A solution is a solar controller about $12 and a 10 watt or larger solar panel(about $30), see below.  I put the panel on the south side of building, angled about 30 degrees down from horizontal. I ran 50 foot wires to a 2pin automotive SAE bullet connector(must be able to disconnect), then to the SOLAR on controller, then from BATTERY to the red and black battery clips.  The controller will maintain either a 6 volt or a 12 volt battery but is most useful on the old 6 volt cutout AC systems(WC, WD, B, CA, etc). Be careful to wire so that correct polarity of red and black is maintained.  It will not overcharge the battery, but it will not charge a dead battery, it is only a maintainer and the battery must be 20% or so to start properly.  Important: connect controller to the battery FIRST, then connect the solar panel, as it must read the battery voltage first to know whether it is 6 or 12 volt!  The red and black wires labeled LOAD are not used here, cut short and tape.  It will not work on 8 volt battery or 24 volt systems, unless one for each 12V battery separately. Use shrink tubing and tape as needed to make wire connections waterproof.  Use not more than 3 or 4 feet of wire from the controller to the battery clips, as the controller must be near and about the same temperature as the battery in order to regulate properly.  If the battery is very low charge to start with, it may need ten or 12 days of good sunshine to bring up to full charge.  

Pics and links above and below...

https://www.amazon.com/Newpowa-Polycrystalline-Efficiency-Module-Marine/dp/B00W80N8TA/ref=sr_1_11?crid=3KQI011BQ0V07&keywords=newpowa+solar+panel&qid=1566426337&s=lawn-garden&sprefix=newpo%2Clawngarden%2C240&sr=1-11#customerReviews" rel="nofollow - https://www.amazon.com/Newpowa-Polycrystalline-Efficiency-Module-Marine/dp/B00W80N8TA/ref=sr_1_11?crid=3KQI011BQ0V07&keywords=newpowa+solar+panel&qid=1566426337&s=lawn-garden&sprefix=newpo%2Clawngarden%2C240&sr=1-11#customerReviews

https://www.amazon.com/Controller-Waterproof-Lead-acid-Charging-Discharging/dp/B00XTQ76WW/ref=sr_1_12?crid=10X1H26IO9S0R&keywords=solar+charge+controller&qid=1566428057&s=lawn-garden&sprefix=solar%2Clawngarden%2C334&sr=1-12" rel="nofollow - https://www.amazon.com/Controller-Waterproof-Lead-acid-Charging-Discharging/dp/B00XTQ76WW/ref=sr_1_12?crid=10X1H26IO9S0R&keywords=solar+charge+controller&qid=1566428057&s=lawn-garden&sprefix=solar%2Clawngarden%2C334&sr=1-12

We used to connect 12v solar panels directly to sealed batteries on remote equipment.  Worked good and a lot less trips out to the equipment.  I'm not sure why you would need the controller you have.

I often thought about putting up my solar panel and run wires around the barn to all of the tractors.

Most , if not all 'solar controllers' have MPP built in. Simply put MPP (Maximum Power Point) is a technique to get the most power from a solar cell into a battery. Getting useful energy from the Sun used to be challenging. It's the 'details' like clouds, temperature, etc. that need to be dealt with MPP does that and more.

Just wiring a '12V solar array' to a 12V battery only kinda works....use  a $10 controller and you get a better charge.

If sending the 12V any distance , use 16GA or lower wire to reduce 'line losses'. A 10% line loss at 12V is 1.2 volts...so you'll never recharge a battery, far,far away.

You need the controller  prevent reverse discharge of the battery at night and to prevent overcharge of the battery and loss of electrolyte and to insure the correct rate of charge and correct float charge of the battery.

You can also use one very large solar panel and wire in parallel to many tractors, but you need one controller for each battery.

I note also that 12V only complete(panel and controller) solar battery maintainers  are available on Amazon but cost is 2X!

Thanks also to CTucker for pictures and for trying this out before I posted!!!

Steve@B&B

I had a 16' tandem axel trailer with battery powered break away brakes.

The system never gave me any trouble and after a few years I wondered about the battery, so I took it in for the winter, and checked it. It was fine.

I decided that I needed a bigger trailer, and I bought a new 20' with the same type axels and brakes, except that it had a system that kept the battery charged.

I soon found that if I didn't disconnect the battery when not in use the electronics would discharged the battery.

Then I had a problem remembering to plug the battery in when I used the trailer.

I assume that was typical of that system, or is there a problem with it?

Other that that it works fine.

Dusty  

re: Dusty  drained battery..

The trailer battery electronics should only be powered when you press the 'test' button. At least that's the way it SHOULD be designed.......

If you put an ammeter between the battery + and the + battery connector, it'll show any drain on the battery. If not zero, then something IS draining the battery.

Jay