Maintenance Free batteries

Any advice on charging a maintenance free battery?  

I assume your question is since you can not check the battery with a hydrometer you need another indication of full charge.  I charge with a voltmeter connected.  You will see about 15.5 volts on charge but I keep my small charger (1 to 4 amps) on until the voltage stabilizes at the highest reading.

You are still left with how to confirm a battery problem.  For that I go to the parts store and have them check the battery on their electronic tester after I have a full charge.  This sorts out the battery from a charging system problem.  

Lead acid batteries begin to "boil" bubble at 14.4 volts and prolonged voltage exceeding 14.4 will shorten the batteries life. Maintenance free batteries usually have a higher calcium content that makes them more resilient to higher voltages but the voltage should not exceed 14.9 volts for any period of time.

Nebraska's voltage stabilising at 15.5 volts is probably due to the fact it is a poor quality charger and that is its maximum voltage.

Newer smart chargers will "hold" the max voltage to about 14.4 for wet lead acid batteries and around 14.1v for gel type batteries. When this peak is reached (boost mode) the charger will continue to charge until the current drops to a preset value (absorption) then change to float mode (around 13.6-13.8v).

Using a smart charger will lengthen the life of your battery.  A typical auto battery is only charged by it's alternator until a light is left on and it becomes "flat" or it is near it's end of life.  An alternator is not the best form of battery charging and if you want to extend your car batteries life, charge it 2 or three times a year with a quality three or four stage automatic charger.

A quick indication of battery state of charge without a hydrometer is by using a multimeter. Leave the battery sit idle at least 1hr after charging, or if you just pulled up in your car wait about an hour.  Fully charged for normal lead acid is about 12.6 - 12.7 volts no load.  12.1 volts no load is a flat battery.
12.4v is about half flat or half full..

While the car is running the battery should be between 14.0 - 14.5v although some new cars have temperature/computer controlled regulators that sometimes regulate around 13.7v depending on temp.

These figures are general, as some batteries have more specific charging requirements.

The sooner you recharge a flat battery the longer it will live.  Hotrodders will typically find their batteris die of sulphation caused by parasitic drain and self discharge due to infrequent use.

Cheers
Dave