Electrical – How to safely charge a 12V deep cycle battery with a wind generator

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.

The "right" way to charge 4 batteries simultaneously would be to buy 4 small "trickle charger" style battery chargers and charge each battery independently. This would allow you to safely leave all of the batteries that aren't in active use to charge at their own ideal rate. This is the approach that your distributor is recommending to you.

It is possible, though far from ideal, to charge several 12V batteries (monoblocks with 6 cells) in parallel. It works much better if they are all the same age and they must be the same brand and same capacity. What you really want is for them to have the same specific gravity in each cell in each monoblock. It sounds like you are using flooded batteries, so checking for proper electrolyte levels often will be very important.

I'd recommend the following guidelines for charging multiple lead acid batteries in parallel:

1) Ensure that the lead lengths between the charger and each cell are close to identical. You would not, for example, want to connect the charger directly to the posts of battery A, then connect battery B to the posts of battery A, etc.) Each length of wire will have a voltage drop associated with it while passing current. You want to ensure that, under charge, the terminal voltages of all batteries (measured at the post) are very close to the same.

2) Add current limiting (like a fuse or fusable link) in each connection (both sides) to each battery. This will provide a basic (not comprehensive!) level of protection against accidental reverse polarity connection).

3) Use a good quality low rate (10-20A) charger

Your copper pipe approach satisfies requirement #1. If you use leads with inline fuses (buy fuse holders from an auto parts store and splice them inline) and ensure that the pipes are mounted in such a way that nothing metallic could reasonably short the two pipes, you've got #2.

As for why your most recent experiment didn't work, it's tough to say without more information. Wheeled automototive battery chargers typically have zero regulation - they are simply a multi-tap transformer (different charge rates) and a rectifier. They are made primarily for jump-starting/battery assist type of applications and aren't a great fit for your application. Research good quality trickle/maintenance chargers, and select one with a 10-20A output (selectable). (#3) You can then more comfortably experiment with the charge rate that best matches your application.

Always wear eye protection when working around batteries.