Electric jack on my trailer acts like its getting no juice whatsoever.  I know the batteries are getting weak, but the trailer is plugged in, and even plugged in the jack doesn't engage at all.  Usually when the batteries are low it at least makes an effort.  Yesterday, got nothing.  Does this mean my batteries are dead and no juice is getting through them to the jack?  Looks like the jack is wired directly to the batteries. 

(quote)....  I know the batteries are getting weak, but the trailer is plugged in,

What's the trailer plugged to, 110 volt shore power? Are your trailer batteries suppose to be charging when the trailer is plugged in? Do you have wet batteries? Check the electrolite level lately? Is there anything else that operates off the batteries that doesn't work? When was the last time you operated the jack? Is it total electric or a electric over hydraulic jack? Have you noticed that the jack has been getting slower each time you use it? etc. etc....

Plugged into 110v shore power.

Hubby added water to the RV batteries, I think he said they were nearly dry so we were skeptical they would hold a charge anyway. 

Shouldn't the RV batteries be charging when the trailer is plugged into shore power? 

Jack was last used in Nov/Dec 2008, and no, it has not progressively been getting slower.  I can tell a significant difference in jack speed when the trailer batteries are low vs when the trailer is plugged into shore power. 

I believe its hydraulic/electric but the jack was installed on the trailer when I bought it and I honestly don't know what it is.  Its faster and louder than friends' electric-only jacks, and has some what looks like hydraulic fluid around the openings which makes me think its hydraulic/electric.

I suspect we have overcharged the batteries by leaving the trailer plugged in the entire time its not in use but not disconnecting the batteries.  

Batteries are in fully-enclosed battery boxes protected under the gooseneck.  Connections are pristine. 

TEST your batteries first.....

The way we test the batteries state of charge is with a battery hydrometer. A hydrometer measures the specific gravity of the acid solution. The higher the specific gravity, the more charge in the battery. A fully charged battery will have a specific gravity of 12.75. A weak battery will have a specific gravity of 12.50 or 12.25. A discharged battery will have a specific gravity of 12.00 or less. When you check the battery, you need to check each cell. All six cells must have the same specific gravity. If five cells test at 12.75 and one tests at 12.25, you have may have a bad cell. Charge the battery for about 30 minutes and test again. If the low cell does not come up, it's bad and you need a new battery. Some variance is allowed between cells but if it is a large variance, you may have battery problems.

The individual cells can also be tested with a voltmeter. Take a coat hanger and make two lead extensions about six inches long and attach them to the meters test leads. Touch the positive lead to the positive terminal and stick the negative lead inside the cell next to it. It should read about 2.1 to 2.3 volts. Now insert the positive lead in the first cell and the negative lead in the second cell. Proceed down the line until you get to the last cell. Here you will put the positive lead in the last cell and the negative lead on the negative terminal. All the cells should read the same, or within 0.2 volts. If one reads 4.0 or more, you have a shorted cell and the battery is no good. If you get a very low reading or a zero reading, the cell is open and again the battery is no good.

Before you do any battery testing, you need to start with a fully charged battery. If it is not fully charged, then any test results you get mean nothing. So always check the specific gravity before you do anything. Also make sure the terminals are clean and tight.

Now some batteries are sealed so you can't do a cell test or check the specific gravity. In this case all you can do is charge the battery for about 30 minutes and do the load test. In sealed batteries you will usually see the "green eye." What this is, is a built in hydrometer. Don't trust it. I have seen hundreds of bad batteries with green eyes telling me they are good.

A little bit about working with batteries. Whenever you disconnect a battery, ALWAYS disconnect the negative cable first. This will prevent sparks that may cause the gasses inside the battery from igniting. Wear safety glasses, batteries contain sulfuric acid and it can splash if you are not careful. If it comes in contact with your skin, wash it off with plenty of water. Take off any rings or jewelry when you work on a battery. I worked with a technician who was taking a battery out of a car and the wrench he was using slipped and touched both terminals. The battery shorted and his gold wedding band was, literally, welded to his finger. He was fortunate that the doctor at the emergency room was able to remove the ring and save his finger.

As with anything, a little common sense and care when working with a battery will save you a lot of grief and aggravation later on.

Your electric jack takes a large amount of current to run, the reason it is wired directly to the batteries. Your converter/charger doesn't put out enough amperage to run the jack on its own. If your batteries are weak or dead, it's unlikely you will be able to use your jack when using your shore cord.

If the batteries were over charged and boiled dry, they're shot now. Head to an auto parts store with them for replacements. The store can check them for serviceability, and when they fail, you can turn them in for your core charges.

Gard

What you are looking for is a deep cycle marine or RV battery, with the largest amount of cold cranking amps that are available in your group size. (1100 + CCAs) These batteries usually have a dual type of post design, that feature both a standard automotive side terminal, and a top post marine/RV type that utilizes a bolt and wing nut.

The newer "Consumer Reports" touts a brand called "Ever Start Maxx" as scoring very well in their testing. For several years I've been using a brand sold by Advanced Auto with excellent results.

Purchase a battery, that has a many year free replacement guarantee. The better ones will range anywhere from three to five years before they become prorated. Then they will have a minimal partial value up to seven or eight years.

Gard

Learned something today.  Even plugged into the truck, the jack didn't work.  Hubby is off to Sears to go buy new batteries.  Hopefully that is the problem. 

I'll take better care of these, did not realize totally dead batteries would leave me dead in the water without a jack.