The misunderstanding that battery supplies normal running load power of Motorcycle !!
Automotive systems mostly use the oldest type of rechargeable battery (invented in 1859), the lead-acid battery. 12 volts is the short common slang for nominal battery voltage. It really is not 12 volts.
Lead-acid cells are 2.1 volts per cell at rest, with a full charge. The six-cell 12-volt battery is 12.6 volts at full charge resting, should be over 14 volts while charging, and over 13 volts immediately after charging is removed (without any electrical load).
Traditional low-voltage (12 volts) electrical systems use a negative ground system, the “ground” almost always being the vehicle’s entire chassis. The highest current ground is to the engine block since that is where alternator and starter currents flow. That ground has to be a very solid low resistance connection. It is a direct connection to the battery with batteries near the engine, and often via the chassis with remote batteries. A ground must always connect from the battery negative to the chassis with all battery installations.
The Battery’s Job
A common misunderstanding is the battery supplies normal running load power. This is not correct, the alternator normally supplies all electrical energy. Of course, the alternator will not support the electrical system when the engine is off, when starting the engine, and under certain operating conditions of extreme electrical loads, especially at slow engine speeds. The battery supplies power any time the alternator is unable to support the full electrical demand. The battery kicks-in instantly, even if needed for a tiny fraction of a second, because the battery directly parallels the alternator’s output.
The battery serves as a giant “electrical flywheel” to smooth voltage from the alternator. Just as the flywheel on an engine smoothes piston thrusts and clutch loading, the battery prevents sudden alternator surges or electrical load changes from radically changing the electrical voltage.
The battery must be kept in parallel with the alternator with solid connections. On a running engine, if the battery is disconnected (accidentally or intentionally) and the electrical load or engine speed abruptly changes, or if the battery is disconnected from the system while charging, an alternator can surge to over 100-volts. The surge voltage can wreak havoc on sensitive electrical parts, including stereo systems, ignition systems, engine controls, and light bulbs. This is why anything we do to the electrical system should always be done in the context of having the most reliable battery connection possible. To avoid damaging voltage surges, the battery to alternator connection must never be interrupted while the alternator is supplying charging current or running current!
“Under drive” Pulley Sets
The things commonly called underdrive pulleys actually slow the accessories. All electrical energy comes from the alternator (or whatever charges the battery if other than an alternator). If the alternator is turning too slow (perhaps through under-drive pulleys at slow speeds), the electrical system will run from the battery. The lack of low-speed charging depletes the battery’s charge. When an electrical system operates from battery charge at slower speeds or idle, the alternator loads the accessory belt and crankshaft heavier than normal at higher speeds to replenish the battery’s idle charge loss.
Slowing the alternator below operating charge speed levels at idle actually increases alternator drive belt mechanical load at higher RPM. This is because the alternator has to replenish battery charge lost at idle or low engine RPM. Underdrive pulley systems reduce engine parasitic alternator load at idle and slow speeds, and increase alternator parasitic loading and waste horsepower at racing speeds.