True, these bikes have alternators. But they have alternators with permanent magnets, not field coil excitation. So, as soon as the magnets (mounted on the flywheel) start turning, they generate voltage into the stationary stator coils. Those coils supply three-phase AC to the voltage regulator, which in turn supplies (about) 14.4 volts DC to the entire remainder of the electrical system, including the battery. If the battery is very dead, it may not accept a charge, so it will appear the system is faulty and did not charge it.  A "smart" charger will see this 14.4 volts and no charge acceptance current as a fully-charged (not dead) battery and will switch to trickle or float charge. Given time the trickle charge (or voltage regulator) voltage will begin to charge the dead battery. Once the battery starts to accept charge current its terminal voltage will drop. The regulator will respond by "dumping" less excess charge current; or the "smart" charger will respond by entering bulk charge mode. At this stage the battery charge system will proceed to a normal charge.

Charon wrote on 05/03/09 at 13:30:33:


Yeah, what he said!

However, the nominal 12 volts must be present, or the CDI won't be making any sparks and the engine won't be running. The whole system is a parallel circuit, and it isn't very likely the 12 volts can be present in only part of it. Given that the bike has been jump-started, the 12 volts was supplied by the external battery. When the engine starts, the voltage regulator tries to supply the 14.4 volts to charge the external battery, and the bike battery in parallel. In the process it supplies the voltage to the rest of the bike. The external battery is removed when the jumper cables are disconnected. The voltage regulator sees this as a fully-charged battery (the dead one on the bike), and "dumps" excess charge. A push-start is explained similarly, in that the bike battery is rarely so dead as not to be able to supply at least a little voltage, which will probably turn on the low-current control circuitry and CDI in the absence of the heavy starter current.

Edited to add: To be pedantic, a generator is an alternator with built-in mechanical switching - the commutator. Both generators and alternators can be made with either permanent-magnet or electromagnetic fields, or even both. A permanent magnet field does not require excitation, but is harder to control Its charge cannot be reduced, so the excess has to be shunted (dumped) somewhere. The output of an electromagnetic field alternator or generator can be controlled by controlling the field current.