A battery load tester applies a large momentary load on a battery. Common automotive-type load testers are rated for up to 500 amps. They include a built-in load and an analog-type volt meter. On most testers, the current rating is only for 12 VDC batteries; when used with a 6 V battery, a 500 A load tester will only draw about 125 A. On a 2 V battery, it will only draw about 30 A. Specially made units can be bought for 6 V or 2 V batteries that will draw higher amperages at these lower voltages, but they are more expensive and harder to find.
To load-test a battery, securely attach the two cable clamps of the tester to the battery’s terminals. It is also useful to connect a quality digital voltmeter to the battery being tested as the load-tester’s built-in voltmeter is usually not very accurate. Press the “test” button for approximately three seconds, read the display on the separate digital voltmeter before releasing the button, and record the value. Repeat this test on each of the individual batteries with the load applied for the same amount of time. All of the batteries should indicate similar voltages when a load is applied. Any significant variation would suggest an unequal charging or a damaged battery or cell.
A “capacity test,” which estimates the amount of energy a battery is able to store, is another useful load test. This test involves significantly discharging the battery with a constant load over a five- to eight-hour period. At remote sites, this can be challenging, as the power drawn by normal system loads varies and the battery will need to be recharged afterward. Typically, this test is only done after verifying that the batteries are fully charged and all maintenance procedures are complete.
For systems using flooded lead-acid batteries, ensuring that the electrolyte levels in each cell remain above the battery plates by regularly adding distilled water is critical to the batteries’ health.
A one-time check of the electrolyte levels on an unfamiliar battery bank can be misleading, since you’ll be unable to determine if the plates had previously been exposed, the cell had been overfilled, or if water was just recently added. Regardless, checking and recording each cell’s current electrolyte level will give you a reference point for future inspections.
Allowing the electrolyte level to drop below the plates can substantially reduce a battery’s capacity, since the area of the battery’s plates that was left exposed to air loses the ability to react with the electrolyte. Overfilling a battery and losing sulfuric acid out through the vent cap can result in the electrolyte becoming diluted, which will also reduce the battery’s capacity and shorten its life.
While you’re checking cell electrolyte levels, note the color of the electrolyte in each cell and any residue on the inside of the battery vent cap. Sulfuric acid in a healthy lead-acid battery should be clear and colorless and you should be able to easily see the plates. The inside of the battery vent cap should have only a clear, colorless liquid residue on its surface.
As a battery ages and is charged and discharged, the color of the electrolyte and residue in the vent cap provide clues about what is occurring within the battery. If the electrolyte liquid is dark gray or brown, this typically indicates significant overcharging or excessive temperatures. The dark color results from particles of the lead plate coming off and mixing with the acid. A milky or cloudy light-gray electrolyte typically indicates excessive sulfation of the plates, which occurs when the battery is routinely left in an undercharged condition for long periods of time.