it got very cold around here and like many others i just bought two new batteries for my truck, these are just factory but AGM upgraded.   now in an audio system lets assume i have enough wiring to transmit power with negligible losses from under hood to back seat.  if i have two batteries at 1150CCA up front and put an older battery in back on the amp rack (say a 900CCA agm) will i run into issues with having a less than new battery back there?  lets say the battery tests out at 80% of its rated cca (so still a decently good battery i think), when the amps pull power will it have a similar effect as a cap can have in that the alternator is now trying to charge the battery up as well as the amp, increasing the load on the alternator or would the fact that its got capacity over ride any negative impacts on the charging system (would it even have any?)

would it be better to just keep the two batteries in front and not add a 3rd in the back?

i know an older battery can drain a newer one when left sitting in parallel, id probably be isolating the rear battery from the front two while off.

 

No it will not act like a cap or need isolated. But your listening style and alternator capability will influence how fast the batteries can recover. 

will different cca ratings effect the electrical system under load? will one battery with higher cca be damaged or negatively impacted by other batteries having a lower cca and not being able to supply as much current?

No, the different size batterys will not effect each other. You are good to go.