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I'm currently using a Scorpion AGM from batterystuff.com. It's now exactly 2 years old & I keep in on a smart charger, hoping it will last longer than the previous one. which lasted exactly 2 yrs & it was 12.76 VDC straight out of the box & I checked it with a multimeter regularly. Since the voltage never dropped below 12.6 VDC, I only charged it once a month or so if I didn't ride enough. That battery never went more than a few weeks without riding the motorcycle or charging. This time I keep it on a smart charger to see if it will do better. I didn't want another walmart battery or a wet cell, and I don't think Yuasa offers an AGM for my motorcycle. I didn't see one anyway.

My motorcycle has a 3-phase charging system & Ninja 500 voltage regulator (credit to loudhvx for the reg-rec).
I use a VC97 digital multimeter & Ctek US 0.8 smart charger.
...

I checked my alternator at 4,000 rpm and it's sending 55 volts on each phase to the solenoid/regulator. This is a bit below the 75 volts stated in the manual. Resistance is 0.7 ohm on each phase, which is a bit higher than the stated 0.48 ohm max. Overall the stator seems OK but the rotor probably has weakened magnets over the decades.

The solenoid checked out fine. Roughly 5 ohm in one direction and 100 ohm in the other.

Just for fun I started the bike on a spare battery and then removed the battery connection. The bike ran fine on just the alternator. Looks like my issue was a worn out battery. I have a new lithium battery on order. The AGM I had previously ordered was not available until Sept so I cancelled it.

The 550 manual has the voltage wrong. 75vac is for the 1-phase model. Your 3-phase stator should have about 55vac.

The most common way the regulator fails on 3-phase kz550's is gradually during a ride. Initial charging may show near 14v on the battery, but during a ride the voltage drops due to a failing regulator. For $7 or $8 its worth it to just throw a used ninja 250/500 reg rec on the kz55 as a test. Just splice the wires.

By the way, dont rev it too much without a battery or big capacitor. The regulator cant regular properly without something to smooth out stator pulses. People have melted headlights etc.

Appreciate the information. I'll make a note in my manual.