Battery keeps discharging!

It's a 1982 KZ750H3. When I got it a year ago, the battery was weak and I eventually bought a new one 2 months ago. Now it has the same problem. When fully charged I can take the bike out and ride to the store (say 1/2 mile away), park it and shop, then come out 20 minutes later and it will barely crank over. Usually, it will eventually start, but I've had to bump start it twice.

I have checked the three output wires from the alternator for proper voltage while running, and that's fine. The voltage regulator seems to be working fine as well. In other words, the charging system itself seems to be working okay. The engine runs fine after getting it started, with a strong headlight, etc. I have tightened the battery terminals without effect. I am now wondering if engine heat is having some effect on the starter (maybe not as after it cools the cranking is still slow).
I've checked all connections (particularly grounds) and find no problems.

So, any ideas?

Verify that the system (battery) voltage is 14V with the engine revving up. If it is less than about 13.6V, you're not getting charge to the battery.

If charging: check the battery. Pull the plug wires and measure the battery while you crank the starter. If the battery drops below about 9.5V, it's done.

I doubt that engine heat would affect the starter, but it could screw up the regulator assembly which is solid state device.

Can you put it on a trickle charger to make sure it's charged up?

You wrote: "Can you put it on a trickle charger to make sure it's charged up?"

I have to charge it up every time I get through riding it. I'll check the idling and cranking voltage tomorrow morning and report back.

It runs with strong engine, headlight and otherlights even at idle. It's like the charging system is working fine, but not quite getting back to the battery (even though the battery is like new). Maybe it's the connector off the regulator?

I always get these weird problems no one else gets, Maybe it's just me!

It sounds like something is wrong with the charging system, or the connections between the charging system and the battery. Make sure that the voltage out of the Reg/Rec is around 14V as stated. If it's below 13.4V it is no good.

I checked the running voltage at the batt today and, sadly, it stays right at 12 volts regardless of rpm. So I suspect it's not charging. Also did the cranking test and got about 9.5 volts.

I also took off the left bottom cover (the sprocket/shifter cover) seaching for an oil leak, so I have good access to the wiring. I did a voltage check for all three combinations of the three yellow wires off the alternator, and got 50 VAC on all at about 4000 rpm (somewhat lower voltage at lesser rpm's), so the alternator itself seems good.

Tommorrow I'll check all connections, but I suspect the regulator. Sound about right?

Yes, if the stator output is 50 volts A.C. in all three combinations, I'd suspect the regulator/rectifier is bad.

All electrical connections clean,no corrosion/insulation problems?

always!!!!!! is the regulator on my previous kawsakis

polkat wrote:

I checked the running voltage at the batt today and, sadly, it stays right at 12 volts regardless of rpm. So I suspect it's not charging. Also did the cranking test and got about 9.5 volts.

The good news is the battery is still usable, the bad news is if you keep discharging it the battery will croak pretty quick.

I vote for the regulator or the stator. That's about all there is.

No grounded shorts, then look at and regulator/rectifier luck Duke

No, I have not yet checked for corrosion or dirty connectors (or loose for that matter). I will do that tomorrow and hope I get lucky. This is the second regulator/rectifier on this bike. Installed it only half a year ago, and it worked fine then. More later.

I just went through this last week on my 900...

Started as normal, drove to cycle gear, shopped for 15 minutes, came out and no starty.... Thank god for kick start, rode home got out the DVM....

Battery was down @ 10.6Vdc, so charge it up over night....

Next day, battery voltage @ 12.6Vdc... Start it, rev it up to 4Krpm, no change still at 12.6Vdc.... Time to get out the manual fro electrical testing....

Disconnect green plug for testing regulator @ 4Krpm, no change still at 12.6Vdc.... Henceforth, rectifier was the problem!!! I should have witnessed the increase in unregulated Vdc with VAC increase RPM swings....

Pull rectifier from KZ1000 (same part # as KZ900) start and run through electrical tests, increases up to 14.4Vdc @ 4Krpm...

All better....

TIP: Rectifiers convert Vac to Vdc and on increased rpm this voltage swing will rise with increased RPM thus input Vac increase to rectifier...

Ex:

@ 20Vac X .7 (half way rectification) = 14Vdc (Low RPM)
@ 30Vac X .7 (half way rectification) = 21Vdc (High RPM)

From this, the regulator keeps the voltage at ~ 14-15Vdc in design....

Maybe will help in your diagnosis....

OMR