I have trouble starting my rider after it sits a while. If I put my
10A charger on it for some 10 minutes, the mower starts fine.

I find no evidence of a drain on the battery.

The battery is relatively new, one year. I replaced earlier battery
with same problem.

The battery shows 10V when it won't start the mower.

The 10A charger causes my meter to show some 15V at the poles when
charging.

If I let it go for a while, then disconnect the charger, my meter
shows 11-12V at the battery poles,

The charger cuts back normally, but I would expect it to start at 10A,
then drop eventually to close to 0. But, it starts out at only 2-3A!
I am wondering if that is a give-away.

Any thoughts?

The old man