Lead-acid battery monitoring technology has matured to the point where the IEEE issued a standard (IEEE 1491) in 2005 to describe the various battery health assessment methods. Most large data centers, for example, spend tens of thousands of dollars for battery monitoring equipment designed to predict whether their UPS batteries will work. Data centers with downtime costs of six figures per minute often make large investments in battery monitors that can provide early warning of weak battery cells. Battery monitor capital costs frequently reach $40,000. Additional money is spent installing the battery monitor systems and interpreting their data. Sometimes, interpretation of abundant battery monitor data is outsourced to a battery service company or the monitor’s manufacturer.

Delivering useful predictive function means choosing the best compromise between performance, cost, and simplicity from the menu of techniques described in IEEE 1491. Choice of Battery Assessment Method The original, and still most accurate, way to evaluate whether a battery can perform is to load test it. We can exploit this fact by taking advantage of the fact that the starting battery gets “tested” under heavy load each time it cranks the engine. Measurement of battery discharge voltage under load is a simple technique and can indicate whether there are battery problems. At least one manufacturer of Genset transfer switches has adopted this technique. The general theory of operation is straightforward: Assume that a 24-volt starting battery normally falls to, say, 19 Volts during an engine crank. During the most recent engine start, it dropped under 19 volts. This is an important change that could indicate that the battery is weakening. Unfortunately, the information is not complete. Another equally viable explanation for the lower voltage reading during engine crank could be a heavier load on the battery – as is likely to occur during winter or if engine oil was changed to a heavier viscosity. Two INTELEC papers from the 1990s presented a straightforward battery failure prediction technique, called “battery middle point voltage comparison” (midpoint monitor) that can reduce the ambiguity associated with simple voltage readings. This midpoint monitoring technique divides the battery in half for measurement purposes and continuously compares the voltage of each half of the battery to the other. An alarm is activated if voltage deviation exceeds a predetermined range. There are three principles of operation: The first is that one weak cell makes the whole battery bad. The second principle is that voltage drop in healthy cells tracks the others during discharge. In contrast, weak cells show early, and faster voltage decay. A weak cell creates an imbalance between the two battery halves, activating the failure alarm. The third principle is that one battery cell almost always weakens before any of the others. The practical difference between the two techniques is the number of warnings of battery failure. Assume a 24-volt battery with 12 cells, one of which has weakened but not failed: when simply detecting total battery string voltage, the weaker cell can be disguised by the performance of the other 11 cells in series. If the starting current demand is light – say a warm engine on a warm day – total battery voltage would remain above alarm limits during engine cranking. In contrast, midpoint monitoring would measure a voltage imbalance between the two 12-volt series batteries and issue an alarm regardless of total battery voltage. This means that the midpoint monitor gives an earlier warning of battery problems with an associated lower risk of start failure.

The following chart summarizes general requirements for battery monitoring in a genset environment:

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