The battery temperature sensor is critical to ensuring proper charging. The sensor needs to be well-attached to the side of the battery, about two-thirds of the way up from the bottom. It should be located toward the middle of the battery bank—not on the end or sides, where it would have more airflow or be influenced by other heat sources. Check the sensor wiring for damage, such as breaks or nicks in the insulation.
Once the visual inspection is complete, move on to taking measurements—the only way to really ascertain what’s truly happening inside of a battery. First, sketch out the battery bank layout and assign a label to each battery and each cell.
In the “Labeling Batteries for Consistent Data” illustration, four 6-volt batteries are wired in two series strings for 24 volts and the two strings are wired in parallel. Each battery is made up of three 2-volt cells with a cap on each cell, which allows individual electrolyte inspection and measurement. Permanently marking each cell of the battery with an identification number or letter is very useful for future reference and comparison.
Voltage is the easiest measurement to make, since it only requires a digital voltmeter to be connected to the terminals of each battery. Measure and record each battery’s voltage in a table similar to the example. Some batteries allow measurement of each individual 2-volt cell, while others with multiple cells in one case allow access only to groups of cells—such is the case with nearly all 6-volt and 12-volt batteries.
The battery should be kept in an open-circuit condition while making these measurements—this is accomplished by disconnecting all charging sources and all loads. Allow the battery to rest for 30 minutes, and then make all of the measurements in as short of a time period as possible. To get accurate state-of-charge measurements, many battery manufacturers suggest waiting six or even 24 hours for the battery voltage to stabilize. However, this can be difficult at sites where the battery system is the only power source. When voltage is used to compare one battery to another, a shorter time period can be used.
The voltage variations between the individual batteries provide valuable information on the batteries’ overall condition. Low-voltage readings can reveal weak or failed batteries, and wide variations can indicate the need for an equalization charge, an intentional overcharging of the battery bank to bring the weakest cells up to their full charge. Equalization also mixes the electrolyte to eliminate stratification of the acid and helps remove any sulfation on the lead plates. This can help to balance capacity differences between cells and allow the battery bank to be charged to its maximum potential. This will be discussed more in the second part of this three-part series.
All batteries have a higher voltage when no loads are connected. Once a load is connected, the battery’s voltage decreases. The amount that the voltage drops is a good indicator of the battery’s health and can be used to compare each battery in a bank to find poorly performing batteries or cells.