Most so-called smart chargers have at least three stages. The initial bulk charging stage or mode (constant current) brings the battery up to approximately 75 - 80% of full charge. The next stage is called the absorption mode (constant voltage), and it's designed to bring the battery up to full charge using a higher constant voltage and tapering off the current until the battery reaches a full charge. At that point the charger goes to the third stage which is a maintenance mode that applies a voltage only slightly higher than the fully charged battery produces, usually around 13.2 volts for batteries using modern chemistry. During the absorption stage some chargers I've seen allow voltage to go as high as 15 - 16 volts. Most chargers don't have a voltmeter, so most people don't realize these things do reach higher voltages than the bike's alternator/regulator produces. For those who have a multi-meter or dedicated voltmeter, attach the voltmeter leads to the battery terminals while your Battery Tender or other brand charger is charging your discharged battery (less than 80%) and note the various voltage levels as the charge progresses. What your meter is reading is the voltage applied by the charger, not battery voltage.
The thing that kills batteries quickest is overheating, thus the old constant current chargers have become pretty much obsolete. Those had to be constantly monitored to check for excessive temperature and outgassing. Modern chargers, unless defective, take all the hassle out of monitoring the process through the use of electronics and feedback systems that self monitor and regulate the process.
I agree with Glenn, since the process is pretty much invisible to the user it would be wise to stick with well known brand names. I've developed a real distrust of the no-name Chinese stuff that can be found everywhere these days. Our friends in China have demonstrated repeatedly they have no qualms about producing knock-offs of low quality or even unsafe products. There is no one on either side of the ocean actually testing and weeding out the fakes or the defective/unsafe products. Never assume the US regulatory agencies check incoming shipments, they do not.
JMHO - Jerry