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.
Parallel charging LiPo packs has become very common in the RC hobby. Granted, there is not a lot of empirical evidence about how good or bad this is. Only the fact is that a lot of people do this on a daily basis.
Personally I have been parallel charging 6s LiPo packs for 2 years now with good results. I have some budget packs that are over 100 cycles, so in that regard, I am happy with the life I got out of the pack.
My parallel charging routine was not very stringent. I likely never charged packs with over a 0.25V/cell difference during this time. I think in the future I will be more careful about the voltage of the packs. My recommendation would be to stay under 0.1V/cell difference.
The problem with parallel charging is that it is quiet easy to make a mistake, and connect packs of dissimilar voltages. So if you are going to do this, I would always double check your pack voltages before connecting.
A very good resource on parallel charging can be found on the Tjin Tech site. This is a very thorough examination IMO. If you scroll to the bottom it also addresses the potential surge currents when connecting the packs, and also includes experimental measurement of current that is released at the initial connection.
Best Answer
The TP4056 has a Vinmax of 8V and can be safely operated, if desired, at 6V input.
TP4056 datasheet here
PV panel voltage regulation:
A reason to limit Vin is that the IC will charge at a decreasing maximum current as Vin increases due to thermal dissipation issues. Charging specifications are given at Vcc = 5V (datasheet page 1) so limiting Vin to 5VDC makes sense. Due to the non-critical voltage requirement, a zener diode of suitable wattage could be used (option A below). If tighter Vin control is wanted, a simple shunt regulator will suffice - eg a TL431 driving a shunt transistor (P Channel MOSFET or PNP BJT) (option B below).
Components are sized for regulation at 5V, 2 watt.
simulate this circuit – Schematic created using CircuitLab
TP4056 modules are relatively low cost (your price is OK but they can be substantially lower than your example if you buy 10 from some sites). It is better to use one module per battery although more than one battery can be connected in parallel with a variable degree of success. If connecting more than one battery, ENSURE that they are balanced first - connect them together via say a 10 Ohm resistor if voltages are about equal and maybe charge them individually first and then parallel them.
Paralleling batteries of different mAh capacity is potentially doable but is inadvisable unless there is some good reason to do so. As above, equalise voltages first.
There are two main styles of TP4056 modules available - those with a low voltage load cutout (which you have cited) and those without the cutout circuitry. Those with the cutout are much preferred in simple circuits where the user does not manage the load and battery, as they prevent cell overdischarge and battery damage or destruction.
Battery protection circuitry:
The diagram below shows a TP4056 charger plus a DW01A battery management IC and FS8205A dual MOSFET. The DW01A disables the path to B- when various conditions are not met. The TP4056 already provides charging and overvoltage control, so in normal use the main specification is battery undervoltage protection, preventing a load from discharging the battery below a safe level. On TP4056 modules with Out+ B+ B- and Out- terminals, the two extra ICs are the DW01A and the dual MOSFET.
Very useful TP4056 & DW01A related application note here
PV PANELS:
The PV panels you cited are encapsulated in epoxy resin. This is potentially acceptable for occasional outdoor exposure. If they are to be used outdoors on a semi-permanent basis, you can expect a lifetime of as little as a summer (say 3 months), typically about a year, and in exceptional circumstances, a few years. A far better choice is "PET" encapsulated panels (or, less liable to be found, a fluorocarbon plastic encapsulation). Depending of manufacturing quality, a PET panel may give in excess of 10 years of full time outdoor use.
eg PET PV panel 6V 1.5W $US3.06 free shipping - quality unknown but LOOKS reasonable.
PET (Polyethylene Terephthalate) is the same plastic used in softdrink bottles. It is laminated to the PV material and PCB backing using EVA laminating plastic (as also used in standard glass-fronted PV panels.) PET chemical bonds resist breakdown by UV light, whereas epoxy resin carbon-carbon bonds are susceptible to UV degradation leading to frosting and crazing of the surface over relatively short time periods.
Sunshine hours and related statistics for Esbjerg, Denmark from www.gaisma.com