Batteries give off two explosive gases - hydrogen and oxygen. Consequently, do not smoke or work with open flames around batteries. Change batteries in well vented areas. Avoid contact with highly corrosive battery acid. Wear acid resistant gloves and safety glasses or goggles. Wash off any battery liquid immediately with large quantities of water or neutralize with a baking soda solution. Remove rings, watches and jewelry. The energy in a bank of golf car batteries can melt a ring if you touch the terminals wrong.
Dirt and moisture on the exterior battery surfaces permits current leakage. It is also harder to inspect dirty batteries for damage. While you can use a commercial battery cleaner, a 1/4 cup of baking soda in 1½ gallons of clean water will work as well. Spray the tops and sides, wiring and battery racks with cleaner. Scrub with a bristle brush soaked in the solution. Let solution stand for at least five minutes for neutralization. Rinse with a low-pressure spray of clean water. If any corrosion remains, repeat the process. Make sure caps are on tight so none of the baking soda gets into the battery since even a few drops can decrease the battery's efficiency and decrease its life.
Next check the state of charge using a hydrometer that measures specific gravity and this chart. A hydrometer gives a better indication than measuring voltage with a voltmeter. Make sure that there is sufficient electrolyte in the battery to cover the plates. If not, add water and operate the battery for several hours to properly distribute the water in the electrolyte so you get a true reading. Measure the specific gravity of the electrolyte in each cell. If the specific gravity varies by more than about 0.025 between cells, replace the battery. Use a hydrometer with a built-in thermometer to correct for variations of specific gravity with temperature.
This test does not tell you about the condition of the battery, just the state of charge. A discharged battery should be recharged immediately. Otherwise, sulfating will occur leading to loss of active plate materials and reduced battery capacity. Lead sulfate, a gray material covering the plates, ruins most batteries before their time.
To determine true battery condition, you need a battery load tester that places high amperage load on the batteries to see if it will handle a load. For the deep cycle batteries used in golf cars, first discharge the batteries. This can be done with a discharge machine, if one is available, or by driving the car until the battery is drained. This is different that for a starting/lighting/ignition (SLI) vehicle battery where the load is placed on a fully charged battery since this is the typical operating situation. If the battery voltage drops off significantly while using the load tester, the battery should be replaced.
If one battery in a bank goes bad, it can be replaced with one of like performance and capacity. If more than one battery is bad, the entire bank should be replaced. Typically, a new battery working with old batteries will not give the same life or capacity as if all the batteries were new. That is because the new battery will be quickly brought down to the performance and capacity of the used batteries.