I’ve been working my way through the various regulator/rectifier threads looking for answers to my own charging system problems, but I quickly realized I didn’t have much of a clue as to how a rectifier works. Incomplete and contradictory explanations led me to look for greater detail and to investigate my own charging system with an oscilloscope.
Here’s what (I think) I’ve found, feel free to critique.
The fig below shows the output from the alternator & reg/rec although in this case the
rectifier Edit:
regulator isn’t doing anything. The rectified sine wave appears as a series of half sine wave pulses, the peak amplitude of which is 15V. Six pulses are produced for every revolution of the crankshaft (RM21 single phase alternator). The battery is at 12V. Current cannot flow to a higher voltage, so when the pulse is below 12V it contributes nothing to powering the electrical loads, the battery is doing all the work. When the pulse is above 12V the situation reverses, the alternator is now providing all the power to the electrical loads and there is a little bit extra to flow through the battery and restore some of its charge. As the pulse voltage reduces the battery takes over again. The pink shaded areas represent energy extracted from the battery and the blue shaded area energy extracted from the alternator. Energy flow switches back and forth between the battery and alternator 12 times for every revolution of the crankshaft. If the pink area is greater than the blue area above the 12V line, the battery is discharging (see note at end).
In the next fig the rectifier has started to have an effect. At 15.5V it shorts the alternator causing an immediate drop in its output, well below 12V. The result is that the battery is having to do a lot more work. It’s not so much the limitation of the peak voltage, it’s the reduction in the duration of the time for which the alternator output is above 12V that hurts.
When I saw the “clipped” wave form on the oscilloscope when my bike was running at <2k rpm I thought it odd that the rectifier would be doing it’s stuff at revs where the alternator might be struggling to keep up with demand. I showed the clipped waveform to the reg/rec manufacturer and they said it was normal and that turning on additional load would prevent the clipping at low revs. That was counter intuitive; the engine is at low revs, the alternator can’t keep up, the battery is discharging so I need to turn on the headlight!? Sure enough, turning on the headlight returns the reg/rec output to full half sine waves.
The explanation for this, I think, is that when the alternator output reaches the point where it exceeds the battery voltage it takes on the entire electrical load. With the headlight on the load is increased, the alternator must provide more current and more current comes at the price of lower voltage. The peak alternator reg/rec output no longer reaches the rectifier threshold so no clipping occurs.
This isn’t the cause of my charging system problems but, hopefully, having a better understanding of how the charging system works might help me pin it down in future. If I’m wrong about this let me know, it could avoid me chasing red herrings.
I’ve only investigated this at <2k rpm, I don’t know to what extent this happens at normal running engine speeds.
Note: I used a sine wave to represent the alternator output because I’ve seen it done that way elsewhere, but this is what the real output from my reg/rec looks like.
A sine wave would never work because the blue area above the 12V line could never equal the pink area and so the battery would always discharge. The steep sided waveform is essential to minimize the time during which the battery is powering the loads.
