Spectrum analysis and battery testing

Andy Sagl, product manager

Why do the best battery test sets have built-in spectrum analysers? In a nutshell, the answer is that the spectrum analyser gives the test set user the ability to examine the ripple content of the current being used to charge their batteries and, by determining the ripple frequency, they can verify the health of the charger. Let’s look in a bit more detail at how this works.

Charger rectifiers

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which always flows in the same direction. Rectifiers have many uses and they are key components of battery chargers.

The devices in a rectifier that convert the AC to DC are called diodes. They convert the AC to DC or “rectify” the AC by blocking the negative or positive portion of the waveform. Almost all rectifiers are made up of several diodes in a specific arrangement for more efficiently converting AC to DC. There are many types of rectifiers that use different diode configurations.

Half wave rectifier

The half wave rectifier uses a single diode in this configuration:

This type of rectifier, which blocks the negative portion of the wave, is used to convert single-phase AC to DC. In most applications, filter capacitors are added to the circuit after the diode to smooth out the DC waveform.

Full wave rectifier

The full wave rectifier uses four diodes arranged in this configuration:

Like the half wave rectifier, this type of rectifier is used to convert single-phase AC to DC. It is, however, more efficient as it uses both the negative and positive parts of the wave.

Three-phase full wave rectifier

A three-phase full wave rectifier, which typical of the type used in many of today’s battery chargers, incorporates six diodes arranged in this configuration:

As with the single-phase rectifier circuits, filter capacitors can be added to smooth out the DC supply.

Ripple frequency

No rectification process is 100% efficient; there is always some AC superimposed on the DC output. The AC voltage superimposed the rectifier’s DC output is called ripple voltage. In most cases, since “pure” DC is the desired goal, ripple voltage is undesirable. In today’s chargers ripple voltages and currents are usually very low, but this can change as the charger ages and components begin to fail. Ripple in excess of 5 A per 100 Ah of battery capacity may cause heating in the batteries, which can dramatically reduce their life span.

Rectifiers with different configurations produce ripple at different frequencies. Sometimes, the methods of rectification are classified by counting the number of DC “pulses” they output for every cycle of the AC input. A single-phase, half wave rectifier circuit would be called a one-pulse rectifier, because it produces a single pulse of DC output for every one complete 360º cycle of the AC input waveform. A single-phase, full wave rectifier would be called a two-pulse rectifier because it outputs two pulses of DC per AC cycle. A three-phase full wave rectifier of the type shown above would be called a six-pulse rectifier.

The normal ripple frequency for each type of rectifier can be determined by taking the number of pulses and multiplying this by the frequency of the AC supply. A full wave rectifier with a 50 Hz input would, therefore, have a ripple frequency of 2 * 50 Hz = 100 Hz. A six-pulse rectifier with a 60 Hz input would have a ripple frequency of 6 * 60 Hz = 360 Hz. By examining the frequency of the AC ripple of a battery charger, it is possible to determine if the rectifier is healthy.

Waveform analysis

A waveform can be described using one of two basic methods, the time domain method or the frequency domain method. The waveform can be described in the time domain by recording how its amplitude changes over time. The waveform can be reproduced if the amplitude values are known for given time intervals.

The other way to describe a waveform is to use the frequency domain method. In this case, the waveform is described as a set of frequencies added together, as shown in the example, see below.

By using a mathematical process known as a Fourier transform it is possible to convert waveform data from the time domain to the frequency domain. This is the technique used in battery tester spectrum analysers. The tester measures the amplitude of the current periodically and then uses a Fourier transform to convert the time domain information to the frequency domain. The different frequencies that make up the charging current can then be displayed in a bar chart, like this:

By knowing what type of rectifier is used in the charger, it is easy to work out what the ripple frequency should be. If the spectrum analyser shows significant levels of ripple at other frequencies, this is a clear indication that there’s a problem in the rectifier circuit of the charger, such as a faulty diode. Battery testers that include spectrum analysers can, therefore, not only evaluate the health of the battery string, but also verify the health of the charging circuitry.