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explanation of charging, alternator rectifiers, the voltage regulator. and no spam.

2K views 2 replies 3 participants last post by  Trillian 
#1 ·
There was a recent post in the General Discussion section that claimed to explain a motorcycle's rectifier but didn't.

Here is a simple (I hope) explanation for everyone confused by that post or just interested in the electrical charging system on our bikes. I am an electrical engineer so it may sound a little techy.

Clarification
Alternator rectifiers and the voltage regulator perform different functions. but on our bikes they live in the same module - the service manual's description of this module makes them sound like one thing.

Location
In an automotive system. the rectifiers (usually six of them) are soldered deep inside the alternator, while the voltage regulator may be removable, hidden under a cover on the back of the alternator.
In our CB500/CBR500 series bikes, the rectifiers for the alternator and the voltage regulator are housed in one module, which is mounted on the bike's chassis, not on/in the bike's alternator.

Function
The rectifiers convert the AC voltage from the alternator to an uneven DC voltage. They do not regulate.
The battery smooths the uneven DC voltage from the rectifiers into the steady DC voltage the bike needs.
The voltage regulator measures the voltage across the battery and adjusts the alternator output current to hold that voltage in a narrow range. Otherwise the alternator voltage would vary with engine speed and changes in the electrical load.

Voltage control
A fully charged lead acid battery will measure around 12.7 to 12.8 volts after sitting overnight at room temperature.
A fully discharged battery = 11.8 to 12.0 volts after sitting overnight.
However when the engine is running above idle speed, the voltage regulator applies around 14 volts to the battery. This voltage allows the battery to charge if it needs it, but does not damage a fully charged battery.

If the battery has been fully discharged, the bike's charging voltage will start off lower than 14 volts. Not sure how much but my guess is if the voltage above idle is less than 13.2 volts after an hour of riding, you have a bad battery (or, much less likely, a charging system problem).

Caution
If you disconnect a battery cable while the engine is running (even for a moment) you may destroy one or more of the alternator rectifier diodes. Same can happen with a loosely connected battery cable that loses contact with the battery terminal.

Magneto? clarifying a point in the original confusing post
There is no magneto in cars/bikes with an alternator. My 1974 motocross bike had a magneto to provide energy to the spark plug coil. There was no battery or electrical system on that bike.
 
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#3 ·
There was a recent post in the General Discussion section that claimed to explain a motorcycle's rectifier but didn't.

Here is a simple (I hope) explanation for everyone confused by that post or just interested in the electrical charging system on our bikes. I am an electrical engineer so it may sound a little techy.

Clarification
Alternator rectifiers and the voltage regulator perform different functions. but on our bikes they live in the same module - the service manual's description of this module makes them sound like one thing.

Location
In an automotive system. the rectifiers (usually six of them) are soldered deep inside the alternator, while the voltage regulator may be removable, hidden under a cover on the back of the alternator.
In our CB500/CBR500 series bikes, the rectifiers for the alternator and the voltage regulator are housed in one module, which is mounted on the bike's chassis, not on/in the bike's alternator.

Function
The rectifiers convert the AC voltage from the alternator to an uneven DC voltage. They do not regulate.
The battery smooths the uneven DC voltage from the rectifiers into the steady DC voltage the bike needs.
The voltage regulator measures the voltage across the battery and adjusts the alternator output current to hold that voltage in a narrow range. Otherwise the alternator voltage would vary with engine speed and changes in the electrical load.

Voltage control
A fully charged lead acid battery will measure around 12.7 to 12.8 volts after sitting overnight at room temperature.
A fully discharged battery = 11.8 to 12.0 volts after sitting overnight.
However when the engine is running above idle speed, the voltage regulator applies around 14 volts to the battery. This voltage allows the battery to charge if it needs it, but does not damage a fully charged battery.

If the battery has been fully discharged, the bike's charging voltage will start off lower than 14 volts. Not sure how much but my guess is if the voltage above idle is less than 13.2 volts after an hour of riding, you have a bad battery (or, much less likely, a charging system problem).

Caution
If you disconnect a battery cable while the engine is running (even for a moment) you may destroy one or more of the alternator rectifier diodes. Same can happen with a loosely connected battery cable that loses contact with the battery terminal.

Magneto? clarifying a point in the original confusing post
There is no magneto in cars/bikes with an alternator. My 1974 motocross bike had a magneto to provide energy to the spark plug coil. There was no battery or electrical system on that bike.
I worked for Bosch for 25years until recently designing automotive electronics. The description here is pretty much right. A couple of minor details to clarify:
  • The rectifier/regulator is mounted on the chassis for cooling the electronics. 90% of the time it doesn’t matter except when there’s lots of load and a low battery to charge in the heat.
  • The disconnection of the battery, known as “load dump” happens when the alternator is generating a ton of energy and then the main load is disconnected. The energy pent up in the alternator has no where to go so it sends the system voltage up until a protection device kicks in or something breaks down and sucks out the energy. All modern automotive electronics have load dump protection otherwise simple things like a loose battery terminal or a fuse blowing would blow your regulator/rectifier up. A full load dump is actually hard to generate because you need no loads except a flat battery taking the charge so you need To some how get the bike started with a very low battery then run it hard and then disconnect the battery. Any other loads like lights, clusters, the engine electronics will sink the energy from the alternator an minimize the disturbance to safe levels.
  • Magneto’s are found in old motors and ones either not having a battery or needing to work without one. Examples are Lawn mowers, Brushcutters, chainsaws and small aircraft engines (eg Cessna’s, Pipers etc.). They have a set of points shorting a primary side of a coil that is next to the flywheel. The flywheel has a magnet on it that , as it’s passes the coil, builds up energy in the coil and then at the right point, the points open and the energy is converted to voltage on the secondary side and creates a spark. They are temperamental and the amount of energy varies with engine speed so starting a cold magneto engine is an art form. Old bikes used them as they didn’t have a battery system.
 
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