Cteks are great chargers and what I prefer for a charger. I use a larger Ctek on occasion to recondition my batteries. The Ctek chargers offer a lot of options!
Depending on what batteries you end up buying the ctek 3300 is way to small of a charger for you.
While all Ctek chargers are good for Lead Acid, Wet, MF, Gel, AGM batteries, the 3300 charger is only rated for 12Ah to 75Ah when charging the batteries, and 12-120Ah while maintaining a full charge on the batteries.
So the only batteries by XS Power that charger would be good for due to the Ah ratings are the:
d375 - 15ah
d545 - 14ah
d680 - 20ah
d925 - 28ah
d975 - 35ah
d1200 - 44ah
d2400 - 70ah
d3400 - 67ah
d5100/d5100R - 60ah
d6500 - 75ah
d1400 (14volt battery) - 50ah
d1699 (16volt battery) - 50 ah
Of coarse if you used 2 d3400 batteries you would have a combined ah of 134, which would make the Ctek 3300 charger way too weak.
If you bought Five d375 batteries they would total up to 75ah which is just at the max ah for charging with the Ctek 3300.
So like I said its going to depend on what batteries you choose to buy, and then how many batteries.
Your other options would be to disconnect all your batteries and charge them one by one each time you are going to charge them, but this could get to be very tedious and old very quick!!
Id strongly suggust stepping up to the Ctek 7002 charger, which is good for 14Ah to 150Ah when charging, and 14Ah to 225Ah when maintaining a charge. They sell for a little under $100 depending on where you buy it.
I love the Ctek 25000 which is a 25amp charger, its compact size and strong, but they sell for around $230 on the internet.
You can always look into the XS Power chargers at this price point too, but you dont get the reconditioning option with them
Now for your low charging voltage at idle, could be due to a couple things.
You could have a bad cell in a battery (you would have to test each battery individually to see which battery has low voltage or use a 100amp load tester you which you can buy for around $50 from most autopart stores. The agm battery cells read around 2.1 volts each (depending on condition and charging habits of your batteries), so if you had a battery with a bad cell, and it was fully charged it read in the low 10volt range.
~You could have a weak or worn out serpentine belt causing belt slip.
~You could have a bad belt tensioner if its a spring loaded one and its not providing enough pressure on the belt.
~If you have a manual adjustable tensioner you could try tightening it up some more for more belt pressure.
~If your using a stock alternator it maybe on its death bed (this could hold true to a high output alternator too).
~Depending on the make and model of your high output alternator it could just have poor output at lower rpm speeds. Usually alternators that are built for more power at higher rpms tend to have poor idle output.
~Depending on your alternator pulley size, you maybe able to go with a smaller pulley to achieve more idle output.
~You could have loose or corroded pos/neg connections on your batteries or alternator, fuses.
As for if you should charge your new XS Power batteries before installing them, the answer is Yes!
The batteries do ship charged, however they could of lost a little voltage depending on how long they have been sitting.
More then likely once you put the battery on the charger it will only take 5-10 minutes on the charger before the charger says it is full.
Hopefully this little essay helps ya out