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Just got this sealed lead acid battery for the Z1.  It came pre-charged but as soon as I got it, I put it on a trickle charge till my smart charger shut itself off.  Next morning voltage had dropped to 10.5 volts.  I put the charger on it again and it brought it up to 12.9 v.  Tried cranking the starter...  cranked at normal speed at first then after just about 5 seconds the starter started slowing.  I charged it again, the next morning I measured the voltage.  The charger had been off it all night.  Was 12.6 v.  Turned the headlight on, it was bright.  As soon as I hit the starter the light dimmed and again only had a few seconds of cranking power.  The thing I don't understand is when it has 12.6 v of power, why does it have no cranking power? 

Leave your meter on the pos/neg of battery while your cranking. I had a battery at rest was just under 12v it would change but never crank the engine. At rest voltage is different then available cranking amps. Sounds like an RMA is needed form Z1

What he said.

Classic signs of a bad battery.

One other thing. Check to see if you find a date on the battery. It might be new but sitting on the shelf for a while…

FWIW, with these shortages of goods and materials we are experiencing it is something we should all be asking and checking. We might be getting old back of the shelf items… just my $.02

I agree.  I think you have a bad battery.  Looking around at various sources, it looks like a lead-acid battery is considered completely discharged at around 11.6 to 11.9V (measured under no load).  If you're measuring 10.5V after just letting it sit after charging, that's a pretty clear sign that the battery is damaged.  

I agree with what others have said, it does sound like the battery has a problem.  To add more to the topic, the starter motor is by far the largest electrical pull on the entire system.  Keep in mind that the starter motor is looking for a certain amount of watts to function correctly.  Watts = volts x amps and the starter does not care how it gets the amount of watts it wants. So if not enough amps it attempts to pull more volts and if not enough volts it attempts to pull more amps to reach the desired watts.  A fully charged and well functioning battery is required to use the starter motor.  I would contact the seller of the battery and get it replaced with a correctly working one.