1977 KZ750B Twin. Thought it was Carbs

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The answer is NO, a bad charging system won't affect how the bike runs until the battery discharges.  And when that happens, it's common to drop a cylinder, the headlamp goes really dim, turn signals won't flash, the electric starter won't engage, stuff like that.

Nessism I would never challenge you on a carb issue but I am going to call you out on this one if the readings stated are correct . You are right in that as long as the battery voltage is higher than the regulator output then the meter will read the higher value however, for the voltage reading to drop to 9v and then return to 13v this can only happen due to a significant load being put on the system momentarily, when you operate the starter system voltage will drop to between 9.5v and 10v. (This is one of the reasons why people fit by-pass relays) There is nothing other than the starter than can draw this big a load under normal circumstances ( nothing else has a big enough cable to carry in excess of 80amps) therefore this would suggest that the charging system is at fault (or a battery lead is chaffed through against the chassis) as this is the only circuit that isn't fused apart from the starter. The issue isn't the battery discharging as the voltage wouldn't then bounce back to normal.
I would suggest rechecking your readings as if what you have posted was correct then you are dealing with a potential fire risk (IMHO)also carefully inspect the yellow stator wires and red lead from the RR to the battery for any sign of damage or heat

Thanks for this. I also might have pinched wires under the tank. thats happened before.
So you are suggesting the lithium battery is defective (its a battery tender early version and they were known to explode, but mine never did after 2 years) hence the fire risk?

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wrote:

The answer is NO, a bad charging system won't affect how the bike runs until the battery discharges.  And when that happens, it's common to drop a cylinder, the headlamp goes really dim, turn signals won't flash, the electric starter won't engage, stuff like that.

Nessism I would never challenge you on a carb issue but I am going to call you out on this one if the readings stated are correct . You are right in that as long as the battery voltage is higher than the regulator output then the meter will read the higher value however, for the voltage reading to drop to 9v and then return to 13v this can only happen due to a significant load being put on the system momentarily, when you operate the starter system voltage will drop to between 9.5v and 10v. (This is one of the reasons why people fit by-pass relays) There is nothing other than the starter than can draw this big a load under normal circumstances ( nothing else has a big enough cable to carry in excess of 80amps) therefore this would suggest that the charging system is at fault (or a battery lead is chaffed through against the chassis) as this is the only circuit that isn't fused apart from the starter. The issue isn't the battery discharging as the voltage wouldn't then bounce back to normal.
I would suggest rechecking your readings as if what you have posted was correct then you are dealing with a potential fire risk (IMHO)also carefully inspect the yellow stator wires and red lead from the RR to the battery for any sign of damage or heat

Something is very wonky with the voltage readings mentioned earlier.  I agree with you about there being almost no way the bike is seeing 9 VDC unless something is shorted or the starter motor is engaged.  I suppose that lithium battery/R/R setup could be really messing up.  I'd never run that sort of battery on a motorcycle since the generator isn't designed for it.  

A test could be run to see what happens.  First disconnect the charging system from the bike's harness.  Then fully charge the battery.  Not ride the bike.  It should run normally for some number of miles this way until the battery power all get consumed.

This isnt a battery drain issue. The battery has NEVER gone dead and floats at 13.5V (lithium). I think something under the tank may have gotten pinched, as it has before, and may be causing a short. Regardless, Im getting rid of the Battery Tender brand lithium battery. Though I have had mine for a few years without issue, Im no longer comfortable with it.

Im hoping this isnt a stator issue. Still no one has said what would make the bike jerk so violently, though I have asked a bunch of times. That would be helpful
 

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This isnt a battery drain issue. The battery has NEVER gone dead and floats at 13.5V (lithium). I think something under the tank may have gotten pinched, as it has before, and may be causing a short. Regardless, Im getting rid of the Battery Tender brand lithium battery. Though I have had mine for a few years without issue, Im no longer comfortable with it.

Im hoping this isnt a stator issue. Still no one has said what would make the bike jerk so violently, though I have asked a bunch of times. That would be helpful

 

If you have a short as I suspect then electricity is fundamentally lazy so it takes the easiest path to ground which would take power away from the ignition and coils - this would give the same symptoms as turning the kill switch on and off which would indeed make it jerk

I can’t see a 16/18 gauge wire under the tank being capable of carrying the amps needed to dead short a battery and drop voltage to 9v on a good battery. It has to be battery or starter cable. 

