Beginner - gate opener battery discharge issue

[stillhunt]

Hello, I was recently asked to look at an issue with an automatic gate opener.  This system operates a gate to block a private road from public access.  The system is hard wired to 120VAC but this feeds a transformer and internally it uses 24VDC and has a small 6Ah battery bank which will open the gate as a fail-safe during power outages.

The problem is the batteries keep being severely discharged and then do not work when there's a power failure.

2 weeks ago I replaced 1 year old batteries that were fully charged and now they are already discharged.  The two 12V (in series) batteries I replaced had been discharged to 6.8 and 9V respectively!

There have not been any power outages so the batteries shouldn't have even been loaded (other than two power failure tests I did after installing them).

The system is supposed to keep these batteries charged automatically, and while the "charging" LED is lit on the large PCB, suggesting it is charging, I'm only showing 20V DC on the terminal where the battery gets connected (lower left in photo).

I am in the process of replacing some older wiring and corroded connections and blowing out the system, since you can see it's dirty with spider webs and such, but unless its as simple as an issue with voltage leaking due to nasty conditions, I'm not sure what to consider from a component perspective.

As may be obvious, I have a limited knowledge of electronics but am trying to learn and would like to try to figure this out.  Do you have any suspicions of components I might look at first?
 

[Gregg]

First thing I would check is the blue automotive style 15 amp fuse near the bottom of your picture.  Second thing I would do is blow out all of the spider webs sand crap in the enclosure and look for bad connections on the board and especially where there are plugs and sockets.;
Do you measure 20V with the batteries connected, or is it voltage produced by the board without batteries connected?  The board should produce approximately 26.6 volts top float the batteries assuming they are lead acid type.  A couple of automotive incandescent light bulbs in series should be able to be illuminated by the power source on the board without batteries connected.
Is there a possibility that someone could have connected the battery bank reverse polarity?  If so, look for burned protection diodes near the two large electrolytic caps.

[stillhunt]

Do you measure 20V with the batteries connected, or is it voltage produced by the board without batteries connected?

I measured 20V on the board terminal with the battery disconnected.  I expect the voltage on this terminal to be above nominal and slide down under load, but starting low seems problematic.

Is there a possibility that someone could have connected the battery bank reverse polarity?  If so, look for burned protection diodes near the two large electrolytic caps.

Yes, very possible.  Before I was asked to look at it I know several other people tried replacing batteries and such, and they have virtually no electronics knowledge.

I will check these out after finishing a thorough cleaning.  Thank you for the suggestions!

[Gregg]

After some more thought, the 4 amp fuse may be between the battery and charger circuitry for the express purpose of protecting the board from reverse battery polarity.  Upon watching a video from Viking, the presenter stated something about 3 amp charging circuit, so a 4 amp fuse would make sense. 
The 20 volts you measured on the open battery terminals may go away with a light load
Another check is the 24VAC line input with only the board powered and with the gate motor powered.
From my experience with Viking Electronics devices, they are quite robust

[Circlotron]

The board should produce approximately 26.6 volts top float the batteries assuming they are lead acid type.

I would say 27.3VDC. 2 x 13.65V.