For Stock, the 2016 rule book states you are limited to a max of two batteries weighing in at a total not to exceed 150 lbs. They must be the same size, type and weight. Brand isn't relevant. I have two 12 volt AGM batteries of the same class size in my Mustang that I built for GT/MA. One is Optima and the other is SuperStart. Both are heavy. One light and one standard is a no-go in Stock.
EDIT: Jeff, I read your first post, which said Stock and your last one which said Stock style Bracket (SSB/). You should be OK mixing battery sizes in ET/Pro or Sportsman, but email your tech before spending the money.
EDIT 2: In regards to charging, if they are wired in PARALLEL I don't think it will make a difference to the charger. It will see one big battery. I found putting two parallel connected batteries on a charger tripled the full charge time. AGM 12 volts on a charger with a AGM setting.
EDIT 3: I was wondering about the safety of using two different size batteries so I found this similar question on a boating forum:
Question: Due to space considerations I want to use two 105 amp hour and two 150 amp hour batteries as a house battery bank. Would this arrangement be detrimental to the smaller batteries?
Answer: My gut reaction is that mixing two sizes of batteries would be a bad idea. Having said that, I really can’t think of a technical reason that it would not work. Let’s see what our battery guru, Justin at
LifeLine Battery says:
“It has previously been said that this is not a good idea; however, we have done some pretty extensive lab testing and we can find no differences in the batteries. Age is important but size is not. You can safely mix and match battery sizes in one bank.
Also, Sabre Yachts does this from the factory. This was one of the reasons we started testing this scenario. Sabre use a GPL-4DL and when they want to upgrade a battery bank they add a GPL-31T.”
Dale