Need help with motorcycle charging system

shortbus= said:

But that isn't what he said.

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It's what I've found on the various ones I've been involved with.

Perhaps this simulation will help to explain things:



Here the RMS AC voltage of the magneto output is about 10.5V, although the positive peaks are far from sinusoidal because they get clamped by the battery. We don't know how your meter reacts to such a waveform.
Note that the positive peaks of I(battery) indicate that the battery is getting charged periodically at several Amps. However, a ~constant 3A discharge through Rload (simulating the lights) means that there is an overall average discharge of around 24mA and a consequent drop in battery voltage.
If, as Nigel said in Post #9, there is simply not enough energy provided by the magneto, then taking more from the battery than you put in will inevitably result in the battery going flat, no matter how you tinker with the coil voltage.

alec_t said:

If, as Nigel said in Post #9, there is simply not enough energy provided by the magneto, then taking more from the battery than you put in will inevitably result in the battery going flat, no matter how you tinker with the coil voltage.

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When the bikes are brand new, they 'just about' cope - the headlight is still crap though, as it's just fed directly from a coil, so dims as you slow. There's no regulation at all for the charging, so I suspect it's relatively low to avoid frying the battery.

Like I suggested earlier - disconnect the ground wires and separate the two coils, add bridge rectifiers to them, change the battery for a larger one, and add a proper charging circuit. Obviously rewire the headlight to run from the new larger battery.

Be an idea to add an ammeter as well, to monitor how well it does. Presumably adding bridge rectifiers should roughly double the power available?.

Still say to check the grounds. While not the same bikes over the years of working on old iron(aluminum?) some charging and lighting issues have found to be a bad ground connection. Acheck from battery terminal to the fram , not the bolt holding that ground terminal to the frame, but the frame itself should be done. High resistance will mess with the battery charge.

shortbus= said:

Still say to check the grounds. While not the same bikes over the years of working on old iron(aluminum?) some charging and lighting issues have found to be a bad ground connection. Acheck from battery terminal to the fram , not the bolt holding that ground terminal to the frame, but the frame itself should be done. High resistance will mess with the battery charge.

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Always worth checking.

Nigel Goodwin said:

There's no regulation at all for the charging, so I suspect it's relatively low to avoid frying the battery.

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Very likely. Guy on another thread had the opposite problem: his batteries kept getting cooked.

alec_t said:

Very likely. Guy on another thread had the opposite problem: his batteries kept getting cooked.

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An earlier road bike I had (a Suzuki T350 2-stroke twin) had no problems with the electrics, but it was 12V with the headlights fed off the battery - I've no idea what kind of regulation was done?. Just had a google, and apparently none according to the wiring diagram, just the alternator and a silicon rectifier. However, there's a connection to it from the switch, so perhaps the current is increased when you turn the headlight on?.

The only 'problem' I had was trying to go home from a friends one night, I started the bike and the battery lead fell off - obviously this is all that stabilises the voltage, which increased and blew all the bulbs. Nowhere to get bulbs from at night, so I had to walk home (about six miles), including getting harassed by the Police wanting to know what I was doing walking around at that time of night - I explained, and suggested he might like to give me a lift home - but he declined!.

Off-topic.

Nigel Goodwin said:

including getting harassed by the Police wanting to know what I was doing walking around at that time of night - I explained, and suggested he might like to give me a lift home - but he declined!.

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This is in stark contrast to an experience a friend of mine had about 35 years ago:
After closing time, this friend was walking home alone from the local pub, maybe a couple of miles.
He'd definitely had his fair share, but wasn't legless. Somewhere along his route, the police pulled up beside him and asked where he had been and where he was going.
So he told them he had been down the pub and was on his way home.
They invited him to hop in the back of the car and they would give him a ride.
When they turned off from the route to his house, he asked where they were going.
They told him "To the nick, it's been a slow night lad."
He got charged with D&D and a small fine, they got to add 1 to their quota for the night.
Any wonder why there's a distrust?
/Off-topic.