SOHC.co.uk Forums > CB750

Charging System Specification

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philward:
I'm currently converting back to electric staeter on the CR750 Replica.
For authenticity (as near as you can expect for a road going replica) I used a race alternator that was narrower than standard in conjunction with a narrower cover. This allowed me to use a nearer to original fairing without cutting a hole in the fairing for the alternator. Down side was that the kit junked the starter ring/clutch (hence no starter motor)
the Electrex World race alternator kit included a voltage regulator/rectifier that I have mounted under the seat with the battery and all other electrics (the inclusion of the replica central alloy oil tank creates space issues). I want to retain this reg/rec (if possible) as I've squeezed it into position and don't want to re-position/make new brackets.
BEAR WITH ME, I'M GETTING THERE!
The Electrex race alternator I'm told (by Andy at 'GoodBits' who sold me the system), that the alternator only kicks in to charge at 3000 rpm. Not an issue while racing.
Question for the electricery experts, is it the reg/rec that controls when the alternator starts charging or the race alternator.
I need to know this so that when I put the original alternator back on, the system charges as per the original Honda spec - as I can't/don't want to always ride the bike constantly above 3000 revs. It hasn't mattered to date as the battery doesn't get taxed as much without the electric starter motor
Pending the answer, I will be able to retain the rec/reg that on the bike or change it to one thats made for standard 750.
Here's the Electrex race alternator spec
https://www.electrexworld.co.uk/acatalog/RPK-1750-Honda-CB750-SOHC-Alternator-System.html
Thanks in advance
PS, I have found a high cranking amp battery that will fit in the current custom battery box under the seat

Oddjob:
The alternator controls when it starts charging Phil, the reg/rec does the converting the AC to DC and the switching on/off of the power coming from the alternator when the voltage gets too high, does this by switching some to an earth IIRC.

philward:
That was my understanding Ken, so is the current regulator switching the charge to the battery starting at 3000 rpm or is the race alternator not producing any charge until 3000 rpm?

Sent from my moto g(50) using Tapatalk

Bryanj:
The answer is both, the generator needs to produce over battery voltage to charge and this depends on speed, number of windings and input to field coil
Reg controls input to field coil, best to ask supplier if it will work on a standard system
A standard system with lights on doesnt charge till over 2500rpm!

Oddjob:
What about without lights on Bryan? Never really got involved with electrics but I worked on quite a few problems back then so maybe I've just forgotten as I got older.

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