I have a 08 SS retrofitted with an AGM battery. I recently became aware that certain types of batteries including AGMs require a higher than normal charging voltage (i.e. 14.7 - 14.9V) than standard lead-acid batteries in order to prevent sulphation. What is the charging voltage in the HHR SS?

 

It can vary but normal is in that 14.5- 14.9 range. Lots of things will affect the demand requested such as battery charge state, amperage demand and such.
I read in one of the service manuals that the charge voltage could go as high as 19.5 volts under extreme load demand.
Will your car know it's a AGM type battery and increase the voltage ? I doubt the HHR's system can tell 1 battery from the other.

 

Where did the information come from for the statement regarding a higher than normal charge rate?

This does not agree with that at all - from https://batteryuniversity.com/learn/..._glass_mat_agm :
"As with all gelled and sealed units, AGM batteries are sensitive to overcharging. A charge to 2.40V/cell (and higher) is fine; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell); a direct replacement with a sealed unit could overcharge the battery on a long drive."

Don't worry about it - lots of members here have switched to AGM batteries without problems. It's not an issue.

also from that same site:
Advantages

Spill-proof through acid encapsulation in matting technology
High specific power, low internal resistance, responsive to load
Up to 5 times faster charge than with flooded technology
Better cycle life than with flooded systems
Water retention (oxygen and hydrogen combine to produce water)
Vibration resistance due to sandwich construction
Stands up well to cold temperature
Less prone to sulfation if not regularly topping charged
Has less electrolyte and lead than the flooded version

Limitations

Higher manufacturing cost than flooded
Sensitive to overcharging (AGM has tighter tolerances than gel)
Capacity has gradual decline (gel has a performance dome)
Low specific energy
Must be stored in charged condition (less critical than flooded)

 

There was a lot of talk on a Toyota 4Runner that I'm sometimes on forum about how the charging voltage from the alternators on the older 4Runners was not high enough to work with AGMs and Silver Calcium batteries so an Alternator voltage booster was required to step up the voltage going to charge the battery on those vehicles. That made me curious to know if anything like that was required for the HHR but if the voltage typically ranges in the 14.5 - 14.9V as firemangeorge says, there's nothing to worry about.

 

If the regulator is asking for a 90% duty cycle the alternator can put out 15.5 volts.