here’s the original question from post #1

 

Your not clear of you questions much, good luck

 

What is not clear? He wants to know if anyone has used an AGM battery. Answer=yes.
The part about a secondary battery is probably because some people like to blow the neighborhood's ear drums with over powered amplifiers that need a second battery. Or, maybe just a misunderstanding of batteries.

 

I was always told you cant mix different types of batteries without causing issues, such as one type ruining the other, due to different resting voltages.

 

They rest at close resting or static voltage, 12.6 volts is normal, our alternators charge pretty much the same .
I have a 2018 GMC Terrain with the push button start and auto engine stop start , which requires the AGM deep cycle battery, the static voltage is 12.6 and the voltage meter shows 14.8 when driving so the battery voltage is maintained.
you need to measure the 90 series battery’s dimensions to ensure you get the right size battery for the OEM location in your HHR.

 

If you want to use 2 batteries in parallel it is best to use 2 completely identical batteries; age, brand, chemistry, process etc.
If you are using them isolated it doesn't matter.
You would not want use 2 12V batteries in series; that would provide 24V.

That was not the question. You can replace the stock battery with an AGM battery, the car does not care what the chemistry or mfg process is.