Have you preformed any of the test procedure mentioned here. Tank cap open or charging system disconnect with a fresh charged battery.

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 Still no one has said what would make the bike jerk so violently

 

I don't think we can answer that question. Voltage too high probably would not directly impact how the engine runs. A big drop in voltage would feel like you were toggling the run/stop switch on and off. I wouldn't think this would cause the bike to buck.  It seems like it would just fall flat and then come back on, but I can't say for certain.

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I can’t see a 16/18 gauge wire under the tank being capable of carrying the amps needed to dead short a battery and drop voltage to 9v on a good battery.

I agree.  I'm not really in favor of the "voltage is dropping due to a short circuit" theory. If he's measuring the voltage directly at the battery terminals of a lithium battery, it would take a LOT of current to drop the voltage to 9V. Something somewhere would certainly be melted and probably would be destroyed.

My first thought is measurement error. Maybe the terminal wasn't clean? Maybe the meter probes weren't securely fastened? Maybe you weren't measuring directly at the battery terminals?

If it's real, as others have pointed out, a healthy battery really can't quickly drop from 14.77 to 9V, no matter what the charging system is doing.

Here is something that I've often wondered about. The newer motorcycle specific lithium batteries claim to have built in protection. The protection is intended to prevent over charging and over discharging. I think the way it works is to include an internal switch to disconnect the cells from the terminals. So if the protection were to kick in, the battery would basically disconnect itself from the motorcycle. What happens then? Are lithium-specific regulators design to run without a battery? I'm pretty sure most traditional regulators would work correctly without a battery to smooth and damp the output.  I don't know.

Anyway, I would measure it again, very carefully, and if you still see quick drops and surges in the voltage I would try a new battery.

wrote:

wrote:

 Still no one has said what would make the bike jerk so violently


 

I don't think we can answer that question. Voltage too high probably would not directly impact how the engine runs. A big drop in voltage would feel like you were toggling the run/stop switch on and off. I wouldn't think this would cause the bike to buck.  It seems like it would just fall flat and then come back on, but I can't say for certain.

wrote:

I can’t see a 16/18 gauge wire under the tank being capable of carrying the amps needed to dead short a battery and drop voltage to 9v on a good battery.

I agree.  I'm not really in favor of the "voltage is dropping due to a short circuit" theory. If he's measuring the voltage directly at the battery terminals of a lithium battery, it would take a LOT of current to drop the voltage to 9V. Something somewhere would certainly be melted and probably would be destroyed.

My first thought is measurement error. Maybe the terminal wasn't clean? Maybe the meter probes weren't securely fastened? Maybe you weren't measuring directly at the battery terminals?

If it's real, as others have pointed out, a healthy battery really can't quickly drop from 14.77 to 9V, no matter what the charging system is doing.

Here is something that I've often wondered about. The newer motorcycle specific lithium batteries claim to have built in protection. The protection is intended to prevent over charging and over discharging. I think the way it works is to include an internal switch to disconnect the cells from the terminals. So if the protection were to kick in, the battery would basically disconnect itself from the motorcycle. What happens then? Are lithium-specific regulators design to run without a battery? I'm pretty sure most traditional regulators would work correctly without a battery to smooth and damp the output.  I don't know.

Anyway, I would measure it again, very carefully, and if you still see quick drops and surges in the voltage I would try a new battery.

It is possible that as a lithium battery is made up of multiple cells that an internal fault could cause voltage drop off, the MOSFET regulators don't have an ignition live so if the battery cut itself through internal protection then the regulator would also shut down. Given the engine braking of a twin wouldn't cycling the kill switch cause significant jerking ?

Had a look at the tank cap vent and disassembled it. Its nice and clean, no blockage. Will move to next steps..

Turns out the battery was dying and the rec/reg too. Swapping them both out...

Glad you got there in the end, I guess the voltage variations where due to the construction of a failing lithium battery  ​​​​​​​

yep. whats a good replacement agm or lead battery for a 77 KZ750B?