Part of a complete battery bank assessment includes testing the battery cells while they are delivering high power. We call this a “load test” (see “Battery Assessment” in HP159). This performance test does not require an expensive, dedicated load tester, nor any hazardous battery clamps. A heavy discharge load can be applied to the entire battery bank by simply plugging in and turning on high-draw appliances, especially electric heaters, hair dryers, hot plates, incandescent lights, etc. (Borrow these items from your on-grid friends if you must.) With a heavy and steady load applied, you can measure individual cell or battery voltages. Unequal voltages will indicate weak cells. You can also locate weak connections this way.
What is a “heavy load”? That is relative to your battery bank capacity. There is no exact requirement, but a good guideline is to divide your battery’s amp-hour capacity by 5, which determines DC amps of load to apply at the battery bank. Confused? Here’s another way to calculate it: Multiply your battery’s Ah capacity by the DC voltage (nominal voltage: simply 12, 24, or 48), to get the capacity in watt-hours. Divide THAT by 5 to get an idea of a “heavy load” (in watts).
For example, say you have 350 Ah batteries in a 48 V string. Your Wh capacity is 16,800 Wh (350 Ah × 48 V). That number divided by 5 equals 3,360 watts. That’s just a guideline, however. Two ordinary 1,500 W electric space heaters would be a good “heavy” load. If you have more than one parallel battery string, remember to multiply all this by the number of parallel strings.
Although the exact load on the battery bank isn’t critical, use at least half of the load wattage that you calculated. Most important is that the load stays constant for the duration of your test. So, during the test, unplug or disable any loads that may turn on automatically, like a refrigerator or well pump, and be sure nobody uses the toaster! Next, disconnect all charging sources, like your PV array. If you have a system monitor that shows amps (the battery current), it will indicate the heavy load, which should stay nearly constant.
Next, use your digital multimeter to measure the voltage of every cell (if you can) or every individual battery. Place the meter probes directly on the battery posts to read the cell (or battery) voltage exactly. As you read the voltages, have a friend record them on a chart to speed the process.
Any cells or batteries that show a lower voltage indicate a weak cell or battery. A difference of 0.1 volt would be significant for a 2 V cell. If you cannot access individual cells, then you will measure individual batteries, multiplying 0.1 by the number of cells. So, for a 6 V battery (consisting of three 2 V cells), a difference of 0.3 V would be noteworthy.
Significant variations may indicate the need for thorough equalization of the bank, or may indicate cell weakness and a threat of premature failure. If the battery bank has more than one parallel string, this test can also reveal inequality between strings, which will lead to premature failure of an entire string. This is a common problem with batteries in parallel strings.
With your heavy load applied, you can now use your meter to locate weak connections. Place one probe on the terminal and the other on the cable lug. Any voltage higher than 0 V will indicate voltage drop across the metal junction due to corrosion (often invisible) or a loose connection. The terminal needs to be disassembled and its surfaces cleaned, and corrosion-protective compound applied before reassembly.
For advice on parallel strings, equalizing, and corrosion prevention, refer to “Top Ten Battery Blunders” (HP114).
I have used this testing method many times to locate weak cells and connections, or merely to assure that all is well. Correcting problems while they are “young” is the only way to prevent premature failure of a battery bank.
—Windy Dankoff • founder (retired) Dankoff Solar Products & Dankoff Solar Pumps