SOHC.co.uk Forums > CB750
Charging troubles
Waggles:
I too remember the Cibie units back in the day with the strange dished glass, don't remember any charging problems with them. I am on a standard bulb ( and standard 1976 charging system / regulator etc ) I ride 6 miles each way to work with headlight on all the time ( and no I don't kickstart it ! ) and my battery is fine. Would have thought as long as you don't up the wattage too far it should be OK.
Could the regulator be at fault? I would want to test it, a full level 1 diagnostic is in order, make it so number 1
Waggles:
Was just moochin' around the US site and found this charge rate chart which may be of help:
http://www.sohc4.net/index.php/cb750chargingratechart/
I assume the reduction in charge above 3,000 rpm is due to the increased ignition current drain? Overall with headlight on and standard bulb you are OK obviously. If you were to increase bulb wattage from, say 40 to 60watts at 13V that would draw around 0.65 extra amps, again this would be OK ( until 8,000 rpm, not a problem I think! ) once you get up to around 80W with an extra 1.3 amps you would get no meaningful charge from 4,000rpm so in this case you would have to restrict revs to around 3,000 and even then accept a 50% reduction in recharge.
This is a bit 'rough and ready' fag packet calculation, but might help quantify the problem, I was quite surprised how low the charge is at higher revs.
SteveD CB500K0:
--- Quote from: Waggles on October 11, 2012, 10:28:44 AM ---
I assume the reduction in charge above 3,000 rpm is due to the increased ignition current drain?
--- End quote ---
No, it shows the regulator doing its job. >14.5V at the battery would fry it, so it reduces the power output (volts fixed at 14.5 so current falls)
The output of the alternator should (in theory) rise in line with revs. If you add more load, the regulator will simply supply more power until you reach the maximum output of the alternator.
So the question is: what is the maximum alternator output?
If you exceed this with ignition + lights + any other stuff you've got hanging off it, then there'll be nothing left to charge the battery.
BigAl (Alan):
A typical CB750 alternator develops 13 amps at peak power when the field coil is feed by 12.8 volts...if the battery is fully charged and all other component are checked out then the bike should run OK...my K4 with a sealed beam has caused me problems after a 30 mile run at night, lamps becoming dim, engine missfiring.
The issue with the field coil is that it consumes 2 amps until you reach 2/3K revs (not forgetting the other electrics you may have on)...so if you are in traffic revs below 2/3K the battery will flatten very quickly...A new good quality battery is probably the best solution.
ST1100:
Would a modern regulator improve the situation below 3000rpm?
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