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Hey everyone, I bought a new stator and a new R/R for my 81 kz550 LTD, but the battery still will not charge. I re-did all the wiring. I will provide the wiring diagram I followed. I haven’t done a proper test with a multimeter yet. Idk if it’s a grounding issue or how to check for some kind of energy drain, because when I put the bike on a charger it will read low right when I turn on the bike; even if the battery is fully charged before turning it on. Any advice will help. Thanks

First thing is to make sure the battery is good & have it load tested. I would charge the battery
with a good motorcycle smart charger then check it with a digital multimeter & do a load test.
I use a Ctek US 0.8 smart charger & VC97 digital multimeter.

Sadly, a brand new battery can be bad. It seems more common these days. Even if it shows proper voltage at rest, it can have a bad cell that will show up as very low voltage under load. Many auto parts stores and every battery store can load test it, usually for free.
If I understand your post correctly, the battery is showing proper voltage after being charged, but as soon as you turn the ignition switch, it drops noticeably. How low does it go? What does your meter say when you push the starter button?

Kind of related. Five years ago I bought a Scorpion AGM as they had been purported to be long lasting. Mine
went bad after exactly 2 years. It always tested at least 12.6VDC, but I only used a charger once a month or so.
Then I got another scorpion agm, but this time I keep it on a smart charger >> it's now 3+ yrs old. knock on wood.

You can start by using a voltmeter to check your stator. If it's OK then check your regulator.

Charging issues are very rare on the Kz550. What regulator and stator did you buy?
Is the ignition stock?

Charging system grounding should happen through actual wiring, not the frame and engine, so make sure the wiring is in tact.

grab your meter

meter on DC V
ignition on
engine off
red meter probe on positive battery post(the post being the metal part of the battery)
black meter probe on the negative battery post
note voltage

fire the bike up
note voltage

raise rpm up to 2000
note voltage

raise rpm to 4000
note voltage

turn off bike

I haven’t done a load test yet. It was too cold last night so the bike wouldn’t stay running. (Carbs need rejetting) but with the battery fully charged with the bike off it reads 12.5v. I turn the bike on it drops to 11.5. I still need to test the stator and r/r. I need to double check my research on how to do that, unless one of you is kind enough to take the time to give me a step-by-step. I have the battery grounded to the engine and the frame. The wiring checks out, I’ve cross referenced a few wiring diagrams. I beginning to assume the battery has gone bad due to it being charged so many times. (I just recently bought a new stator.)

It really could be anything.
Several months ago someone had charging issues as well and it was a disconnected ground wire off of the regulator/rectifier.
The pull down on the battery when you switch on is a bit much, but it shouldn't prevent your bike from trying to charge the battery.
When you have a chance do the rpm test and report back.

With a digital multimeter a fully charged battery should be at least 12.6 VDC, but a good battery can be higher.
A charger can give the battery a surface charge, but under load, it may not be up to snuff.

High beam load test (engine off). After charging the battery, it should be 12.6 VDC.
Turn ignition switch to ON (engine off) & headlight high beam ON >> the voltage should drop to about 12.2 VDC.
If it drops below that, the battery may need replacing. If it drops much below 11.85 VDC, the battery is probably shot.
The motorcycle needs a good, fully charged battery to start & run right.

You can follow the links in my signature to download several manuals.

Some manuals have the test results backwards.
A 1-phase system (2 yellow wires) should have near 75Vac in the test at 4000 rpm.
A 3-phase system (3 yellow wires) should have near 55Vac in the test at 4000 rpm.

Some manuals have the numbers flipped.

I agree with Martin, A good battery will be no less than 12.6 at rest. You can do a backyard mechanic test on your battery. Like mentioned charge the battery, Turn on the ignition, stop run switch off, and headlight high beam on. All has been mentioned previously. The test comes in by using a meter with red on battery positive and black on Negative, meter set to DC, watch the voltage drop. Turn everything off when battery voltage drops to 12.0 volts. Remove meter and let the battery rest for about 10 minutes. It should recover by itself to 12.6 volts minimum on its own. If it wont recover to 12.6 then the battery is likely bad.