>> put out 22V on a sunny day

> It's not really all that bad for charging a lead acid battery

I'm a little thrown off balance there. My understanding was that
the sequence is constant current, constant voltage and then float

I appreciate this panel is not a high-tech charger, but surely there
must be some risk of gassing and/or ruining a battery with excess
voltage ? Although I note your "As long as the battery is sized big
enough so that it can handle the maximum current even when full"

Still seems unwise to me, 22V and a 6V battery
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