Electronic – safely trickle charge a 24v garage actuator battery with a pair of 12v solar panels

battery-chargingsolar cell

I have a Hormann Promatic Akku Garage door actuator, since my garage doesn't have mains electricity. Generally speaking I'm very impressed with how well it works. Unfortunately while it lasts around two months between charging, it gives almost no notice about when battery charge is getting low.

I would like it to last longer between charges, so a while ago I bought some low power solar chargers. My concern is that there may be something I've missed in the naive way I'm intending to wire them up.

Background

The Hormann battery unit (lower right in the picture) has a pair of XLR sockets, so you can plug the actuator power lead into either one. I presume that Hormanns own (ridiculously expensive and not available in the UK) solar charger plugs into the other socket. Internally, there are a couple of deep discharge 12v batteries wired in series.

Hormann Promatic Akku Garage door actuator, used without permission, but with attribution Attrib. Hormann.

When fully charged, the battery module measures 26.3v and the actuator refuses to work when the voltage drops to 23.3v.

The solar panels I have are just 1w, but are have an integral diode. They were sold as solar 12v battery re-chargers, and the research I did at the time suggested that such a small wattage should count as trickle charging the batteries, so over charging shouldn't be an issue. I note though that open circuit voltage under bright sunlight can reach 18v though.

On one of the panels the diode is blown, so any advice on replacing it would also be useful (I removed the old diode, but it was hot glued to the panel and removing it scratched off some of the ID, I'm guessing it's a 1N4007 though).

How I intend to wire them up

So, naively, I was just going to wire up both panels in series, just as the batteries are wired up, through an XLR plug and leave them connected permanently.

I'm also considering whether it might be better to leave the diode off the second panel and just wire the two panels together with a single diode between them.

My concerns

Assuming a rare British summers day, I know that at just 1w @ 18v means a pretty low current to the battery, but might 36v from a pair of panels damage the actuator circuitry? I'm guessing that being connected to the battery will drop that voltage to just above the battery voltage, but it would be nice to know for sure.

Also, when the actuator is actually drawing current, it is drawing quite a few watts for 20-30 seconds. Could the solar cells be in any way damaged? Looking at the diode datasheet, it looks like each of the diodes would be more than capable of coping with the current and voltages we are talking about.


I've reviewed the relevant solar-cell tagged and battery-charging questions, but non that I can find seem to address my concerns.

Best Answer

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.