Many moons ago, when the Battery Tender chargers first came out I decided to do a little test.
At the time, wet cell (lead acid) batteries were the norm. Nowdays, most of the bikes I work on have some form of maintenance free batteries.
With a 'dumb' charger, the output voltage was constant, and you could see the acid bubbling when the battery was on charge.
Problem was, if you left a lead acid battery on a dumb charger, it would eventually boil out the acid.
When the 'smart' (Battery Tender) chargers came out, they promised to eliminate this problem.
I put two lead acid batteries on the bench next to each other. One was connected to a dumb charger, one to a Battery tender.
In a matter of weeks, the one on the dumb charger lost a significant amount of acid. The one on the Tender stayed at the full mark.
Tender claimed that their smart charger would start at a relatively high voltage and amperage, but that once the charger sensed that the battery was charged, the amperage would throttle back to trickle charge levels and the voltage was reduced to the low 13V range which is below the 'gassing' point of the acid.
I connected a Tender to a fairly dead batter with an ammeter and voltmeter connected.
Watching the readouts, sure enough, the Tender did as advertised. Once the battery was charged, the voltage and amperage scaled back.
What they have trouble with, is a really dead battery. I keep a few Optimate brand chargers on hand to bring back dead batteries. They have a Desulfate feature that 'burps' the batteries and can bring back a pretty dead battery.
Once the lithium batteries came on the scene, I checked with Battery Tender to see if their chargers were compatible. They answered that the older Tenders maintenance voltage wasn't high enough to keep a Lithium battery happy, but that their more recent (prob 10 years ago) ran a slightly higher maintenance voltage, and were up to the job.
One last note, the Desulfate cycle on the Optimate (and similar) chargers will cause a lithium battery to catch fire or explode, which I find undesirable.