Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: the-chauffeur on August 24, 2012, 10:57:14 PM
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Not the first time this topic has come up, I'm sure, but I'm getting a little fed up with the lack of charging power on the K2. To be fair, I've always known the charging circuit wasn't up to much, but I'd not realised just how bad it was until I rode home from work tonight. It's a 17 mile journey and for most of it, I had barely a candle glow from the headlight - and for a large part of the journey the battery didn't have enough power to run the headlight and run at anything over 50mph. Not a pleasant trip.
Some background - my K2 has a modern reg/rect in place of the original parts, which would now be 40 years old. I've also swapped the tail-light/brake light bulb for a LED replacement. The trouble is the draw from the headlight - I switched the ludicrously low powered sealed beam unit for a halogen replacement, but it seems that unless I keep the bike at high revs the whole time the light is on, it drains the battery in minutes. Everything's grounded properly and I've installed relays to take the switching load off the handlebar switches.
So other than change back to a sealed beam unit, what - if anything - are my options? And should I really be suffering this badly with the draw from the headlight? I had no idea the halogen bulbs were quite so power hungry . . .
Thanks all.
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If the generator is in good nick it should cope with a Standard H4 halogen bulb at over 3,500-4,000 rpm.
Have you got a multimeter either analogue or dige and i will give you some tests not in the book mate
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You need to do some basic faultfinding.
Check the battery voltage at the terminals:
- Ignition off
- Ignition on engine off
- Tickover
- 3000 revs
Post results
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:o its the stator it cant make more power than it was designed for.like bryan said they will cope with h4 h/light bulb
you are taking to much power out of the battery before long you will be pushing the bike home, modern regulator and rectifier dont change anything you need to go back to standard lights, make sure all your bulbs are correct ratings (check in handbook or manuel) as these bikes are forty years old and not able to cope with modern electrics ;)
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My F2 is f---king awful at night , i will not ride this bike in the dark anymore , you just cant see a thing, i went out for a ride with mates at night and got left behind cause i felt it was really dangerous to be going the speed they were on there newer bikes, at one point i thought the bulb had gone and it was just on side lights, but when i checked it was on just useless,
If there is a cure i will alter mine as well
Pete
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Not wanting to be awkward but many years back i had a 500 with a standard car H4 and halogen bulb and that charged OK, admittedly i was younger and a raving nutter so the engine was rarely below 5,000 rpm anywhere BUT the 500 has a smaller output and it coped albeit just, and i never had to externally charge the battery
If all the connections are in good order and the parts OK it SHOULD charge over 4-5,000 rpm.
It really needs methodical checking with a voltmeter and if the OP can get one to use i will happily go through tests NOT in the manual to check it all out.
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So I haven't been able to do any running tests yet, although I have established a couple of things:
Battery immediately after charge - ignition off: ~13.2v
Same battery 6 hours later - ignition off: ~12.8v
Same - ignition on (all other circuits off): ~12.1v
Not sure whether that tells us anything, but after re-reading the chapter in Hondaman's book on the charging system, I went back and checked the ground wire loop that connects to the frame under the seat lock bracket. Turns out that there was minimal contact between the metal on the loop and the frame - I hadn't removed any of the powdercoat, so the only real contact point was where the loop touches the the fastener that holds the bracket to the frame.
I've corrected that now . . . and I think the idiot lights glowed brighter when I turned on the ignition. I'm guessing that lack of connectivity won't have helped the effectiveness of the electrical system . . .
More news as I get to run more tests.
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Those voltages look fine. All that proves though is that it is charged!
You need 14-14.5V at 3000(ish) revs to prove the charging system.
Sent from my iPhone using Tapatalk
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Quick update . . . not much to add 'cos I've spent the day pulling the back end off my ST1100.
Take it from me that it's a whole lot of work just to replace an o-ring that seals the alternator (one of the most complex jobs short of pulling the engine). Still, it's given me an excuse to get the swingarm re-powdercoated - it's the bike's 20th birthday this year, and fortunately I waxoyl'd the swingarm as soon as I got it (about 8 years back). At nearly 70,000 miles, there's some minor surface rust that'll clean off with beadblasting, before it's re-powdered and finally re-waxoyl. And then another 5 hours or so putting the whole lot back together, with new bearings and all.
Anyways, the main earth on the 750's fine - the engine has been out a couple of times, which has taken the powdercoat off the main earth join. Battery's held charge at around 12.6v overnight, so it looks like the battery's good despite its depletion when riding with the light on . . . which brings me on to . . .
