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. Next time we'll examine the alternator and see how the battery gets its juice.