Yam XJ900 with sick electrics

[Oz_gaz]

My reliable 1984 Yamaha XJ900 is so rarely used that the battery is usually flat and I resort to clutch starting it (never with any difficulty).

For the first time ever, I recently charged the battery whilst it was in situ (fully connected), and probably for too long (15hrs or so).

I then did nothing further for a day before seeing if it would start. Sadly, when the ignition is turned on, there is an electronic sort of a whine from somewhere below the petrol tank, no ignition lights, and the tacho indicates 2000rpm without the motor going!

Can someone advise if charging in situ is OK, and if not, what have I done?

Any clues appreciated.

[rainman]

Just a guess, mind, as I don't know your bike model, but I'd bet you have comprehensively cooked your electrickery. Zapping your bike electrics and electronics with a charger for 15 hours ain't a good choice... If it's any consolation I'd guess your battery will be OK, though.

I guess you now have to a) find a good bike electrics expert, or b) diagnose the fault yourself. Do you have knowledge of bike electrics and a detailed manual?

[Warr]

I have lots on info in the XJ900, used to have one.
Your troubles dont sound too good.
Are you sure the charger was connected the right way round ? ..
You need to get a multi meter onto the battery first up to see what it tell you

[Oz_gaz]

A belated follow up........ I did nothing for quite some time, but then did check with a local bike shop who said it was OK to charge the battery without disconnecting it from the bike.

I did eventually get round to checking that the battery was in fact, hardly managing even a tiny spark when shorted. Tried recharging again without any difference.

In due course, I bought a new "gel" battery on ebay, and voila! She started up without any problem.

The tacho indicating 2000rpm without the motor going was a red herring I guess, but I've never seen anything like it in over 30 years of rinding.

[davereid]

Just a guess, mind, as I don't know your bike model, but I'd bet you have comprehensively cooked your electrickery. Zapping your bike electrics and electronics with a charger for 15 hours ain't a good choice...

Good to hear you got it sortd so cheaply, but rainman is correct, and your bike shop is wrong.

You should always remove the earth lead before charging a battery on a motorcycle, unless using a very current limited charger like a battery tenda.

Most motorcycles use a simple shunt regulator.

As the alternator spins, its output voltage goes up with rpm. A shunt regulator acts like a variable load, increasing the alternators load, thus pulling the voltage down to a safe value.

For reasons known only to themselves, but likely related to avoiding extra high current wiring, most manufacturers wire shunt regulators on bikes so they are always connected, even when the key is off.

So when you try to charge a battery, the shunt regulator will attemt to regulate your charger.

3 Outcomes...

1. The regulator will win without damaging the charger. The battery will charge, just more slowly.
2. The regulator will win, and fry the charger.
3. The charger will win, destroying the regulator. And possibly anything else unswitched by the ignition key.
3.

[Oz_gaz]

Thanks for the meaningful explanation and indicated three outcomes for clarification.

For what it's worth, the charger I use is an old car charger that had a fast/slow switch that has effectively been disabled, making it a trickle charger.

On reading your explanation, I'll never charge it again without disconnecting (though the ZXJ900 is a challenge to my dexterity disconnect, or more particularly, to strap the battery back in place after charging).

[Warr]

The XJ900 has a proper alternator.
Proper in that the regulation is achieved by reducing the voltage to the stator NOT by shunting the field winding.

At least on this model Yamaha were prepared to make a controllable alternator.

[davereid]

The XJ900 has a proper alternator.
Proper in that the regulation is achieved by reducing the voltage to the stator NOT by shunting the field winding.

At least on this model Yamaha were prepared to make a controllable alternator.

You might have that upside down methinks..

A shunt regulator is connected to the Stator windings, as most motorcycles don't have field windings.

If the bike has field windings, these replace the fixed magnets - and alternator control is achieved by controlling field current.

[Warr]

You might have that upside down methinks..

A shunt regulator is connected to the Stator windings, as most motorcycles don't have field windings.

If the bike has field windings, these replace the fixed magnets - and alternator control is achieved by controlling field current.

Yes YOU are correct my bad !!

The XJ900 does not have permanent magnets for the Field it has a field coil and I recall no slip rings. Cunning plan by Yamaha. Cant find a diagram for it tho