Topic: Battery Detached Mid Ride

[jaannaktin]

 

I was riding back to work when I hit a big bump. About 2 blocks later the bike just died at a stop sign. Nothing - no lights, no juice. I lifted the seat and saw that the positive battery lead had pulled free from the little o-ring clamp part. Luckily, I needed no tools to push the male free end back into the female part of the ring. Came right back to life. The battery is new and fully charged.

I don't understand the electrical part of this. I thought a bike can run even without the battery, as per a recent thread on "ditching the battery" Does this mean that another part of the electrical system is faulty? I told someone at work, and they said the "alternator" is shot. I don't see that the CB750 technically has an alternator? Is it the generator or ignition coils?

Sorry for being thick, I tried FAQ without this specific scenario 

[dusterdude]

i know on a car,if you keep the rpms up the engine will still run,but at idle it will die.the alternator doesnt push enough amps at idle to produce electricity to fire the plugs.

[Cvillechopper]

Same on a bike.  At idle, there's just not enough juice to keep her firing.  If you had stayed up over about 2k she probably would not have died.  (At least that's my understanding)

James

[SteveD CB500F]

From the charging table:  http://sohc4.us/?q=node/62 

it shows a battery discharge of 6.5 amps at 1000rpm with the lights on (I assume the minus sign is missing...), with 0 amps at 2000 rpm. So if your battery is disconnected, you'll have to run at a minimum of 2000rpm for the regulator to provide enough juice to power the bike.

But then again, I could be wrong.

[bistromath]

Also, the battery works as a really great filter to take the three-phase power from the alternator and suck it up as DC. So without the battery, who knows what the main 12V line looked like, and how that could affect the ignition.

[TwoTired]

I believe both SteveD and Bistromath are correct.  I have always speculated that the rectifier diodes have blown when the battery was taken out of circuit and the voltage/current spikes from the bike's alternator surged over the diode's withstand limits.

Difficult to prove as I only see these things after the fact. And, not during the millisecond it takes for them to blow.  Taking the battery out of circuit by disconnection, or letting a cell go dry in the battery may well be a cause for the rectifier to die.

Take a close look at your rectifier to see if any of the six diodes did not survive the ordeal.
Such current/ voltage surges may also damage an electronic ignition if the bike is so equiped.

As to why the bike died, it was explained it pertainent parts of the "ditching the battery" thread.  You just have to weed out posts that are misleading.  At low RPM, the alternator cannot supply enough power to the bike's ignition, lighting, AND meet the power requirement's of it's own field coil to make the magnetic field needed to create any power output.

[techy5025]

The alternator on the bike is a three phase alternator.  As such, without a battery,  the waveform out
of the rectifier bridge never drops below 86 percent of the peak voltage.  What you
will see if you look at the output on an oscilloscope using an independent source to excite the
alternaor,  is a waveform that goes from a peak to 86 percent of peak in a sinusoidal manner.
The peak voltage is controlled by the amount of voltage applied to the stator of the alternator
 and this is controlled by the regulator. The ripple frequency is a function of the rpm.

Without a battery to smooth out the ripple, there will be large swings in output voltage,
but probably not enough to hurt anything.  Once the rpms drop below a certain point,
the alternator excitation will drop off...the rectified output will follow and the whole
process will collapse to zero volts and the biike will die from lack of coil voltage.

Even if you could spin the engine and thus the alternator....it would not generate any
output because it takes an initial excitation from the battery to get the process started.

....as least this is how I think it works.

Jim

[Quail "Owner of the comfortable k8"]

Very well written.  And correct.

[TwoTired]

Techy5025 is correct as far as he goes.  It's a very nice analysis.  But, consider the regulator is rather like a mechanical relay and responds relatively slowly to rapid changes in system voltage.  While I don't know what the regulator's response time is, suddenly dropping off the battery with its voltage transient stabilizing effects, may not allow it to curtail field voltage in time to limit overvoltage spikes seen by the rectifier diodes.  Removing the battery from a system in operation, might even cause it to break into oscillation at certain RPMs where the load is in approximation with the alternator output.  It seems unlikley the regulator could respond at the frequencies the alternator operates within.  Further, any over voltage would be fed back to the field coil without the battery to quash it before the regulator could respond.   Would the alternator put out even more power and voltage when presented with 150 percent more voltage at the field?  Maybe not if the hystereisis of the magnetic mass is slow.  Who knows what that response time constant is?   Regardless, the 60s and 70's power diodes were not very tolerant to overvoltage spikes unless you paid a lot more money for them.  Allowing for such failure modes as dry cell in battery or suddenly removing it from the system, may not have been part of the system design criteria.  And, the added expense for those diodes may have been avoided to cut costs.

Cheers,

[jaannaktin]

  Brilliant. I have to go and crack open an old physics textbook ... Thanks.