. . . the advice I'm getting that I'm probably not riding right to keep the battery charged. From what I've read, most folks who mix modern and older bike find the biggest headache is adjusting riding style to cope with braking changes, but for me it's the opposite; I'm uncomfortable keeping engine revs high. Maybe it's something to do with my style of riding or choice of bikes (particularly the ST) but I don't much find the need/opportunity to exceed 4k revs, and generally find that around town requires much less. I know there's nothing wrong with it, but adapting my riding to keep the revs up on the 750 really doesn't feel right (outside of motorway-type riding).
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If you don't want to keep the revs up you will have to go back to the standard Stanley headlamp with puny bulb mate and that needs 3,500 rpm to give the battery a charge as well!
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Quick update . . . not much to add 'cos I've spent the day pulling the back end off my ST1100.
Tell more about it here: http://ww2.my-mc.com/ ;)
but I don't much find the need/opportunity to exceed 4k revs, and generally find that around town requires much less.
I'm facing similar issues when doing too much commuting with my ST(s)...
After 1~2 weeks urban use the starter turns noticeable slower, likely also due the rather short distances, taking more energy out the battery, then charging back in... whilst while on tour the pinion barely engages to get that 4V firing up instantly...
But back to old SOHC generators:
what's the opinion/experience with flywheel magnets getting lame over the decades?
I'd observed such issues with starter motors and alternators of GL1000.
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Its not a permag, its excited by the power fed into the field coil which creats a magnetic field dependant on the voltage/current applied. The wavy groove then makes the field move about over the stator causing generation.
The normal cause of low output is in order of commoness
(1) bad connections causing volt drop/power loss
(2) broken wires in loom/connector blocks
(3) incorrectly set reg box or bad connections at reg box espectialy earth or sense wire
(4)Bad diode in rectifier--More common in aftermarket than original, sometimes brocken wire connector
(5) VERY RARELY a stator winding gone bad--invariably due to accident and frequently easily repairable.
This is why i said i can give you some extra tests to the manual basic ones if they say you are not getting full output.
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Interesting, like the chauffeur I am uncomfortable revving higher than I need to ( I reason that every extra engine rev I don't do now I can do some other time! ) but obviously I don't labour the motor either, I have a standard headlight which is on all the time and round town I ride at 2000 to 3000 rpm and my battery stays charged just fine.
Not sure what I will do when the bulb goes, but surely as long as the wattage is the same as standard I should be OK ........
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Experience using both K2 and F2 in the 70's 80's and both types supported H4 headlights with no problems at all, either Cibie Z180 or Standard fit Honda labelled STANLEY H4 on the front glass.
I know all the H4 55/60 watt halogen bulbs will have equal draw whatever unit they are used in, but from this it was clear that the spec would support that and charge the battery so you are maybe needing some more detailed checks as Bryan says.
The F2 standard unit was easily one of the best by a long way in that era.
When you've run for a while with lights on, check the battery earth lead to see if it's getting warm? if it is you need to improve it in some way.
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OK, If a bike is used regularly and not just for short runs it should stay charged. My original 500 i rode 7 mileseach way to work and back with a halogen headlamp and a regulator that would "stick" occasionally at no charge and that never failed me, but in the trade we did have customers that used their bikes infrequently for short journeys and went through a battery a year!
On batteries I have found that a Yuasa battery is 20-30% dearer than a generic cheapie but lasts a lot longer and holds charge better
ALSO now that they have removed the component that helps the batteries gas (sorry the name escapes me at present) if you let a battery get very discharged it is almost impossible to get them to accept a charge again-----I have had limited success starting with a 24 volt charge for 10 mins but this can be dangerous if not controlled properly
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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
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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/ (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.
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I assume the reduction in charge above 3,000 rpm is due to the increased ignition current drain?
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.
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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.
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Would a modern regulator improve the situation below 3000rpm?
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The answer is No. The regulator is simply switching current ON&OFF to the field coil.
The early CB 750 around KO had a regulator that simply switched ON/OFF the later type (similar construction on the outside) had a mid way position switch
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The later electronic ones tend to charge as a smooth curve instead of just on-half-off but the maximum output does not increase
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Obvious as max current is related/limited by the alternator's capacity, but shouldn't the smooth, thus steadier voltage be a benefit?
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Electronic regulators will switch the field coil ON in a curved pattern as suggested...lets say 2,4,6,8,10,12 amps as apposed to the standard regulator 0-8-13 amps, but it is still simply switching the field coil on or part on or off.
When batteries are aging and starting to loose there efficiency they are not as good at receiving the full charge due to the deterioration of the battery plates/electrolyte and also the bridge rectifier not providing a true DC output