Charging system Question

That is definitely wrong, not normal. Voltage should stay at 14 volts or near that until the battery is close to fully charged, then will slide down to about 13.5 volts, and stay there while engine is running. Also, a fully charged battery should show about 12.6 volts at a minimum, especially a new battery.
I don't know what is going on with your system. Take it back to Merc- were they aware of how it is acting, voltage-wise? Doesn't sound like it. One small possibility: is your alternator belt not tight enough?

Get a volt meter, start your car and see how many volts your car is producing while running. Should be around 14 volts. May want to watch the meter for a while since your electric seems to fluctuate. Most voltage regulators are internal of your alternator. If you are getting a lot more or less than 14 volts while running, replace the alternator. If it is not charging, voltage will drop as vehicle runs, until battery is dead.

amps or volts
11 to 12 amps indicates nothing but if it is volts then it indicates that the alternator is not charging the battery
Battery voltage is 12 -13.5 volts and when charging is 14.5-14.8 volts dc
the rest of your statement indicates that the alternator or voltage regulator is faulty and you should have the charging system checked by an accredited auto electrician

Check all your grounding straps especially the one from your block to the frame if it is worn rusty or in bad shape replace it check your connections id bet it is just your ground strap

unfortunately the alternator is only a part of the charging system and it should be tested by an auto electrician not a parts shop
replace the alternator and then take the vehicle to an accredited auto electrician for a charging system check charge rate should be 14.5-14.8 volts DC at around 1500rpm
they can check the alternator out put , voltage regulator or if the regulator is part of the ECM they will be able to test that as well especially when the system is hot as that is when the problem occurs

it may have deffective diode on the charging system(the alternator). 14 volts is an over charge in a battery. ampere reading must be on what the battery designed.

ok when the pcm detects something wrong whit the charging system the pcm show you in your board CHECK GAGES if this is not on, dont worry about

When the new alternator was installed did you perform a charging test to ensure it works properly. If it's not the alternator may overcharge the battery causing corrosion to the posts and possible cracking of the battery case. If you don't have the tester at home most auto parts stores can connect theirs and it usually is free.

Good luck to you

Try checking the cables on your battery to make sure there not lose or corroded, a lose connection from the battery could cause this problem, if there's any corrosion it will look like white fluffy deposits, if so clean them off with a wire brush or a battery terminal cleaner. you can perform a basic test on your battery by using an ohm meter, with the vehicle off the battery should read about 12.4 volts, with the vehicle started the battery should read above 12.4 volts, usually around 14 volts, also try checking the drive belt for loseness and wear, if it does'nt have enough tension it could be slipping on the alternator pulley,

This vehicle has a "Regulated Voltage Control" system. The computer reduces the charge voltage based on several parameters to improve gas mileage. Normally it shouldn't be a problem, but the computer will increase the voltage if you switch on the headlights. Also, if you're in Tow/Haul mode the voltage will increase.