What is battery diagnostics


What is battery diagnostics

A working battery is required to ensure that the engine starts reliably in all conditions. This is obvious, but not every service technician routinely checks the condition of the battery. Most often, battery diagnostics are carried out only when the battery refuses to cooperate.

Classic measurement of electrolyte density

Testing the density of the sulfuric acid solution that is the electrolyte has always been one of the primary methods for testing batteries. Today, due to the proliferation of maintenance-free batteries, it is of decreasing importance (it is impossible to measure the density of the electrolyte in them due to lack of access), but still many batteries have plugs that allow measurement.

The advantage is undoubtedly the possibility of using one of the cheapest workshop instruments for testing batteries, the areometer. In addition to measuring the density of the acid, this inexpensive instrument equipped with a transparent tube allows you to visually assess it.

The acid solution should be colorless, turbidity may indicate contamination or refilling of the electrolyte with tap water. Gray electrolyte indicates sulfation, which you can try to remedy by charging for a long time with a low current. The brown shade will be a sign of systematic overcharging of the battery and is due to the release of plate material (accompanied by accelerated water loss). On the other hand, a black coloration of the electrolyte (or dark brown) qualifies the battery for replacement due to a significant drop in active mass.

The right density of electrolyte

The correct value for the electrolyte in a fully charged battery is considered to be 1.28 g/cm3. Density values are given for room temperature; density decreases with increasing temperature by 0.01 g/cm3 for every 15 degrees. We take the measurement a minimum of 30 minutes after turning off the engine for each cell separately. The density of the electrolyte decreases with the degree of discharge. A value of less than 1.25 g/cm3 signals the need for charging. If the density of the electrolyte deviates significantly in any of them, it almost certainly indicates a problem with the battery. If the electrolyte level is too low then we replenish it approx. 1-2 cm (or to the marked level) above the plates by adding the appropriate amount of distilled water. After this procedure, let’s wait a few hours for the electrolyte to homogenize before taking another density measurement.

Load test

The resting voltage indicates the state of charge of the battery. The desired voltmeter reading is 12.6 volts or more before the engine starts. If the battery voltage is lower than 12.5 volts then additional charging should be done before assessing the efficiency of the battery.

A popular way to assess the condition of a battery is to check the battery voltage under load. The load test helps assess the starting capacity. It involves measuring the voltage together with a high value load (on the order of 100 A) for several seconds. If the voltage does not fall below the threshold specified in the instrument’s instructions (typically about 9 – 10 V) the test is successful. The disadvantage is limited load selection, while the battery’s rated capacity and starting capacity can vary widely.

Is it enough?

Although the methods described can indicate whether a battery is suitable for continued use, they do not always explain why the battery life is lower than expected. The cause of the premature end of the battery can be very different. Poor selection of the capacity of the battery under test for a given car is one of the causes. If the battery’s capacity and inrush current are too small, its life will be affected. Other causes could be a malfunctioning starter or improper charging system voltage.

The user can harm the battery by leaving the vehicle with some sort of power receiver on. Leading to a discharge extremely harms starter batteries. Their design (thin plates with a large surface area) provides the ability to briefly draw current of very high value, but is not suitable for cyclic operation. If a discharge has already occurred, it is better to connect the discharged battery to a charger as soon as possible.

Another problem is, in the case of service batteries, allowing a large loss of electrolyte. Evaporating water causes the cells to work with over-concentrated acid, which leads to corrosion of the plates.

Modern battery diagnostics

The greatest diagnostic capabilities are obtained by using electronic testers that operate on the principle of measuring conductivity (conductance). Such a device is the DBT-12+ manufactured by DTE. Checking the battery becomes simple and fast. Just connect it to the clamps, and the tester allows you to evaluate the inrush current in seconds and compare it with the value specified by the manufacturer.

At the same time, we learn how deeply the battery is discharged and can distinguish this condition from an internal short circuit, in which the voltage also drops. Since battery temperature plays a role we have the ability to compensate for it by specifying a range of values.

In addition, the DBT-12+ tester offers tests of the charging system and engine starting system. When examining the charging system, we measure the voltage without load and under load (after switching on loads of several tens of amperes). The tester records the voltage pulsation which will help detect a fault in rectifier diodes even if the voltage is normal. If the charging system is malfunctioning, don’t be surprised if the battery becomes sulfated when the voltage is too low. A common and relatively simple to fix problem is insecure or oxidized terminals of battery terminals or ground connections.

When testing the start-up system, we also record the start-up voltage waveform. A common cause of battery damage is starter problems. Each time the starter is started, a large current flows from the battery. If it is too large it will reflect on the state of the battery.

Battery diagnostics should not be limited to checking the battery itself, but also the environment in which it operates. Let’s use the functions provided by modern testers, not limited to measuring the parameter of starting capacity.


Author: Product Engineer Piotr Libuszowski


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