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.
So, you have panels wired all in series to give up to 8A at 4V.
The first stage will be a boost converter to give you up to 8/3 amps at 12V. You can buy these, which is generally cheaper and much easier than trying to build one. Note that for 8A it'll probably require a fan, and should itself be kept out of the sun to keep cool.
Let's have a look at some references: http://www.powerstream.com/SLA.htm and http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Key facts to take from there are:
- the voltage must be slightly more than 12V to charge the battery, but not too high
- total charge duration should be 12-16h
That implies that a 12amp-hour battery should not be charged with more than 1A. That presents a problem for the charger design as you have (in ideal conditions) too much power available from the cells. So you want a boost converter that's current limited to 1A on the output, preferably designed for battery charging.
Best Answer
When dealing with battery power and current, you MUST include battery voltage. I'm going to assume that your batteries are 12 volt units.
Furthermore, you need to specify the state of charge (SOC) for your batteries. I'll assume that you will treat your batteries gently, and not discharge them more than 20% max - that is, the batteries will never be discharged to less than 80% of full charge.
This paper suggests that, for a particular set of lead-acid batteries, the incremental charge efficiency above an 80% SOC runs about 50%. Using this as a guideline, the maximum possible effective charge rate for your system at full sunlight will be $$C = \frac{2\times 35}{12} \times 0.5 = 2.9 Ahr/hr$$ However.
PV cells have a roughly constant current over a fairly broad voltage range, and maximum power output is calculated at the point where the voltage starts to drop off. Depending on your charge circuit, this may cause problems. For instance, if the PV develops 35 watts at an output of 18 volts, this implies a current of ~2 amps. If the PV cells are used as a current source for the batteries, then 2 arrays will supply a total of 4 amps peak, and an efficiency of 50% will give an effective charge rate of $$C = 2\times 2 \times 0.5 =2 Ahr/hr$$ rather than 2.9. This is reflected in the assumption in my first equation that all power is converted to battery current (2x35/12) which is not true for a simple charger. If the PV cells feed a DC-DC converter with a variable output this can, in principle, be compensated for, but now you have to factor in the efficiency of the converter, which may well be in the 80% to 90% range.
Assuming a DC-DC converter in the charge circuit with an 85% efficiency,$$C = \frac{2\times 35}{12} \times 0.85 \times 0.5 = 2.47 Ahr/hr$$
You will, of course, have to provide your own estimate of full-sunlight/day equivalent for the PV array. This will have to take into account misalignment of the array due to seasonal shifts in sun elevation (since I assume a 70-watt array is too small to merit a tracking installation), along with estimates of the effect of bad weather.
The uncertainties associated with weather, PV performance, charge efficiencies, etc, mean that you MUST take all of the above numbers with a grain of salt. Quite specifically, numbers like 2.47 are misleadingly precise, and if you use such numbers without constant awareness of just how imprecise they really are (despite the apparent precision of 3 significant figures) you will get yourself in deep trouble.
ETA - Examples of why these equations aren't precise. I've used 12 volts as a power conversion voltage, while actual lead-acid charge voltages usually run 13 to 14 volts. Meanwhile, I don't actually know the operating point of the PV power number, and PV cells actually put out slightly higher current at lower voltages. Depending on device type, PVs do not necessarily have a linear response to different sunlight levels, so calculating effective response during cloudy periods is not simple if you want high precision.