In the early '90s BMW included a charger with their motorcycles. Part 95 61 1 250 700 is a large black Schumacher charger which charged until the voltage reached 14.0V, then it turned off and resumed charging after the voltage had dropped. This was updated with 95 61 1 250 800. Either is a perfectly adequate charger except that it gives no indication that it's working, other than a small ammeter. If the unit fails, you can't see from across the garage that it is dead. This charger, for conventional (flooded) batteries, was later superseded by 71 60 7 676 472 and 72 60 1 470 156 (a Deltran charger).
In 2003 BMW started using gel batteries in the motorcycles and released gel-specific chargers. 72 60 7 679 040 is the original "blue aquarium pump" gel charger. 99 99 0 005 370 is a 5-battery charger for dealer use and 99 99 0 005 656 is the single-battery charger sold to end users, both made by Deltran. All can be used on older batteries as well as on gel batteries, but the older chargers have higher output. Deltran also markets a gel charger under their own "Battery Tender" label. I have seen BMW chargers with labels peeling off, revealing the Deltran label underneath.
On the R1200GS, BMW used the YTX14BS battery, an AGM design. Because the accessory socket is controlled by ZFE (the onboard computer), another special charger is needed which will talk to the computer and convince it to turn on the accessory socket. This charger is the "gray aquarium pump" part 71 60 7 688 865. Other chargers will work until they go into float mode, at which point ZFE will turn the accessory socket off for good. BMW authorizes use of their older chargers 7 676 472 and 7 679 040 (superseded by the 656 charger) if they are connected directly to the battery terminals, while any non-BMW charger used must have a maximum charge voltage of 14.4V and the battery must be disconnected from the motorcycle.
865 was replaced by 90 88 6 611 590, which has a higher output to keep batteries charged during CIP programming, and is selectable for gel or non-gel. It also has some test functions. The number on the charger label is 61 1 581.
Charger 82 11 0 026 663 is for automobiles.
NOTE: the rest of this is just snippets of old emails, for now. Sorry about the total lack of editing but I wanted to get this part started.
Voltage-sensing alone isn't a great way to monitor charging, but it's often the only way. Temperature affects this, more so in a sealed battery. Picture a battery getting hotter and hotter, with the voltage dropping lower and lower, drawing more and more current. It can (and does) happen. Unlikely with a bike, though, since the battery can cool itself a bit.
Personally, I'd love to have an accurate voltmeter on a charger.
All battery chargers operate at a voltage higher than the fully charged OCV (open-circuit voltage) of the battery. Without this, there is no driving force for the charging and no way to compensate for self-discharge.
When the battery is charging, not all of that energy is going into heat. Remember, the whole reason for charging is to drive a chemical reaction. The excess is turned into heat. In a severely discharged battery, the charging efficiency is very high (close to 100%) and there's no heat. Once you approach the fully charged state, the efficiency will drop. You can get heat (probably "bleed" current through the battery, not sure about this) and gassing (from electrolysis). Both of these waste energy, but they only happen when the battery is almost charged. So the charger won't be putting out full current anyway.
Some modern batteries (and we're not talking bleeding edge here) can be recharged with 110% of the energy which was removed. And while the faithful standby criterion for a fully-charged flooded battery is that it should be gassing freely, the sealed (no free acid) batteries can't be charged this way or they dry out. Batteries only gas if they're above a certain voltage (dependant on the grid alloy, mostly) and you can charge them at a low rate (just under this point) so they hardly gas at all but still have a sufficient voltage differential that the charging happens. Very efficient, but you don't usually see this sophistication for cars and bikes because it's expensive and mostly unneeded. The BT and other smart chargers are like this.
About chargers: like you said, once the battery reaches the threshold voltage of the charger, the current will go to a very low level. This happens with any decent charger. Smart charger, wall transformer, whatever. The typical charger is constant-current/constant-voltage meaning there's a maximum on each. At low states of charge, it puts out full current. As the battery charges and the voltage rises, the voltage peak is reached and the charger reduces current to maintain the voltage limit. This is the same for any of the ones Roger discussed.
Also, there's a lot of conjecture in the post about charging. At low states of charge it won't matter how smart the charger is; amps are amps. Fully discharged, assuming all batteries in the discussion pull the maximum amps from the charger, there's no difference. If you're hinting that the smart chargers have a different current than the "dumb" chargers when they're in constant-current mode, that's generally true but not related to the fact that they're smart.
Lastly, charging and maintaining are two different things. If I wanted to revive a dead battery in the least amount of time, I wouldn't use a BT. I'd use a big honking current source and juice the snot out of it. Not good for the life of the battery, though. Smart chargers take their time and do no damage. Most people need a charge maintainer, not a charger. The BT will recharge dead batteries, but the "smart" aspect of it is either unused or unneeded (until the recharge is complete and becomes a maintenance charge) and they typically have a fairly low output. It's fine for maintaining, but power is what charges a dead battery in a short time.
As the charging process converts the active material, battery voltage rises and the rest of the energy is dissipated in heat and/or electrolysis. BTW, "fully charged" is not a point, it is a zone where certain design aspects are met. This is a real stumbling block for a lot of people. Most batteries NEVER EVER are 100% converted into PbO2 and Pb.