Electronic – Direct Connect Solar Panels to 12v Chargers (for misc electronics)

but might 36v from a pair of panels damage the actuator circuitry?

So here's the deal. Lead-acid batteries look electrically like a voltage source/sink with a small series resistance, with the voltage level a function of state of charge. 2V/cell (there are 6 cells in series in a 12V battery) is nominal, and if I remember right, their open circuit voltage is something like 1.9V empty, 2.1V full. That covers 90% of their behavior.

Considering that, the "1W@18V" spec of the solar panel isn't going to be able to "win" against the battery, and the solar panel's voltage will be pulled down to battery voltage, delivering probably 0.055A (=1W/18V) at whatever the battery voltage is.

When a battery gets completely full, however, its series resistance goes up dramatically, and the voltage goes up, until there's enough voltage to start electrolysis of the fluid and you get H2 and O2 generation at the terminals and loss of the electrolyte. A lead-acid battery, depending on the type + manufacturer, has a certain recombination rate of H2 + O2 => electrolyte that it can handle; if you electrolyze at a higher current than that, it leads to permanent electrolyte loss (+hence capacity loss)

So there is a safe current that can be delivered to a lead-acid battery continuously, where its own self discharge due to electrolysis balances the charging current. It depends on the manufacture + construction. I wouldn't feel worried about a C/10 or C/20 rate of charge (where C = the current needed to discharge a battery in 1 hour). Garage door batteries are probably > 1Ah capacity so you should be safe with 55mA charging current.

HOWEVER -- I would probably put a (zener diode and resistor in series) in parallel with each battery, the zener diode being about 14V and resistor being maybe 10 ohms or so, so that it keeps the battery terminals from getting charged too far.

Also: if you can, wire each solar panel to each battery (and keep the diodes), rather than the pair of panels in series wired to the batteries in series -- i.e. try to connect the center taps. By doing so, you'll charge each battery independently. Otherwise, what can ruin battery life is if the battery voltages diverge -- the one with the higher voltage will tend to get overcharged, while the other one will tend to get overdischarged and not completely charged.

Such a control circuit is possible, but a better idea is to leave the panels fixed in one configuration and deal with the resulting change in voltage with the right kind of switching power supply. If you want to get fancy, you could even implement maximum power point tracking, or something reasonably close.

The switching power supply can't make more power than what the panels produce, but it can convert (with a little loss) from whatever voltage and current the panels want to produce to a different controlled voltage or current (with the other limited by the available power).

If the series configuration always produces more voltage than you need even in low light, then that's how you should configure the panels. The switching power supply then always makes a lower voltage, which keeps it simple. That is called a "buck" regulator, with much information about that out there.

It would be helpful to say what voltage you ultimately want, and what current range is useful at that voltage.

Added:

You now say this is to power a 12V system, which presumably runs from lead-acid batteries although you didn't say that.

One useful feature of lead acid batteries is that they can take reasonable charge current even when full and still regulate the voltage well enough for most purposes. Given that, a really simple solution is to wire the solar panels in series to get the higher voltage you talked about, which is always a bit more than the 12V battery level even on cloudy days (when light is really low the voltage will be lower, but then there is so little power to be irrelevant), and just connect this to the 12V rail with a Schottky diode. That will not use the panels most efficiently in high illumination, but probably not so bad on cloudy days from the numbers you give.

A buck converter that runs the panels at the best efficiency for the given insolation and then dumps whatever current it can onto the 12V rail should be a bit more efficient. With a decent converter design, the extra loss in the switcher should be more than offset by running the panels at their optimum efficiency.

However, if you have more than enough power in full sunlight and the real problem is when it's cloudy, maybe the dumb series connection (with a Schottky diode to prevent reverse current when dark) will do it. I'd probably be tempted to try that first and see what you get and how efficient the whole system is on cloudy days when it really matters.