Small Solar Maintenance Charger Advice Pretty Please [emoji120]

[the evil twin]

A shunt regulator is another option in this type of application. It simply clamps the voltage at the set level thus preventing to going above that limit. It would be placed at the panel, before the blocking diode so should be set to about 13.9V (the ideal 12.9V plus the 0.6V drop across the diode). Probably not readily available to buy off the shelf, but anyone into electronics could knock one up for the cost of a beer or two.

Good point and agree shunt regs will a) work and b) clamp the voltage but in a practical application like the one discussed the panel output voltage should never rise to 13.9V.

At more than 100 to 1 the battery internal resistance should ensure a current flow such that the panel never gets anywhere near open circuit voltage.
If it did either the panel rating is too high or the battery AH capacity too low for a non-regulated use.

Could maybe consider voltage clamping so that it won't matter what size battery is used (IE works even with smaller capacity) to make it a bit more versatile tho so good idea for that case.

Personally I like to keep my battery maintainer panel setups as simple as possible so no reg of any description is necessary but that may not suit everyone of course.
In the garden I just hook the panel to a 2nd hand battery from work and walk away and they work for years.
For my boat I just have a small fly lead from the panel that plugs into an aux power socket on the boat IE same principle as the dash mat style car panels that sit on the dash and plug into the ciggy lighter.
After a day out on the water (sigh... a rare happening these days) I wash the boat down and as the batteries are fully charged or close to it anyway I just plug in the fly lead and check that it is holding 12.8 every week or so.