event of an accident, (or grow legs) you should be good to go.
The Optima (yellow, red, Rockford...) batteries are very different from standard car batteries in the way they are constructed. A standard battery uses flat plates that are separated by paper, with a liquid electrolyte (the acid) surrounding the whole mess. These are adequate for most applications, but are VERY unhappy when discharged more than 20% (somewhere around 10.4 volts). Once this happens, the life of the battery is reduced by as much as 50%, and it will NEVER provide full output current again. They also do not like heat - the kind of heat under your hood... (Hmmm. I wonder why they don't use different battery technology for cars, yet?)
Oh, wait! This is where the Optima "Spiral Cell" technology comes into play. (Don't I sound like a commercial? LOL) The way the Optima batteries are built, is they take the lead plates, and roll 'em up with a paper separator, but then they saturate the paper with the same acid, but leave none free inside the battery. This is why you can use them in any position. Because of the rolled technology, the plates will not warp or crack (as easily) with abuse, such as (semi)deep discharging, or high temperature environments.
On to the differences between standard (red-top) and deep cycle (yellow-top) batteries. NOTHING. They are the same, save the THICKNESS of the plates they are built with - deep cycle being up to 25 percent thicker. Because the current a battery can produce comes from a chemical reaction, the number of plates (or total surface area of the plates - 25% thicker means 25% fewer plates, means 25% less peak current...) will DIRECTLY affect the amount of peak current it can produce, because you have fewer plates, right? If you have a battery case of X volume, and you place battery plates within it, it will make X current, right? If you have the same battery case, but you place thicker plates in it, you will be able to get LESS peak current, right? Now, red and yellow top batteries are the same size case... the deep cycle version, (yellow top) has FEWER plates, right? Which one is better for CURRENT? The red top, right? What does a car amp need to produce power? Current, that's right. Now which one will you choose? Me? I'm saving the cash, and getting a better battery for my application, and gonna buy the red top.
Now, before somebody argues with me, I will also add that the rolled cells prevent the warping caused by deep discharging, WITHOUT the plates needing to be any thicker - so I don't NEED to buy a deep cycle battery... Optima already gave me the deep(er) cycling capacity when they rolled the plates. Also, I never run my batteries to that deep a discharge cycle anyway.
No, I am NOT a salesman for Optima, I am a network administrator in the health care field. BUT, I have 16 years of formal electronics experience and training,
and I take everything the salesman tells me with a grain of salt... I'm their WORST nightmare - I used to BE in sales. BWAHAHAHAHAH!
Happy Halloween!
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."